They are small squares, usually bright colors and black crosshair, like those you see in video games. I always wondered what exactly they are used for.
In: 2
Probably reflective survey markers, put at a specific known location and height, so someone with survey equipment (such as a laser sight) can point a laser at it and know precisely how far they are away from that point.
If they have two markers to target, they can work out their exact location.
If you keep them in place, you can measure several from a fixed point over long periods of time to see if there is any movement of land or buildings going on.
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