[FWTools] Calculating ground distance from altitude and focal length : off topic

[David Fawcett]

This question doesn't relate to FWTools functionality, but this seemed like
the best place to ask a question like this.  I am using gdal to get some of
the image info...

I have a case where I have an image shot from an airplane.  I have the below
info:

height of plane = 1221 meters
focal length = 24mm (digital camera, so ~36mm film)
EXIF value for X and Y Resolution is 240 (inches)
The image dimensions are 2592 x 3872 pixels

I am trying to calculate the approximate dimensions of this image in ground
units (meters).

I believe that I can calculate the scale of the image by:

focal length in mm / 1000 mm per meter/ altitude in meters or

36 / 1000 / 1221 =  0.036 / 1221  =  0.000029484

If the resolution of the image is 240 px per inch, I should be able to
calculate an image size in meters:

image height= 2592px / 240 px per inch / 39.37 inches per meter = .274321
meters

image width= 3872px / 240 px per inch / 39.37 inches per meter = .409787
meters

So using the scale value, I should be able to calculate meters on the ground
by taking:

.274321 meters / 0.000029484  = 9304 meters

width on ground =  .409787 meters / 0.000029484  = 13898 meters

When I look at the same image that I have georeferenced in a GIS, the image
measures about 1000m x 685m.

I am sure that I am totally screwing this up, and may even be in a case of
can't-get-there-from-here.  I would appreciate any input that anyone has.

I fully understand all of the limitations to this methodology, I am trying
for something that is quick, dirty, and automated.  I have a bearing and
will be doing rotation later, I just first want to get the ground units
correct.

Thanks,

David.
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