[kamocat]

How high in the air are you? 252 inches is 21 feet.

With a laser-based sensor, you'll have to do some processing to deal with the noise in your reading (because you're measuring distance in a straight line, and there's ALWAYS clutter on the ground)

EDIT:
I suppose the other option is to use the ground-based vehicle as a distance reference. You can triangulate.
(To be honest, time-based methods with light are a pain to measure because it's such a minuscule length of time)