The chassis will be rolling and there will be lots of vibration (engine running and blades turning). Likely running a sort of grid pattern over a patch of ground.
Distance constraint will likely be up to 50ft or so. Height needs to be as close as possible to the set "zero reference". perhaps +/- .25"
water level sound like a very interesting idea..... will have to think about this some..... it would have to be off of the ground so it doesn't get rolled over and chopped by blades though.