Photogrammetry

PhotogrammetryPhotogrammetryProf. W.N. Wilson Department of Geography

DefinitionsDefinitions

The science or art of obtaining reliable measurements by means of photography (American Society of Photogrammetry, 1956)

The art, science and technology of obtaining reliable information about physical objects and the environment by recording, measuring and interpreting photographic images (American Society for Photogrammetry and Remote Sensing, 1987)

The use of photography for accurate measurement is called photogrammetry (simple definition)

Aerial photographs are Aerial photographs are notnot mapsmaps

Aerial Photographs are single-point perspective views of the Earth’s surface, whereas maps are orthogonal representations of the surface

Sizes shapes and positions of objects are distorted in aerial photographs

However, aerial photographs can be used to construct maps and to accurately measure distances, heights and elevations.

Orthogonal representations of Orthogonal representations of the surface (MAP)the surface (MAP)

Figure - 1

Single-point perspective views of Single-point perspective views of the Earth’s surface the Earth’s surface (AERIAL (AERIAL PHOTOGRAPH)PHOTOGRAPH)

Figure - 2

If the terrain is completely flat and horizontal, the vertical photograph is same as map. If difference in height occur, then relief displacement becomes appearance. Pole AB appears as a point on map and as line on aerial photo. So aerial photo cannot be used as a map.

Figure - 3

Then how to prepare maps from Then how to prepare maps from aerial photographs?aerial photographs?

To overcome relief displacement, which is shown above, two photographs are taken of the same area at different positions. These photographs are overlapped and viewed stereo viewing devices and using this principle a map can be prepared.

Figure – 4

There are two kinds of photographs used in photogrammetry, aerial and terrestrial.

◦Aerial photographs are usually acquired from aircraft but can also come from satellites, hot air balloons or even kites.

◦Terrestrial photographs come from cameras based on the ground (Photo Theodolites), and generally are used in different applications from aerial

There are two main data extraction methods used for analyzing these photographs:

 ◦Quantitative: that is size, length,

shape, height, area etc.◦Qualitative : geology, vegetation,

drainage, land use etc.  This section is primarily concerned with

the quantitative evaluation of survey photographs.

The Aerial CameraThe Aerial Camera

The rays of light pass through one central point in the lens of a survey camera, and the distance between this point and the film is calibrated to decimals of a millimeter. This distance is known as the focal length.

The film plane is as flat as possible, often using flattening devices to achieve this.

The camera body also carries reference marks known as fiducial marks that define a coordinate measurement axis and allow film stretch to be determined.

The camera is also very large, the size of the negatives is 23cm by 23cm which is somewhat bigger than the average snapshot.

The cameras are constructed in this manner so that measurements of a high accuracy and precision can be obtained.

Air-photo terminology

Fiducial marks: Marks on the photograph margins used to locate principal point in photo.

Nadir point:-The vertical line passing through the perspective centre (camera lens) intersects the negative plane of at nadir point (n).

Principal point: The orthogonal projection of the perspective centre on the negative (photo) is the principal point. In other words it is the geometric centre of photograph.

Principal distance:- The distance from projection centre to negative plane (c).

Iso centre: The angle is called the angle of the maximum tilt. The point where the line bisecting this angle intersects the negative plane is the isocentre (i).

Principal axis:- The perpendicular line from the perspective centre on the negative plane.

Principal line:- The line joining the nadir point and principal point.

Principal plane:- The plane containing perspective centre , nadir point, iso centre, principal point, principal axis and principal line.

Iso line:- A line representing the intersection of the plane of a vertical photograph with the plane of an over lapping oblique photograph.

Axis of tilt :- A line through the perspective centre perpendicular to principal plane.

Conjugate principal point: Point in overlapping photo that is equivalent to principal point of adjacent photograph.

Photo base: Distance between principal point and conjugate principal point measured on a single photograph.

Ground (air) base: Ground (air) distance between principal points of overlapping photographs.

Parallax: Apparent shift in relative positions of objects when viewed (photographed) from different vantage points.

Exercises: 1. On photo, the distance between nadir

point and principal point is 0.5cm. Focal length is 200mm. Determine the tilt.

Tan a = 5/200 a = 1 25’ 55.6”

2. The distance between principal point & nadir point 20mm. C=210mm. Where is the iso center?

Comparison of aerial photographs

vertical, oblique and high oblique

Photographs

High oblique

Low oblique Vertical

Characteristic

Horizon on photo

No horizon photo

Tilt smaller than 4 degrees

Coverage Greater (wide)

Less (normal) Least (narrow)

Area (shape) Trapezoidal Trapezoidal Rectangular

Scale Decrease from for ground to back ground

Like as high oblique but lesser extent

Uniform for one level

Comparison with map

Greatest difference

Less Least

Advantage Economic Easiest to map

The Geometry of the Aerial Photograph

The average scale of the aerial photograph can be computed either by taking the ratio of the flying height above the ground and the focal length (f/H), or by taking the ratio of a known distance in the photograph and the distance on the ground.

As the flying height above the ground is not usually known accurately, the second method is employed where a more reliable scale is needed.

Exercises: 1. Focal length = 6” Flying height = 12000ft Scale = 6”/ 12000’ = 1:20000

2. c = 210mm and Z = 4200 m Scale = 210mm / 4200m = 1:2000

3. The measured distance between points A & B is 215mm. The given co-ordinates are XA =205346.39 m, YA = 10793.16m ; XB =205100.11m, YB = 11587.98 m. Find the scale

Neither of the two scale calculation methods give an accurate scale as there are two distortions that affect measurements made on a single aerial photograph.

a. Height distortion:Because a lens and a photograph give a perspective or central projection, objects that are above or below the plane will be shifted by an amount approximately

b. Tilt distortion:Although a lot of care is taken in the flying of aerial photography the photographs are rarely taken exactly horizontal. The tilts that occur in the aircraft, although kept to a minimum by the levelling of the camera system, do affect the position of objects on the photograph.

There are other distortions such as film shrinkage, earth curvature, refraction effects and so on but these are only significant when dealing with precise photogrammetry.

Determining the height of an object from shadow length

h = L tanwhere H = flying height

h = height of the vertical object

L = length of the shadow

= sun elevation angle

f = focal length

Conditions:- The shadow on which the ground falls is level. The object is vertical

Sun elevation angle:Sun elevation angle:The sun angle above local

horizon can be derived using a solar ephemeris table, given the latitude and logitude of the site and photograph acquisition date and time.

Relief displacementRelief displacement

Any objects that are higher or lower than the principal point are displaced from its true planimetric (x,y) location on a vertical aerial photograph. This displacement is referred to as relief displacement or terrain distortion.

The relief displacement is outward from the principal point for objects whose elevations are above the local datum, and toward the principal point for objects whose elevation are below the local datum.

The direction of relief displacement is radial from the principal point.

Height from Relief Height from Relief DisplacementDisplacement

Figure

Simple definitionSimple definition::

Relief displacement is the distance between the position of a point on the photograph if it was on the reference plane and its actual position due to relief.

Conditions for Relief Conditions for Relief Displacement Method to find Displacement Method to find object heightobject height

Aerial photographs must be vertical or near vertical (<3° of tilt). Namely, the principle point is the photo nadir.

The top and the bottom of the object are visible

The object is on level base and vertical.

Exercises:

1. On flat horizontal terrain there is erected a flag pole of 25 m. The distance from the nadir point of the vertical photograph to the pole is 300 m. The flying height is 1000 m. Principal distance is 200 mm. Compute the relief displacement

2. On a photo of flat terrain there is an isolated hill 200m high. Flying height is 2000m. What is the greatest value of relief displacement which we may find ?