Modeling a bomb/mines field

[GPC]

Can you fly over the beach area with a helicopter? If you can drop something along the beach shore, like a paint bucket or barrel, something large enough to hit with a reflector-less total station, then you can control the entire area. It just needs to be tall enough that side facing the safe area is somewhat normal to the laser measurement. Fly over nice an high and just drop them.. so if there are ordinances you're at a safe distance away.

[JMR]

Differential GPS is not possible on cheap UAV 

If by cheap you mean trimble X-100 I'd say you are wrong in two senses. 1. xwing is far from cheap, imho. and 2. dgps for sub-dm accuracy is actually possible on board of small uas... just wait a couple of months. It's gonna take longer for kinematic, but is very on the way for post-processed solution.

On the other side, have you considered manned solutions like parafan or ul (ultralight aircraft)? this could fit perfectly and you could carry with you surveying-grade gps stuff.