Setting Scale and the Pixel Aspect Ratio

[LenaLy]

Dear ImageJ users,

I am conducting a project where I need to measure distances between hens. I am uncertain of how to correct the perspective of the image... The instructions have told me to divide the width by the height, does height refer to the distance from the ground to top view of the camera?

Setting a horizontal scale means that the distances are only accurate when lines are drawn horizontally, and the same for when I set vertical distances. Will setting a pixel aspect ratio correct this?



Going by the measurements that I have of this image, my field of view is 38m squared, so what should I be setting my pixel ratio at?