Aerial Photo (Image) vs Map
• Images
central projection, non-uniform scale
actual features
• Maps
orthogonal projection, uniform scale
symbols
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
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
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
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 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
Smallest detail and resolution
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.
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
C-factor and desired minimum contour interval
Example:
The C-factor of the instrument is given to be 1500. If the desired contour interval is 1 meter, determine the photoscale.
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.
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
Enlargement from photo to map
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.