Assume the global position

Does anybody know anything about GPS?

Specifically: I want to map my backyard, so I can find a clearish spot right in the middle where I can indulge my hippie granola fantasy of someday building a marijuana farm drum circle in the woods. It’s dense wood with heavy undergrowth, so every time I walk through it (as opposed to around it) I get completely lost; I have no clear perception of how long a walk it is from the trail at the property edge to the house, because every time I try walking it I miss the house completely and end up at the river..

So I was thinking of getting a cheap GPS unit to map out the perimeter, then crash about in the bracken with it until I’ve found a likely spot. But here’s my question: would trees block the satellite signals? There are some clearings here and there, but most of it is pretty thick; if I’d have to climb a tree to tell where I am, it’d be a waste of time.

I guess the other question is, does anyone local already have one, that I could borrow for a couple days?