I have seen several scales/scalebars for use in fieldwork/scientific photography that include quartered circles (similar to targets). If I understand correctly, those would be useful to correct images that were taken with the camera at an angle regarding the surface being photographed - a circle being photographed at an angle would appear as an ellipse in the final image.
Nevertheless, I've never been able to find a clear explanation of how this is done in post-processing/image editing software. Can anyone help me out with this? Many thanks!