[Rolando]

it is not really much different to pot someone standing than someone on the ground, unless there is cover.

So the big difference is that a person flat on the ground may claim more visual cover than someone standing in the same type of ground.

A person in an empty, uniformly illuminated, parking lot is almost as easy to spot crouching or prone, even standing probably.

There are also other considerations, like point of view, someone flat on the ground may be easier to spot from above for example.