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Thats fine IF you are working the size out from a print-out, when working on data resolution is required, the picture requires calibrating as well! Counting pixels is totally meaningless unless you have some means of reference! The Only thing you can do without resolution is work out the percentage of pixels to the whole document.

 

If there is a scale on the picture, have you resized the picture to reflect the size of the scale if not your area calculations are meaningless?

IE if you measure the scale on the picture in pixels does it give you the same size as shown on the scale, are there any perspective/barrel distorsion parameters to be taken into account?