[FWTools] Calculating ground distance from altitude and focal
length : off topic
David Fawcett
david.fawcett at gmail.com
Thu Dec 20 16:15:39 EST 2007
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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