[nine_k]

I don't see why apps like that should give you an exact distance instead of some rough estimate.

"X is a few miles away", "X is less than a,mile away", "X is several yards away" (which is, say, within 30-50 yards) — and more precision is not needed. If users want contact, they need to explicitly coordinate.

This approach, of course, should not be centered on the exact user location, but use some rough and slightly irregular grid, so that the best one could do is to determine the cell of that grid (again, like 50 yards wide), without any idea where in that cell the target user is.

This, and query throttling so that scanning more often than once in 5 minutes isn't possible.

[nlitened]

It’s easy to measure statistics of a device noising among several grid cells to find with high probability the exact coordinate. Think of a person standing right where grid lines intersect, so with 25% probability each time the position will be reported to be one of four neighboring cells (due to errors in GPS measurement), which gives their exact location right on the intersection. If the probability weights’ distribution is different, it’s easy to infer the coordinate as well. Or if the person is moving, you can build their trajectory with high confidence.

It’s not that easy to obfuscate the coordinate in a way that the position is still relatively accurate (for the purpose of distance), and not “hackable” with some basic statistics.

[Gunax]

I think the goal is to divide the earth into grids. Then report which grid you're in.