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Comment by userbinator | Hacker News Reader

even using GPS coordinates

How can GPS coordinates be ambiguous? I can see different units of measure/notations being used, and the accuracy might be a problem for extremely close addresses, but wasn't it the point of GPS to provide an exact reference for any point on the planet?

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GPS coordinates don't generally include altitude, correct? If he's in a six-floor apartment, how could GPS define which floor he's in?

Then the full name should be anough to determine the floor. More problematic though is the inaccuracy of GPS. Where I live, my smartphone's GPS encircles 3 buildings.

Bingo. In my apartment building none of the units had numbers. I could point you to my unit, which was one of two doors on my floor, the other leading to a unit directly above me. So even GPS + Floor wouldn't quite me enough, although that's close enough that we could figure out who the package was for.

Then it's not a GPS coordinate, it's a lat/long one. GPS measurements include altitude; whether or not one chooses to use it is a different matter.

In the UK, locations may be encoded as Ordinance Survey map references (I've heard of people using OS references as addresses). It is what most geo databases in the UK were keyed on, until a few years ago.

The OS uses a different datum to the GPS system, so if you convert to lat/lon you'll be out by about 150 metres (where I am, at least). You need to re-project the coordinates onto the other datum to get the real location.

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