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Introduction
The photogrammetry has been derived from three Greek words: o
Photos: means light o Gramma: means something drawn or written o
Metron: means to measure
This definition, over the years, has been enhanced to include
interpretation as well as measurement with photographs.
Definition The art, science, and technology of obtaining
reliable information about physical objects and the environment
through process of recording, measuring, and interpreting
photographic images and patterns of recorded radiant
electromagnetic energy and phenomenon (American Society of
Photogrammetry, Slama).
Originally photogrammetry was considered as the science of
analysing only photographs.
But now it also includes analysis of other records as well, such
as radiated acoustical energy patterns and magnetic phenomenon.
Definition of photogrammetry includes two areas:
(1) Metric:
It involves making precise measurements from photos and other
information source to determine, in general, relative location of
points. Most common application: preparation of plannimetric and
topographic maps.
(2) Interpretative:
It involves recognition and identification of objects and
judging their significance through careful and systematic analysis.
It includes photographic interpretation which is the study of
photographic images. It also includes interpretation of images
acquired in Remote Sensing using photographic images, MSS,
Infrared, TIR, SLAR etc. Definitions
Aerial Photogrammetry Photographs of terrain in an area are
taken by a precision photogrammetric camera mounted in an aircraft
flying over an area. Terrestrial Photogrammetry Photographs of
terrain in an area are taken from fixed and usually known position
or near the ground and with the camera axis horizontal or nearly
so. Photo-interpretation Aerial/terrestrial photographs are used to
evaluate, analyse, and classify and interpret images of objects
which can be seen on the photograph
Applications of photogrammetry
Photogrammetry has been used in several areas. The following
description give an overview of various applications areas of
photogrammetry (Rampal, 1982) (1) Geology:
Structural geology, investigation of water resources, analysis
of thermal patterns on earth's surface, geomorphological studies
including investigations of shore features.
engineering geology
stratigraphics studies
general geologic applications
study of luminescence phenomenon
recording and analysis of catastrophic events
earthquakes, floods, and eruption.
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(2) Forestry:
Timber inventories, cover maps, acreage studies (3)
Agriculture
Soil type, soil conservation, crop planting, crop disease,
crop-acreage. (4) Design and construction
Data needed for site and route studies specifically for
alternate schemes for photogrammetry. Used in design and
construction of dams, bridges, transmission lines. (5) Planning of
cities and highways
New highway locations, detailed design of construction
contracts, planning of civic improvements. (6) Cadastre
Cadastral problems such as determination of land lines for
assessment of taxes. Large scale cadastral maps are prepared for
reapportionment of land. (7) Environmental Studies
Land-use studies. (8) Exploration
To identify and zero down to areas for various exploratory jobs
such as oil or mineral exploration. (9) Military intelligence
Reconnaissance for deployment of forces, planning manoeuvres,
assessing effects of operation, initiating problems related to
topography, terrain conditions or works. (10) Medicine and
surgery
Stereoscopic measurements on human body, X-ray photogrammetry in
location of foreign material in body and location and examinations
of fractures and grooves, biostereometrics. (11) Miscellaneous
Crime detection, traffic studies, oceanography, meteorological
observation, Architectural and archaeological surveys, contouring
beef cattle for animal husbandry etc.
Categories of photogrammetry
Photogrammetry is divided into different categories according to
the types of photographs or sensing system used or the manner of
their use as given below:
(1) On the basis of orientation of camera axis: (i) Terrestrial
or ground photogrammetry When the photographs are obtained from the
ground station with camera axis horizontal or nearly horizontal
(ii) Aerial photogrammetry
If the photographs are obtained from an airborne vehicle. The
photographs are called vertical if the camera axis is truly
vertical or if the tilt of the camera axis is less than 3
o. If tilt is more than (often given intentionally), the
photographs are
called oblique photographs. (2) On the basis of sensor system
used: Following names are popularly used to indicate type of sensor
system used in recording imagery.
Radargrammetry: Radar sensor
X-ray photogrammetry: X-ray sensor
Hologrammetry: Holographs
Cine photogrammetry: motion pictures
Infrared or colour photogrammetry: infrared or colour
photographs
(3) On the basis of principle of recreating geometry When single
photographs are used with the stereoscopic effect, if any, it is
called monoscopicphotogrammetry. If two overlapping
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photographs are used to generate three dimensional view to
create relief model, it is called stereophotogrammetry. It is the
most popular and widely used form of photogrammetry. (4) On the
basis of procedure involved for reducing the data from
photographs
Three types of photogrammetry are possible under this
classification: (a) Instrumental or analogue photogrammetry It
involves photogrammetric instruments to carry out tasks. (b)
Semi-analytical or analytical Analytical photogrammetry solves
problems by establishing mathematical relationship between
coordinates on photographic image and real world objects.
Semi-analytical approach is hybrid approach using instrumental as
well analytical principles. (c) Digital Photogrammetry or softcopy
photogrammetry It uses digital image processing principle and
analytical photogrammetry tools to carry out photogrammetric
operation on digital imagery. (5) On the basis of platforms on
which the sensor is mounted: If the sensing system is spaceborne,
it is called space photogrammetry, satellite photogrammetry or
extra-terrestrial photogrammetry. Out of various types of the
photogrammetry, the most commonly used forms are
stereophotogrammetry utilizing a pair of vertical aerial
photographs (stereopair) or terrestrial photogrammetry using a
terrestrial stereopair.
Classification of Photographs
The following paragraphs give details of classification of
photographs used in different applications
(1) On the basis of the alignment of optical axis
(a) Vertical :
If optical axis of the camera is held in a vertical or nearly
vertical position.
(b) Tilted :
An unintentional and unavoidable inclination of the optical axis
from vertical produces a tilted photograph.
(c) Oblique : Photograph taken with the optical axis
intentionally inclined to the vertical. Following are different
types
of oblique photographs:
(i) High oblique: Oblique which contains the apparent horizon of
the earth. (ii) Low oblique: Apparent horizon does not appear.
(iii) Trimetrogon: Combination of a vertical and two oblique
photographs in which the central photo is vertical and side ones
are oblique. Mainly used for reconnaissance. (iv) Convergent: A
pair of low obliques taken in sequence along a flight line in such
a manner that both the
photographs cover essentially the same area with their axes
tilted at a fixed inclination from the vertical in opposite
directions in the direction of flight line so that the forward
exposure of the first station forms a stereo-pair with the
backward exposure of the next station.
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Comparison of photographs
Type of photo Vertical Low oblique High oblique
Characteristics Tilt < 3o Horizon does not appear Horizon
appears
Coverage Least Less Greatest
Area Rectangular Trapezoidal Trapezoidal
Scale Uniform if flat Decreases from foreground to
background
Decreases from foreground to background
Difference with map Least Less Greatest
Advantage Easiest to map - Economical and illustrative
(2). On the basis of the scale
(a) Small scale - 1 : 30000 to 1 : 250000, used for rigorous
mapping of undeveloped terrain and reconnaissance of vast areas.
(b) Medium scale - 1 : 5000 to 1 : 30000, used for reconnaissance,
preliminary survey and intelligence purpose. (c) Large scale - 1 :
1000 to 1 : 5000, used for engineering survey, exploring mines.
(3). On the basis of angle of coverage
The angle of coverage is defined as the angle, the diagonal of
the negative format subtends at the real node of the lens of the
apex angle of the cone of rays passing through the front nodal
point of the lens.
Name Coverage angle Format size (cm)
Focal length (cm)
Standard or normal angle 60o (i) 18
(ii) 23 (i) 21 (ii) 30
Wide angle 90o (i) 18
(ii) 23 (i) 11.5 (ii) 15
Super wide or ultra wide angle 120o (i) 18
(ii) 23 (i) 7
(ii) 8.8
Narrow angle < 60o
Information recorded on photographs
The following information is recorded on a typical aerial
photograph 1. Fiducial marks for determination of principal points.
2. Altimeter recording to find flying height at the moment of
exposure. 3. Watch recording giving the time of exposure. 4. Level
bubble recording indicating tilt of camera axis. 5. Principal
distance for determining the scale of photograph. 6. Number of
photograph, the strip and specification no. for easy handling and
indexing. 7. Number of camera to obtain camera calibration report.
8. Date of photograph
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Figure 1: Typical aerial photograph
Types of projections
1. Parallel : The projecting rays are parallel. 2. Orthogonal :
Projecting rays are perpendicular to plane of projection. This is a
special case of parallel projection. Maps
are orthogonal projection. The advantage of this projection is
that the distances, angles, and areas in plane are independent of
elevation differences of objects.
3. Central : Central projection is the starting point for all
photogrammetry. In this projection rays pass through a point called
the projection center or perspective center. The image projected by
a lens system is treated as central projection although in
strictest senses it is not so.
Figure 2: Various types of projections
Introductory definitions for photographs Vertical photograph
A photograph taken with the optical axis coinciding with
direction of gravity. Tilted or near vertical
Photograph taken with optical axis unintentionally tilted from
vertical by a small amount (usually < 3) Focal length (f)
Distance from front nodal point to the plane of the photograph
(from near nodal point to image plane). Exposure station (point
L)
Position of frontal nodal point at the instant of exposure (L)
Flying height (H)
Elevation of exposure station above sea level or above selected
datum.
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Click on the figure for a larger view
Figure 3: Aerial photographs showing various elements as defined
(a) Elements of vertical photograph (b) Section of imaging geometry
showing various elements
X-axis of photo
Line on photo between opposite collimation marks, which most
nearly parallels the flight direction. Y-axis
Line normal to x-axis and join opposite collimation marks.
Principal point (o)
The point where the perpendicular dropped from the front nodal
point strikes the photograph or the point in which camera axis
pierces the image plane. Camera axis
It is a ray of light incident at front nodal point in the object
space and at right angles to the image plane. Fiducial marks or
collimation marks Index marks usually four in number, rigidly
connected with the camera lens through the camera body and forming
images on the photographs to which the position on the photograph
can be referred. Photographs center
The geometrical center of the photograph as defined by the
intersection of the lines joining the fiducial marks. Format
It is the planar dimension of photograph (9" x 9", 7" x 7", 23
cm x 23 cm, 18 cm x 18 cm, 15 cm x 15 cm). Photogram
Photograph taken with a photogrammetric camera having fixed
distance between negative plane and lens and equipped with fiducial
or collimating marks. For photograms the bundle of rays on the
object side at the moment of exposure can be reproduced. To achieve
this the following data known as the elements of interior
orientation must be known:
Calibrated focal length
Lens distortion data
Location of the principal point with reference to the photograph
center (normally these two coincide)
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Hence, a photogram is a photograph with known interior
orientation Difference between near vertical photographs and
map
1. Production : Quickest possible and most economical method of
obtaining information about areas of interest. Boon for difficult
areas. Enlarging and reducing easier in case of photographs than
maps.
2. Content : Map gives an abstract representation of surface
with a selection from nearly infinite number of features on ground.
Photograph shows images of surface itself. Maps often represent
non-visible phenomenon (like text) This may make interpretation
difficult for photograph. Special films like color and infrared
films can bring about special features of terrain.
3. Metric accuracy: Map is geometrically correct representation,
photos are generally not. Maps are orthogonal projections, photo is
central projection. Map has same scale throughout photo has
variable scale. Bearing on photographs may not be true.
4. Training requirement: A little training and familiarity with
the particular legend used in the map enables proper use of map.
Photo-interpretation requires special training although initially
it may appear quite simple as it gives a faithful representation of
ground.
Perspective geometry of vertical photographs
The figure shows camera axis SP of a camera, perpendicular to
the photographic plane ABCD, tilted at angle from the vertical at
exposure so that the plane of the photograph itself is inclined by
an angle to the horizontal plane CDEF, representing a level ground.
S represents perspective center as defined by the inner or rear
node of the lens system.
Figure 1: Perspective geometry of aerial photo
One can identify pair of points v and V (ground and photo nadir
point), i and I (ground and photo isocentre), p and P (ground and
photo principal point) on photographic plane and ground plane
respectively. These point pairs are called homologous pair. In fact
any corresponding point pair on photo and ground plane is a
homologue pair.
Perspective Axis
Line CD where the two plane meet is called the perspective axis
or horizontal trace.
Principal lines
A line UP drawn perpendicular to the perspective axis along the
photograph plane. This projects as Up on ground plane (CDEF) and is
also perpendicular to perspective axis. These lines are called the
photo and ground principal lines respectively.
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Principal plane
A plane containing P, V, and S is called the principal plane.
Photo principal line (VP) and ground principal lines (vp) are
contained in this plane.
Plate parallels or plate horizontals
Horizontal lines drawn in the photo plane are called plate
parallels or plate horizontals. Line iI is the bisector of tilt
angle . It meets photo plane at i and ground plane at I. These
points are known as photo isocentre and ground isocentre
respectively.
For a truly vertical photograph taken from exposure station S,
various photo plate parallels are lines I'I", P'P", etc. The plate
parallel through I is also called isometric plate parallel.
Scale of a vertical photograph
Due to perspective geometry of photographs, the scale of
photograph varies as a function of focal length, flying height, and
the reduced level of terrain over a certain reference datum. In
figure 2, for a vertical photograph, L is exposure station, f is
its focal length, H is the flying height above datum, h represents
the height of ground point A above datum. Point A is imaged as a in
the photograph. From the construction and using similar triangles
Loa and LOAA, we can write the following relations (Wolf and
Dewitt, 2000)
Figure 2: Scale for vertical photo
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havg = Average terrain elevation Savg =Best single scale to use
for a photo or group of photographs
Determination of Scale of photograph
Scale of photograph can be determined by various methods such as
1. By using known full length and altimeter reading, the datum
scale can be found. 2. Any scale can be determined if havg known.
havg can be obtained from a topographic map. 3. By comparing length
of the line on the photo with the corresponding ground length. To
arrive at fairly representative
scale for entire photo, get several lines in different area and
the average of various scales can be adopted. 4. Use the
formula
Scale along plate parallels Referring to figure 1, the scale
along various plate parallels are as follows:
1. Scale through the plate parallels passing through principal
point, P with q as tilt angle
2. Scale along an isometric parallel through nadir point, V
3. Scale along an isometric parallel through isocentre I
This shows that the scale along plate parallel through isocentre
of a tilted photo is same as that over the whole surface of a
vertical photo if ground surface is plane. For any other plate
parallel, scale will depend on the tilt angle. Also, the scale
along any plate parallel is constant
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Ground co-ordinates for vertical photographs In figure 3, X and
Y are ground co-ordinates with respect to a set of axes whose
directions are parallel with the photographic axes and whose origin
is directly below the exposure station, x and y indicate x and y
photo coordinates with respect to the photo coordinate system with
origin at o axes as shown. Using similar triangles, we can write
the following relations:
Click on the image for larger view
Figure 3: Ground coordinates from vertical photographs
Flying height for vertical photographs
The flying height can be calculated by two approaches
Direct
Indirect
Direct Method
In this method, if the ground coordinates of two points, A and B
are given (XA, YA ) and (XB, YB ), then a quadratic equation can be
formed to derive the flying height as given below:
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Indirect Method
In this method, one can find the flying height by an iterative
approach. For this one can use equations (1) and (2) where hAB =
average elevation of points A, B, H app is approximate height and
AB = known ground distance Get H app by equation
1. Using Eq. (1), get Happ. 2. Using Happ, get XA, X B, YA, YB
using formulae for measurement of coordinates. 3. Get computed AB
using XA, XB, YA, YB. 4. Again use Eq. (2) to get new H value. 5.
If the required precision been obtained (i.e. old Happ and new H
values have converged and do not differ by more than a
threshold value), then stop else go to (b) and repeat.
Relief displacement on Vertical photographs
In figure, L is the perspective center of the camera system. A
is the point on ground at an elevation of h with respect to the
datum. a is the image of ground point on photograph. a' is the
location of projected point A' on the datum. These figures indicate
that although point A is vertically above point B, their images are
not coinciding and are displaced on photographic plane due to
relief.
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Figure 4: (a) Relief displacement on vertical photo (b) Radial
nature of relief displacement (Moffit, 1959)
The displacement of the point a on the photograph from its true
position, due to height, is called the height or relief
displacement or relief distortion (RD). This distortion is due to
the perspective geometry.
It can also be noticed form these figures that the relief
displacement is radial from nadir point. In case of vertical
photographs, the nadir point and the principal point coincide.
Hence, in this case relief displacement can be considered to be
radial from the principal point also. The following derivation
using figure 4(a) provides the magnitude of relief distortion
Differentiating d with respect to H gives From these equations,
the following observations can be made:
1. For a given elevation, RD of a point increases as the
distance from principal point increases. 2. Other things being
equal, an increase in H decreases RD of a point. (This point is
important for mosaicing or combining
photographs along common features). For the same reason, for
satellite imagery, RD is very small since H is large. 3. If ground
point is above datum then RD will be outward or positive; if point
below datum then h has negative sign and
RD will be inward or negative. 4. RD is radial from nadir point
regardless of unintentional or accidental tilt of the camera. This
is a fundamental concept of
photography. It has important implication for photo
rectification (this concept will not be discussed in this course).
5. Large relief displacement is objectionable in pure plannimetric
mapping by graphical methods but advantageous in
contouring with stereoplotting instruments. Most effective way
to control RD is to select proper flying height.
Numerical problems 1. The distance on a map between two road
intersections in flat terrain measures 12.78 cm. The distance
between the
same two points is 9.25 cm on vertical photograph. If the scale
of the map is 1: 24,000, what is the scale of the photograph?
2. Fifteen photographs were taken in a strip each covering an
area equal to 25.75 sq. km. If the longitudinal overlap is 60%,
find the total ground area covered by the strip.
3. A aircraft takes photographs at a scale of 1:10,000. Photo
size is 23x23 cm. Overlaps are: longitudinal 65% and lateral 30%.
The photography consists of 5 strips of 21 photographs each.
Calculate: (a) Ground area covered by a single photograph. (b)
Ground area covered by the first strip. (c) Ground area covered by
the whole photography.
4. A vertical photograph was taken with H above datum = 2400 m
and f = 210 mm. The highest, lowest, and average elevation of
terrain appearing in the photograph is 1330, 617, and 960 m
respectively. Calculate minimum, maximum, and average photographic
scale.
5. An aircraft flying at an altitude of 4600 m above MSL
photographs 5 strips of 20 photographs each of a terrain having
havg = 300 m above MSL. If f = 205.53 mm, find the scale of
photograph and area covered by each photograph of size 23 x 23 cm.
Assuming 60% forward and 20% sidelap, find total area covered by
the photography.
6. Points A and B are at elevations 273 m and 328 m above datum,
respectively. The photographic coordinates of their images on a
vertical photograph are: xa = -68.27 mm xb = -87.44 mm
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ya = -32.37 mm yb = 26.81 mm What is the horizontal length of
the line AB if the photo was taken from 3200 meters above datum
with a 21 cm focal length camera?
7. An image of a hilltop is 87.5 mm from the centre of a
photograph. The elevation of the hill is 665 meters and the flight
altitude 4660 meters from the same datum. How much is the image
displaced due to elevation of the hill?
Answers:
1.
2.
3.
4.
5.
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6.
7.
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Scale of a tilted photograph
Click on image for larger view
Figure 1 Tilt-swing-azimuth relationship in tilted photo
(Moffit, 1959)
The figure shows a tilted photograph. In this figure, rotate y
-axis till the new y -axis or the y '-axis coincides with the
principal line having a positive direction no as shown (Moffit). =
Amount of rotation = 180 - s s = swing angle
In the same figure, P is the point lying at an elevation hp
above datum, p is its image on tilted photo, x, y ;
photo-coordinates, x and y are measured with respect to axes
defined by collimation marks. O - principal point, n - nadir point,
s - swing angle.
Since is clockwise it must be negative. Co-ordinates in rotated
system can be given below. Translating x' axis from oto w, the
co-ordinates of p after translation
Construct line wk perpendicular to line Ln. This is a horizontal
line (because Ln is a vertical line). Since wp is perpendicular to
the principal line, it is also a horizontal line. Therefore, plane
kwp is a horizontal plane.
Also, PP' = WW' = NN' = hp(from construction) and therefore,
plane nwp is also horizontal. In s Lkp and LNP
But Lk = Ln - kn. = f sec t - y' sin t. Therefore,
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But kp/NP = scale for a point lying in a plane kwp. Since p lies
on this plane
Where St scale of a tilted photograph at a point whose elevation
is h. f focal length. t tilt angle H flying height above datum. y'
y -co-ordinate of the point with respect to a set of axes whose
origin is at the nadir point and whose y' axis
coincides with the principal line.
Ground co-ordinates on tilted photograph
Referring to figure 1, the coordinates can be derived by
assuming the following situation:
1. Y-co-ordinate axis lies in the principal plane. 2.
X-co-ordinate axis passes through ground nadir point.
Therefore, ground nadir point is the origin of co-ordinates wp =
x', WP = X Therefore, scale relationship between plane kwp and KWP
is given by
X, Y Ground co-ordinates x', y' Co-ordinate of point with
respect to axes whose origin is at the nadir point and whose y'
axis coincides with the principal line t tilt Flying height for
tilted photo
The flying height can be derived in a similar as that for the
vertical photograph explained earlier. The procedure is given
below:
1. Get photo-coordinates of the end points of control line with
respect to axes defined by the collimation marks. 2. Photographic
length of line is scaled directly. 3. Use ratio of photographic
length to known ground length to get first approximation as
where, f - focal length h AB - Avg. elevation of A and B
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AB - ground length. ab - scaled photo length. By using Happ,
together with other scale data, one can solve the following
equations
4. Calculate distance based on first set of ground co-ordinates.
5. Compare this computed distance for better approximation of H.
Where H is new value of flying height, AB is the value of length
based on first approximation height
6. Repeat the procedure till computed value of length agrees
with the known ground length within the desired precision
Relief displacement on a Tilted photo
Figure 2: Relief displacement on a tilted photo
Figure 2 shows the relief displacement on a tilted photograph.
It should be noted that:
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1. On a tilted photo relief displacement (a'a) are radial from
nadir point (n). 2. The amount of relief displacement depends upon:
(i) Flying height (ii) distance from nadir point to image (iii)
elevation of
ground point (iv) position of point with respect to principal
line and to the axis of the tilt. 3. Compared with an equivalent
relief displacement on vertical photo, the RD on a tilted photo
will be
o less on the half of the photograph upward from the axis of the
tilt, o identical for points lying on the axis of the tilt and o
greater on downward half of the photo.
4. Image displacement due to the tilt (explained later) tends to
compensate relief displacement on the upward half and will be added
to RD on the downward half.
5. Because tilts in near-vertical photos rarely > 3o,
therefore the value of RD is given with sufficient accuracy with
following
equation. However, the radial distance should be measured from
the nadir point rather than from the principal point
Tilt displacement (TD) on a Tilted photo
Figure 3: Tilt displacement on a Tilted photo (Moffit, 1959) 1.
Tilted and corresponding vertical photo taken from same flying
height and with same focal length will match along the axis of
the tilt (passing from the isocentre) where they intersect. 2.
In the figure, image points A, B, C, D appear as images a, b, c, d
on tilted photo, and as a', b', c', d' on an equivalent
vertical
photo. If we rotate equivalent vertical photograph (EVP) about
axis of tilt, till it is in the plane of tilted photo then for any
point other than lying on the tilt axis, it will be displaced
either outward or inward with respect to equivalent position on a
vertical photograph:
o If point lies on the half of photo upward from the axis of
tilt, it will be displaced inwards. o If lies on downward half,
then displaced outward.
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Figure 4: Magnitude of TD (vertical section through principal
line (Moffit, 1959)
The TD is a"a, b"b, c"c, d"d. These displacement occur along
radial lines from isocentre.
Although a and c lie at the same distance from axis of tilt, but
the displacement of a is greater than that of c. This ratio =
secant of cia
Tilt displacement also depends upon the position of point with
respect to both the axes of the tilt and the principal lineno.
For c lying in principal line, TD is given as c"c
is obtained by measuring the distance to the point from a line
through the principal point and parallel with the axis of the tilt.
When point lies on principal line, as does the point C, the
distance is measured from principal point itself to the image
point. This distance is then divided by focal length to obtain tan
.
The following observations should also be noted for the tilt
displacement:
1. Tilt displacement for a point not lying on the principal line
is greater than that of a corresponding point on principal line. 2.
Above ratio is equal to the secant of angle at isocentre from
principal line to the point. Therefore, tilt displacement on
upper
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half of tilted photo is inward and is given as:
For a point lying in lower half or nadir point half is
outward
I = angle measured at the isocentre from principal line to the
point. t = tilt = angle in the principal plane formed at the
exposure station. Another expression for tilt displacement For a
point b on upper half of photo and using similar triangles Lqb and
ibb'. q is intersection of horizontal line through L meeting
extension of tilted line ab (Wolf and Dewitt, 2000)
Figure 5 Tilt displacement with respect to an equivalent
vertical photograph ( Wolf and Dewitt, 2000 )
Using figure 5, the following derivation can be accomplished for
TD given as d.
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For any point a lying on lower side of photo
For any general point The above equation holds for any point
lying on principal line. For any other point lying off the
principal line, TD is d/cos where is angle between the line passing
through off axis point and principal line at isocentre. For any
off-principal line pointb, TD is given as
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TD is maximum when = 0, i.e. point lies along principal line,
minimum when point lies along isometric parallel.
The general equation can also be written as
Figure 6: General format for TD ( Wolf and Dewitt, 2000, ) angle
in plane of photo, measured clockwise from positive end of
principal line to the radial line from isocentre to imaged point d
Tilt displacement. ri Radial distance from isocentre to imaged
point. f Focal length. t Tilt angle Angle in plane of photo,
measured clockwise from positive end of principal line to the
radial line from isocentre to imaged point. 1. The algebraic sign
of ri is always considered +ve. 2. The units of d will be same as
those used for ri and f. 3. The correct algebraic sign of d is not
obtained automatically but must be assigned. +ve : if point lies
above the axis of tilt. -ve : if the point lies below the axis of
tilt. For b between 0 - 90 and 270 - 360 , pt lies above axis of
tilt.
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Figure 7: Combined displacements of points due to relief and
tilt
In figure 7, position of each point: o position marked as 1 is
datum photograph position o position marked as 2 is the position
after image has undergone relief displacement o position marked as
3 is the position after image has been displaced due to tilt.
For points a and b, the two displacements tend to cancel; for
points c and d, these are cumulative; for point e lying on tilt
axis, there is no tilt displacement. This complexity is further
compounded by the fact that amounts and direction of tilts are
random and difficult to be computed.
Stereoscopic vision
Method of judging depth may be classified as stereoscopic or
monoscopic.
Persons with normal vision are said to have binocular vision.
They can view with both eyes simultaneously. These persons are said
to have stereoscopic viewing
The term monocular vision is applied to viewing with one eye
only.
Depth perception
Monoscopic viewing provides only rough depth impression which is
based on the following clues: o Relative size of objects o Hidden
objects o Shadows o Differences in focussing of eye required for
viewing objects at varying distances.
For stereoscopic depth perception usually two clues are involved
o Double image phenomenon. o Relative convergence of optical axis
of two eyes.
The human eye functions in a similar manner to a camera. The
lens of the eye is biconvex in shape and is composed of refractive
transparent medium. The separation between eyes is fixed (called
eye base). Therefore, in order to satisfy the lens formula for
varying object distance, the focal length of eye lens changes. For
example, when a distant object is seen, the lens muscle relax,
causing the spherical surface of the lens to become flatter. This
increases the focal length to satisfy the lens formula and
accommodate the long object distance. When close objects are
viewed, a reverse phenomenon happens. The
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eye's ability to focus varying object distance is called
accommodation.
The figure shows two situations where eyes separated by eye base
(b) are focussing on two objects A and B located at two distances
dA and dB respectively.
In binocular vision, there is convergence of axes of eyes when
focusing at point A and B. The corresponding angles are 1 and 2.
Angle 1 tells the mind that object A is distance dA. Similarly for
point B. These angle are calledparallactic angles for points. The
nearer the object, the greater the parallactic angle and vice
versa. Difference ( 1 - 2 ) tells the mind that the distance
(depth) between two points is e = d B - d A.
For average separation of eye and distinct vision of about 10
inch, the limiting upper value of 16. Lower limiting value of
ranges from 10 to 20 seconds and represents a distance of about
1700 to 1500 ft for average eye separation. It is called
stereoscopic acuity of the person.
Figure 1: Binocular vision (Wolf and Dewitt, 2000)
To understand the stereoscopic vision by viewing photographs,
let us replace the eyes by two cameras and take photographs as
shown in figure 2 assuming that the photographic plates are between
the objects and lenses. If these photographs could be placed in the
same position and seen with both eyes, one would get the same
depth. Since it is not possible to keep eyes at such a wide
separation (called air base B), a scaled version is shown in figure
(ii) where camera position are replaced by a scaled down
geometrical arrangement (eyes separated by eye base-b e ), the two
images are fused and the brain perceives the scaled down depth
between objects.
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Figure 2: Stereoscopic vision (i) object imaged with cameras
(ii) scaled down version with eyes (Wolf and Dewitt, 2000)
Stereoscope
If two photographs taken from two exposure stations L1 and L2
are laid on table so that left photograph is seen by left eye and
the right photo is seen with right eye, a 3D-model is obtained.
However, viewing in this arrangement is quite difficult due to
following reasons:
o Eye strain and focusing difficulty due to close range. o There
is disparity in viewing. The eyes are focused at short range on
photos lying on table where as the brain
perceives parallactic angles which tends to form the
stereoscopic model at some depth below the table.
By using stereoscope, these problems can be alleviated.
Different types of stereoscopes are available for different
purposes - from pocket (inexpensive) for viewing small area to
mirror (expensive) for viewing larger area.
Figure 3: Photograph showing mirror stereoscope (Wolf and
Dewitt, 2000)
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Figure 4: Principle of stereoscope (Wolf and Dewitt, 2000) (a)
pocket (b) mirror
Conditions of good stereo viewing
1. The two images must be obtained from two different positions
of the camera. 2. The left photo and right photo should be
presented to the left and right eye in the same order. 3.
Photographs of the object should be obtained from nearly the same
distance, i.e. scales of the two adjacent photographs
should be nearly same. Normal eye can accommodate 5 to 15%
variations in scale of photograph. 4. During stereovision, keep the
area under vision in near vertical epipolar planes as far as
possible. The plane containing
optical centers for eyes (S1 S2), object point A and
corresponding images a and a' on left and right photographs
respectively is called the epipolar plane. Thus camera axes should
be approximately in one plane, though the eyes can accommodate the
differences to a limited degree. The term epipolar planes is used
if it refers to eyes; the term basal planes is used if it refers to
cameras.
5. For a good stereovision, the pair of photos should be in the
same orientation with each other as they were at the time of
photography.
6. The brightness of both the photographs should be similar. 7.
The ratio B/H should be appropriate. If this ratio is too small say
smaller than 0.02, we can obtain the fusion of two pictures,
but the depth impression will not be stronger than if only one
photograph was used. The ideal value of B/H is not known, but is
probably not far from 0.25. In photogrammetry values up to 2.0 are
used.
Geometry of overlapping vertical photos
Figure 5 shows two photographs taken from two exposure stations
L 1 (left) and L2 (right). Ground point A appears in these two
photos at a and a' in left and right photos. The image coordinates
of these photo points a and a' are (xa, ya ) and (x'a, y'a )
respectively.
A pair of vertical photographs which have some common area
imaged is known as a stereopair. This stereopair can be used for
estimation of depth by measurement of parallax which will explained
next.
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Figure 5: Geometry of overlapping vertical photos
Figure 6: A stereopair
Parallax
It is the central concept to the geometry of overlapping
photographs and is defined as the apparent shift in the position of
a body with respect to a reference point caused by a shift in the
point of observation
In photogrammetry, it refers to the relative difference in
position of an image point that appears in each of the overlapping
photo.
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Absolute Parallax:
Absolute X parallax (P X ) is given by (x l - x r ),
where x l and x r are x coordinates on left and right
photographs respectively P x = x l - x r
Other names: X-parallax, horizontal parallax, linear parallax,
absolute stereo parallax, or just parallax For the following
photograph pair, the parallax is given as follows. P x = x b - (-x
b' ) It can be noted that the value of parallax has been used with
sign
Figure 7: Parallax in photograph pair Parallax and its relation
to height
From figure 5, the absolute parallax PA at point A is given as
follows where B is air base.
Similarly for another point B, we can write
Height differences between two points ( dh BA = h B - h A ) is
given by:
dP BA = P B - P A is the difference in absolute parallaxes
between two points. Above equation can be modified to give
where dp BA = p B - p A is the difference in parallax bar
readings between two points. Thus the above equation says that dPBA
= dpBA under the assumption that flying height for both photographs
is same (i.e. H1 = H2 = H) and photograph is truly vertical.
It may, however, be noted that for a given point, there may be
parallax in both directions X and Y. The Y-parallax is caused due
to the following reasons:
o Unequal flying height o Photographic tilt
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o Misalignment of flight line o Misalignment of stereoscope o
Great difference in parallax between adjacent images (in highly
mountainous/rugged terrain)
Other forms of parallax equation If the principal point are
assumed to have same elevation as known point A, then the parallax
of principal point is the same as the
parallax of point A i.e. P A = b m, the mean principal base.
Using this assumption
dp = absolute parallax of unknown point - absolute parallax of
known point
If dp is small then
Some other forms of parallax equation are given in adjacent
formats
Sources of errors in using parallax equation
Locating and marking the flight line on photo.
Orientation of stereopair for parallax measurement.
Parallax and photo co-ordinate measurements.
Shrinkage and expansion of photographs.
Unequal flying height.
Tilted photo.
Error in ground control.
Camera lens distortion, atmospheric refraction etc.
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Measurement with Parallax bar and floating marks
From the parallax equation, we note that o parallax of any point
is directly related to its elevation o parallax is greater for
point which are higher in elevation
If we could measure the parallax of the point, its height could
be measured easily. Unfortunately there is no device for this.
However, difference in absolute parallaxes which can be related to
height differences. Therefore, if the elevation of one point is
known, the elevation of other point can be measured.
Parallax bar (figure 8) or stereometer is used to measure the
difference in parallaxes between corresponding points on stereopair
which have been oriented as they were at the time of
photography.
Figure 8: Parallax bar (Wolf and Dewitt, 2000)
If the two photographs are placed in a properly oriented
position (this process is known as base lining) on a table and
viewed through stereoscope, the distances between corresponding
(conjugate) points on left and right photos can be measured by
parallax bar in terms of parallax bar reading. The difference
between two measured distances will provide parallax differences
between two points. The measurement by stereometer is called
parallax bar reading.
While measuring the parallax, the stereometer uses the principle
of floating mark.
Floating mark is used to carry out quantitative measurements on
stereo photographs. It is made up of two identical half marks or
dots which are placed over conjugate images on the two overlapping
photographs
The dots may be black circular spots or crosses (etched on two
clear glass plates), for example parallax bars used with
stereoscopes or many stereoplotters. The dots may also be luminous
circular spots, crosses or annuli which are used in stereoplotters,
stereocomparators and particularly the digital image display.
The floating marks are placed on the photographs such that the
right mark is on the right photo and the left mark is on the left
photo.
Within a stereomodel, the floating mark will appear to lie on
the surface of the terrain (fused form) when the dots are placed
exactly on conjugate or corresponding images.
If the dots are moved slightly closer together, the floating
mark will appear to rise or float off the surface.
If the mark floats too far above the surface then stereofusion
will be broken because the eyes can not accommodate the range in
convergence, and a double image of the terrain will result
If the dots are moved slightly apart the floating mark will
split into two separate dots as the brain of the observer can not
imagine seeing the floating mark beneath the surface
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Again if the dots are moved too far apart a double image of the
terrain will result.
Figure 9: Floating mark
Base lining
In order to get fused 3D images, the orientation relationship
between the two photographs and stereoscopic lenses should be same
as that between the photographed ground and the camera lenses at
the time of photography. Base lining is the process to achieve such
orientation. The step by step procedure to achieve base lining is
as follows:
o Find out the photographic center of the first photograph by
joining the opposite fiducial marks and getting the intersecting
lines. Let it be A. Mark a corresponding point A' on the other
photo of the stereopair.
o Find the photograph center of the second photo B and locate
corresponding point B' on the first photograph of the
stereopair.
o Join A, B' to get AB' and B and A' to get BA'. o Fix a white
sheet (called the base sheet) on table and draw a straight line on
this sheet in the middle. o Set up stereoscope on this sheet such
that the line is midway between stereoscope legs. o Put the first
photograph in such a way that the line AB' coincides with the line
on the base sheet. o Now by trial and error, and viewing through
the stereoscope, put the second photograph (right photograph)
in
such a way that the image of point A coincides with A' and B
coincides with B' i.e line AB' coincides with A'B. o This
arrangement will give 3D view of stereopair. o Now with the help of
stereometer, the floating mark of left plate is put over a well
defined point on the left
photograph. Now by giving lateral and longitudinal motion to the
other plate (with the help of stereometer drum), we try to bring
the floating mark of other glass plate on the corresponding point
of second photograph. When the images of left and right floating
marks are fused, the reading on the parallax bar can be recorded as
the parallax bar reading for that point.
Numerical examples 1. Two ground points A and B appear on a pair
of overlapping photographs, which have been taken from a height of
3650
m above MSL. The base lines as measured on the two photographs
are 89.5 and 90.5 mm respectively. The mean parallax bar reading
from A and B are 29.32 mm and 30.82 mm respectively. If the
elevation of A above MSL is 230.35 m, compute the elevation B.
2. In the above problem if the lengths of base lines are not
known and the absolute parallax of A is measured to be 89.80 mm,
compute the elevation of B. Also, find the height of another point
C whose parallax bar reading is 32.32 mm.
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Answers
1.
2.
Base lining
The flight planning is the process of making all relevant
preparations and taking certain decisions for taking photographs to
satisfy certain application requirements. This may include the
following:
o deciding about flying height above datum o spacing between
successive exposures o separation between flight lines
After careful decision about these elements, the flight lines
are carefully laid on the map of the study area to be photographed.
This map is called the flight map.
Computation of flight plan
The following information is required for effective flight
planning (Tiwari and Badjatia, 1985; Sadasivam, 1988): o Camera
focal length o Photographic scale o Flying height above datum o
Permissible scale variation o Relief displacement o Tilt of
photographs o Size of photograph o Area to be photographed or
ground coverage o Air base o Base-height ratio
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o Flight line spacing o Number of flight lines o Forward or
longitudinal overlap o Side or lateral overlap o Ground speed of
the aircraft o Number of photographs per flight line o Drift angle
o Exposure interval, maximum exposure time
Focal length (f)
It is the most important parameter for flight planning and must
be determined by proper calibration.
Photographic scale (S)
It is specified to ensure that the user can resolve the smallest
objects to be identified. Choice of scale is a function of project
at hand and the experience. This is a function of the focal length
(f) and altitude of aircraft (H) at the exposure time. Scales vary
from large to small. Very large scales are used for cadastral
surveys and small scale for topographic mapping. The larger the
scale, the better and more accurate the interpretation and
plotting. But this will result in too many photographs and hence
more time and money for covering the same area.
It is often more convenient to use an average scale
corresponding to the average elevation in the study area o (Sa = f
/ ( H - ha ).
Flying height (H)
This is the height of the camera exposure station and recorded
by altimeter (aneroid barometer) and sometimes with a statoscope.
It is recorded on the picture itself for easy reference.
For a given focal length, average scale, and focal length, the
flying height H is fixed. Several interrelated factors affect
choice of flying height. For example, scale, relief displacement,
and tilt.
Permissible scale variation
Scale variation is caused by combination of change in ground
elevation or flying height. Scale variation affect ground coverage.
Large scale variation also affect the capability to view images in
stereo mode.
Relief displacement
Large relief displacement create difficulty in forming
continuous interrupted picture. Relief displacement decrease with
height although increase in height reduces scale. Hence, these two
effects have to be balanced.
Tilt of photograph
The tilt in a photograph can be resolved into two components:
x-tilt and y-tilt, along x and y directions respectively. In a
photo with y-tilt, the forward overlap will be higher on one side
and lower on opposite side. The x-tilt causes the side lap to
decrease on one side and to increase on another. Large x-tilt
affects flight line spacing.
Crab and Drift
Crab is the angle formed between flight line and the edges of
the photographs in the direction of flight. It reduces the
effective width of photographic coverage. This can be rectified by
rotating the camera about the vertical axis of camera mount.
Drift is caused by failure of the aircraft to stay on
predetermined flight line. It leads to serious gaps between
adjacent flight lines.
Ground coverage
After choosing scale and camera format, the ground coverage with
a single photograph can be calculated. If the longitudinal and
lateral overlaps are known, the ground coverage by a stereomodel
can be calculated. This coverage is important since it provides
approximate mapping area.
Airbase (B)
This is the distance between two adjacent exposure stations. On
photographs, it is the distance between successive principal points
which is also called the 'advance'.
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Base-height ratio (B/H ratio)
The ratio of airbase (B) and flying height (H) is called B/H
ratio. Normally the vertical scale of stereo model than the
horizontal scale. This scale disparity, which helps determination
of heights and identification of objects better is calledvertical
exaggeration.
The average adult eye base (b) is about 6.5 cm, the
corresponding variable h is difficult to measure. Experiments have
provided an approximate value of 42.5 cm. This gives approximate
b/h value as 0.15. The vertical exaggeration V is defined as V =
(B/H) x (h/b)
Spacing of flight line spacing
It is defined as the distance between two adjacent flight lines
or strips, at the photographic scale or ground scale. The direction
of flight lines is also important. If there are number of ridges
and valleys, it is better to fly parallel to the ridges. The strips
are arranged in N-S or E-W direction keeping in mind the movement
of sun throughout the day and effects of shadows.
It is better to run the strips in E-W course, as the dip of the
magnetic north towards north (or south) makes accurate course of
flight difficult if navigation is with compass only.
Number of flight lines
It depends on total width of area to be photographed. To avoid
any possible gap in the flight line spacing and extra line can be
added at ends
Number of photographs per flight line
It depends upon the total length of flight line and is given by
the length of the line divided by airbase. Generally, additional
two additional photographs are taken at the end of line for factor
of safety.
Exposure interval
This is the time interval between two successive exposures and
is a function of longitudinal overlap and aircraft velocity. It is
equal to the time taken by aircraft to cover airbase. This can be
done with a device known as intervalometer, which automatically
make an exposures at fixed interval of time.
Maximum exposure time
Larger diaphragm opening (f - stop setting) allows more light to
enter the camera, giving better image. A low shutter speed allows
light for long time. The time interval which is small fraction of a
second, during which the diaphragm is kept open is called exposure
time. A small value of exposure time results in poor illumination
and darker image. But a higher value is also problematic since it
gives streak in an image instead of a point while imaging. So one
needs just sufficient exposure time (shorter) without any
appreciable image movement.
Assuming a permissible image movement of about 0.02 mm, the
maximum exposure time can be calculated. The image movement allowed
on the photograph, when converted to ground scale and divided by
the aircraft speed gives exposure time, and is normally expressed
as 1/t seconds.
Examples The overall flight planning procedure can be understood
by a few simple examples that follow. Example 1 : An area 45 km
long and 36 km wide is to be photographed to an average scale of
1:12000, using an aerial camera of f = 21 cm. The speed of the
aircraft is 200 km/h. The photographs are 23 cm square, with a
longitudinal overlap of at least 60% and lateral overlap of 30%,
average elevation of the terrain is 500 m above MSL. Calculate the
following:
The flying height above mean sea level (MSL).
Distance between successive exposures
Distance between flight lines for successive strips.
Flight line spacing on flight map at a scale of 1 cm = 600 m
Interval between successive exposures
Number of photographs per strip taking one extra photograph at
either end
Number of strips with only one extra strip as a safety
factor
Total number of photographs
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Solution:
The flying height (H) can be calculated by using
Spacing of the flight line (W)
Number of flight lines (N fl ) required
Actual spacing between flight lines
Spacing of the flight lines on a flight map
Ground distance between exposures
Exposure interval
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Since required overlap is at least 60%, hence exposure interval
can be kept as 19 seconds.
Adjusted ground distance between exposures L = 55.55 x 19 =
1055.45 m
No. of photographs per flight line (N p )
Allowing two extra photographs at each end, the total
photographs per flight lines = 44 + 2 + 2 = 48.
Hence, total no. of photographs = 48 x 20 = 960
Example 2: The following data is given for flight planning:
Format 18 x 18 cm
Focal length = 21 cm
Scale = 1/20,000
Longitudinal overlap = 60%
Lateral overlap = 20%
East-west terrain length = 100 km
North-south terrain width = 50 km
Flight direction: East to west
Aircraft velocity = 296 km
Permissible image movement = 0.02 mm
Wind velocity = 10 m /s from SSE direction
Calculate the following
Exposure interval
maximum exposure time
Ground speed of aircraft
Drift angle
Solution
Airbase = 0.4 x 18 cm at photo scale. On ground
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Exposure interval
Image movement = 0.02 mm on photo. On ground,
Time taken to cover this distance = maximum exposure time
(shutter speed)
Wind speed = 10 m/s = 36km/hr; flying speed = 296 km/hr. The
ground speed of the aircraft and the angle of drift can be found
out by vector operations. Effective ground speed = (296
2 + 36
2 - 2x296x36xcos67.5)
1/2 = 284.176 km/hr.
Drift angle ( ) is given as
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Developments in photogrammetry
In a typical photogrammetric process, the information is
obtained by establishing rigorously a geometric relationship
between the model created by images and the object, as it existed
at the time of the imaging event (Mikhail and Bethel, 2001).
The geometric relationship can be established by various means,
which are broadly classified as: o analog photogrammetric methods o
analytical photogrammetric methods o digital photogrammetric
methods
Further, based on the type of platform used for the data
acquisition, photogrammetry can be classified intotopographical
photogrammetry and non-topographical photogrammetry (also known as
close-rangephotogrammetry). Topographical photogrammetry uses
satellite and/or airborne imagery, whereas non-topographical
photogrammetry uses imagery from ground based platforms (Mikhail
and Bethel, 2001).
Data processing system is used for data reduction from geometric
information (e.g., image coordinates of targets) on the image to
object space information.
Depending on the type of input used and the desired output,
there are three alternatives for the data reduction: analog,
analytical and digital.
Analog methods use optical, mechanical and electronic components
for modeling and processing. In this mode of processing, the data
reduction is carried out using analog instruments (e.g., plotters
such as Kelsh K-480 stereoplotter). A typical instrument using this
principle is shown in figure 1.
Analytical methods use mathematical modeling assisted with
digital processing. In this mode, the image coordinates are
obtained using monocomparators or stereocomparators (equipments to
measure coordinates) and further processing is done through
computer. Images used in both the above methods are in hard copy
form (e.g., on film). A typical instrument known as analytical
stereoplotter which uses this principle is shown in figure 2.
In recent times, with the advent of digital scanners and digital
cameras, images in digital form are used instead of hard copy form.
In digital methods, the modeling is based on analytical principles
except that they use digital images as input. Hence, digital
photogrammetric methods use strengths of both analytical and image
processing methods for stereoplotting and related photogrammetric
works. In digital mode of processing, the entire process is carried
out using computer programs. These programs constitute digital
photogrammetric software. A typical instrument known as digital
photogrammetric work station (DPWS) is shown in figure 3.
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Figure 1: Kern PG2 mechanical projection stereoplotter equipped
with pantograph for direct manuscript plotting, and interfaced with
computer for digital mapping (Wolf and Ghilani, 2002)
Figure 2: Zeiss P3 Analytical Plotter (Wolf and Ghilani,
2002)
(a) (b)
Figure 3: (a) Digital photogrammetric work station (b) Digital
Video Plotter DVP (Wolf and Ghilani, 2002)
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In general, a photogrammetric system is defined by its three
components 1. Data acquisition 2. Data reduction 3. Data
presentation
1. Data acquisition
Data acquisition systems are concerned with procuring the data
or information. The data can be acquired in terms of images. The
data acquisition system is generally classified into two
categories: Conventional Imagery and Non-Conventional Imagery.
In conventional imagery, the imaging system has a lens and an
image plane such as frame photographs, which is based on the
central projection of the object onto an image plane. These can be
obtained by using both metric andnon-metric cameras. In
Non-conventional imagery the imaging system does not use a lens and
image plane. Holograms, X-rays, T.V. systems etc. are categorized
to this type of imagery.
The metric cameras are those manufactured specially for
photogrammetric applications. In these types of cameras, the
elements of interior orientation are known. The elements of
interior orientation include: the focal length and location of the
centre of the photograph. The metric cameras are further classified
as single and stereometriccameras. A single metric camera is
mounted on a tripod (figure 4 a) whereas a stereometric camera
consists of two identical metric cameras mounted rigidly at the
ends of a fixed base for photography (figure 4 b).
A non-metric camera is characterized by the off-the-shelf
cameras which are often used for conventional photography (Figure 5
a and b). These are not the cameras especially made for the
photogrammetric purposes. In these types of cameras, the elements
of interior orientation are unknown or partially available.
(a) (b)
Figure 4: Terrestrial cameras (a) Zeiss TMK6 camera (b) Tripod
mounted Zeiss SMK 40 + SMK 120 cameras
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(a) (b)
Figure 5: Non-metric cameras for photogrammetric application (a)
Sony video camera (b) Sony still camera
Digital close range photogrammetry (DCRP) is the latest
development in photogrammetry, which is especially used to obtain
3D spatial information about objects placed near the camera. In
this close range process, the cameras are generally positioned
within 100 meters from the object and camera axes essentially point
towards the center of the object (Atkinson, 1996). From multiple
positions, the user is able to acquire imagery at many convergent
angles.
The introduction of digital cameras into DCRP (as image
acquisition systems) has given rise to on-line systems, which
facilitate both real time and near-real time 3D coordinate
measurement. With the availability of wide range of digital cameras
including camcorders, CCD (charge coupled device) video cameras and
digital still cameras at affordable prices, the utilization of
these cameras as data acquisition systems in DCRP has increased
considerably (Samson, 2003).
DCRP systems employing wide range of cameras coupled with
automated image measurement and mapping have attracted wide usage
over the past decade for precise deformation measurement in
industrial and engineering applications. For example,
Engineering Applications o Monitoring of dam structures o
Highway applications (DTM and GIS for alignment by computer) o
Measurements of sand deposits in Hydraulics channel for different
flow conditions
Biomedical Applications o Design of prosthesis for below knee
amputees o Facial reconstruction studies o Physical education -
monitoring the movements of athletes
Architectural Applications o Depicting the existing state of
monuments/buildings and preparing working drawings o Studying the
deformation decay and damage to buildings/structures of importance
by periodic monitoring o Preserving the cultural heritage of
various epochs in the form of stereo - photographs o Reconstructing
and restoring and architectural monuments to their past glory o
Mapping of sites and relocating/recapturing the landscape and
location of monuments
Industrial Applications o Automobile industry o Shipping
industry o Antenna calibration
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2. Data reduction
The data reduction is concerned with the process of extracting
desired information from photographs. Photogrammetric techniques
have changed a lot with time.
In the earlier stages, analogue approach was used in which the
imaging geometry is reconstructed by orienting two images in such a
way that a three-dimensional model of the object is formed through
optical or mechanical devices. With development in optics and
mechanics the analogue photogrammetric instruments have improved
and can attain very high accuracy.
With the evolution of computers, analogue instruments have been
replaced by analytical plotters, where single or a pair of
photographs is placed in X-Y measuring system which digitally
records image coordinates (using mono or stereo comparators). The
relations between image points and object points are described
through numerical calculations based on the collinearity
equations.
The collinearity condition specifies that the exposure station,
ground point and its corresponding image point must all lie along a
straight line. Figure 6 shows the collinearity condition. Let O be
the exposure or perspective centre, P be the location of a point in
object space whose corresponding point in image plane is p.
Therefore, O p P represent the collinearity condition. The basic
transformation equation describing the relationship between two
mutually associated 3D coordinate systems is given by (Wolf and
Dewitt, 2000)
Figure 6: Collinearity equations
Where,
x, y coordinates of the image point,
X, Y & Z coordinates of the object point,
f focal length of the camera,
m11,.. m33 elements of rotation matrix,
X L, Y L and Z L coordinates of exposure station in XYZ
system
The introduction of digital photogrammetry has changed the world
of photogrammetry completely. The advances in electronics and
computer science have permitted new approaches to obtain
photogrammetric solutions more effectively, while the basic
mathematics, optical theory and many of the basic practices remain
same.
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3. Data presentation
It consists of preparing and presenting the results in a
suitable form. The final output can be in the form of contour maps,
pictorial representation of objects, digital model,
three-dimensional (3D) spatial coordinates etc.
The developments in digital photogrammetry has resulted in
digital photogrammetric work stations (DPWS). In the late eighties,
the International Society for Photogrammetry and Remote Sensing
(ISPRS) had defined a DPWS as "hardware and software to derive
photogrammetric products from digital imagery". Some of the well
known DPWS are Autometric, LH Systems, Z/I Imaging, and ERDAS.
A typical DPWS comprise standard hardware components such as
stereo viewing devices and a three-dimensional mouse with a core of
specialized photogrammetric software. Majority of DPWS are PCs with
windows operating systems though UNIX based systems are also there.
Such systems carry our various operations such as vector, raster
and attribute data storage and processing, image handling,
compression, processing and display, and several photogrammetric
applications such as image orientation, generation of digital
terrain models (DTM) or the capture of structured vector data, and
the user interface level (Heipcke, 2003)