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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>David –<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I suspect the EXIF numbers are just hooey. Your image is 2,592
pixels by 3,872 pixels, and that’s the number of pixels on the sensor chip in
the camera. You’re unlikely to have a CCD in there that’s 11 inches by 16
inches or so! At a quick glance it looks like this is the major problem; you
should be working with the actual sensor size (just as you would work with the
actual size of the negative in a film camera). <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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</span></span></span><![endif]><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Ed<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Ed McNierney<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Chief Mapmaker<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Demand Media / TopoZone.com<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>73 Princeton Street, Suite 305<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>North Chelmsford, MA 01863<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><a href="mailto:ed@topozone.com"><span style='color:blue'>ed@topozone.com</span></a><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Phone: +1 (978) 251-4242<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Fax: +1 (978) 251-1396<o:p></o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
fwtools-bounces@lists.maptools.org [mailto:fwtools-bounces@lists.maptools.org] <b>On
Behalf Of </b>David Fawcett<br>
<b>Sent:</b> Thursday, December 20, 2007 4:16 PM<br>
<b>To:</b> fwtools@lists.maptools.org<br>
<b>Subject:</b> [FWTools] Calculating ground distance from altitude and
focallength : off topic<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>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...<br>
<br>
I have a case where I have an image shot from an airplane. I have the
below info: <br>
<br>
height of plane = 1221 meters<br>
focal length = 24mm (digital camera, so ~36mm film)<br>
EXIF value for X and Y Resolution is 240 (inches)<br>
The image dimensions are 2592 x 3872 pixels<br>
<br>
I am trying to calculate the approximate dimensions of this image in ground
units (meters). <br>
<br>
I believe that I can calculate the scale of the image by:<br>
<br>
focal length in mm / 1000 mm per meter/ altitude in meters or<br>
<br>
36 / 1000 / 1221 = 0.036 / 1221 = 0.000029484 <br>
<br>
If the resolution of the image is 240 px per inch, I should be able to
calculate an image size in meters: <br>
<br>
image height= 2592px / 240 px per inch / 39.37 inches per meter = .274321
meters<br>
<br>
image width= 3872px / 240 px per inch / 39.37 inches per meter = .409787 meters<br>
<br>
So using the scale value, I should be able to calculate meters on the ground by
taking: <br>
<br>
.274321 meters / 0.000029484 = 9304 meters<br>
<br>
width on ground = .409787 meters / 0.000029484 = 13898 meters<br>
<br>
When I look at the same image that I have georeferenced in a GIS, the image
measures about 1000m x 685m. <br>
<br>
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. <br>
<br>
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. <br>
<br>
Thanks, <br>
<br>
David.<o:p></o:p></p>
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