Ge 178 lecture 5 (principles of aerial photography)

GE 178 Lecture 5: Principles of Aerial Photography and Photo Scale Determination

Aerial Photo (Image) vs Map

• Images

central projection, non-uniform scale

actual features

• Maps

orthogonal projection, uniform scale

symbols

Orthogonal vs Perspective Projection




Vertical Photography

Vertical Aerial Photograph

Characteristics • tilt ≤ 3° from the vertical

• scale is approximately constant throughout the photo

• p = i = n

• within limitations, a vertical air photo can be used as a map substitute

• most common format is a 9 by 9 inch photograph

Negative plane

Positive plane

Mean ground elevation

f (focal length) = C (principal distance)

Hmge

(flying height)

f = C

O

p = i = n

Vertical Aerial Photograph

Elements of a Vertical Photograph

Fiducial Marks

• optically projected geometric figures located at either the four corners of a photograph, or on the four sides of a photograph

• define the coordinate axes and geometric center of a single aerial photograph

• Intersection represents the principal point of the photograph

Fiducial Marks and Principal Point

Fiducial Marks

Three Photo Centers

1. Principal Point – geometric center of the photograph; intersection of the line normal to the image plane through the PC

2. Nadir – point vertically below the camera at the time the photo was taken; intersection of the plumb line through the PC with the image plane

3. Isocenter – point halfway between the principal point and nadir; point intersected by the bisector of the angle between plumb line and optical axis

Three Photo Centers

Ground

Na

dir

Is

oce

nte

r

PP

oin

t

Kinds of Photography or camera according to focal length (f)

• Wide-angle (f = 6 inches)

• Normal-angle (f = 12 inches)

• Superwide-angle (f = 3.5 inches)

PhotoScale

Photoscale of Vertical Photo

distance on photo fphotoscale

distance on ground H

Recall:

But what if not all the required values are given initially, and instead some other parameters are known?

Determining Photoscale

Photoscale may also be determined according to:

• Smallest detail and resolution

• C-factor and desired minimum contour interval

• Expected accuracy

• Enlargement from photo to map in the instrument

Smallest detail and resolution

1

p

resolutionphotoscale

s smallest detail

• Resolution – smallest distance that a feature on the ground is still discernible on the image/photo


Example:

The smallest detail that needs to be seen on the photograph is 1 foot in length. If the resolution of the photo is 0.1 mm, determine the photoscale.


Solution:

1

3000photoscale

C-factor and desired minimum contour interval

• Contour interval – difference in elevation between consecutive contour lines

• C-factor range from 1200 to 1500

flying height HC factor

contour interval h


Example:

The C-factor of the instrument is given to be 1500. If the desired contour interval is 1 meter, determine the photoscale.


Solution:

1

9000photoscale

Expected Accuracy

40 1 0 0001 10-

hm . H . H H

Mean square error of horizontal position of points:

%ο – per mil; equivalent to 1/1000

For Cadastral Survey:

mh = 10 cm (urban)

= 30 cm (rural)

Expected Accuracy

Example:

Determine the photoscale for an urban area if the camera to be used is a wide-angle camera.

Expected Accuracy

Solution:

1

6000photoscale

Enlargement from photo to map

Zenlargement

C

• Using the stereoplotter, there will be an enlargement from the photo to the stereomodel:

where:

Z = projection distance for stereoplotter

C = f = projection distance of camera


Example:

A map with scale 1:5000 was derived from a stereomodel with a scale of 1:8000, using a stereoplotter. The projection distance of the stereoplotter is twice the focal length of the camera. Determine the scale of the photograph that was used to generate the stereomodel.


Solution:

1

16000photoscale

END OF LECTURE

Ge 178 lecture 5 (principles of aerial photography)

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