Shadow Height Measurements
The height of several features imaged on the ground shots was determined as follows. First, the sun elevation () and azimuth () was established at the time the aerial photography was exposed. The azimuth was measured directly on the aerial frame after it was accurately aligned with true north. Shadows cast by buildings and trees were then measured, and an average taken. It was found that on the frames of the GX120 mission, = 90 degrees. That angle, in conjunction with the geographic coordinates of the camp and the time of year, was then used to compute the local sun time (t) and elevation angle. The results were: t =0634 AM, and = 24 50m.
The last step was to measure on the aerial photos the length of the shadows cast and then to compute the heights of the features casting them. This is given by elementary trigonometry by:
Other Height/Ranging Measurements
FL = Nominal focal length of the camera | |
H(o) = Estimated height of the object | |
Where: | H(i) = Measured image height of the object |
SF = Scale factor to compensate for enlargements |