# [FWTools] Calculating ground distance from altitude and focallength : off topic

David Fawcett david.fawcett at gmail.com
Thu Dec 20 17:19:00 EST 2007

```Ed,

I was actually hoping that you would respond...

The sensor measurement for the D200 is 15.8 x 23.6 mm

When I run the calculations based on sensor size, I get 535 x 800 using a
focal length of 24mm and 803 x 1200 using a focal length of 36mm.  It looks
like the sweet spot may be somewhere in between.  (Whether the issue is with
the true focal length or not.)

David.

On Dec 20, 2007 3:41 PM, Ed McNierney <ed at topozone.com> wrote:

>  David –
>
>
>
> 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).
>
>
>
> -          Ed
>
>
>
> Ed McNierney
>
> Chief Mapmaker
>
> Demand Media / TopoZone.com
>
> 73 Princeton Street, Suite 305
>
> North Chelmsford, MA  01863
>
> ed at topozone.com
>
> Phone: +1 (978) 251-4242
>
> Fax: +1 (978) 251-1396
>
> *From:* fwtools-bounces at lists.maptools.org [mailto:
> fwtools-bounces at lists.maptools.org] *On Behalf Of *David Fawcett
> *Sent:* Thursday, December 20, 2007 4:16 PM
> *To:* fwtools at lists.maptools.org
> *Subject:* [FWTools] Calculating ground distance from altitude and
> focallength : off topic
>
>
>
> 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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