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?
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.
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.