I does the aimer on the "holographic dot" mirror work?

Anyway, this does indeed all depend on resolution. But high-res photography is getting much cheaper, and will continue to get cheaper as time goes on. I'm presuming we get down to one pixel per inch or so letting you spot people, clothes, some tracks, and of course cars. Plus any symbol (like SOS) marked out with rocks or tracks or branches or whatever.

My plan to design a glyph would work well if we could get people to know the glyph. To do this, I would gather lots of existing aerial photos, and then try test glyphs until you find one that does not occur in any of those photos (ie. not in nature or by accident.) It would need to be the simplest glyph that's easy for a computer to spot but not likely to occur in nature.

However, I think the cell phone proposal is the best one, and I will be expanding on it.