This is new to me but from what I have read and experienced so far, it seems that the biggest problem is that looking at an aerial image is not something most humans do very often. And if these images are in grayscale, most humans can only distinguish between 8-10 shades of gray. They are difficult to measure and they are highly subjective to interpretation. This would also not be a job for someone who is colorblind. I'm sure these are only a few of the problems associated with interpreting aerial photographs.