I have a photo of an object with a scale and I want to calculate its area. I guess I have to create a polygon of the area to be calculated to use the 'field calculator'. But how do I tell the program what a centimeter (cm) is?

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    What GIS software are you using? – PolyGeo Oct 28 '13 at 11:49
  • Is 'cm' centimeter? – artwork21 Oct 28 '13 at 11:49
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    Is this an aerial image? If it is, the pixilation will probably limit the accuracy, depending on the height the photo was taken. I am assuming it is something like that since this is a GIS Q&A Site and you mention Qgis. Perhaps a little more information might help in terms of where the images came from and how you arrived at a scale. – Mark Cupitt Oct 28 '13 at 13:54
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    Edited my answer to add a 'gis' element to it. – Barrett Oct 29 '13 at 21:31
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    @Mark C Your workflow should work just fine. In fact, with some care (by placing multiple control points in suitable locations) you can detect and correct nonlinearities in the image (which are to be expected due to perspective distortions). This is why people routinely place coins instead of (or with) rulers in photos: they provide scale info in two dimensions, not just one, and this is critical. Even better might be to use multiple photos of the object and create a "DEM" from stereo pairs, so you can adjust the area calculation to be correct in three dimensions. – whuber Oct 29 '13 at 22:34

Here is a non-GIS option which should work unless the image is georectified:

With the AnalyzingDigitalImages software you first measure the scale on your photo, then you can measure features within the photo and it calculates the area using the scale.

Workflow - PDF

Digital Earth Watch Software link

If you are focused on using GIS:

Georeference control point 1 at the beginning of your scale bar and set dstX & dstY to 0

Georeference control point 2 at the end of the scale bar and set dstX to the scale length (1200ft, 300m, etc), dstY to 0. (Also, manually edit srcY to match control point 1 srcY)

That should scale your photo to match the scale bar. Use the measuring tool to check.

Reference GIS: Imaginary Plane World for coordinate system info.

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