Latitude/Longitude storage and compression in C

Question

Does anyone know the most efficient representation for lat/long coordinates? Accuracy level should be enough for consumer GPS devices.

Most implementations seem to use double for each unit, but I'm suspicious that a float or fixed point format should be sufficient. I'd be curious to hear from anyone who has tried to compress and or store large arrays of these values.

EDIT:

In other words, whats the minimum accuracy required to represent lat/long for a consumer level device?

Answer 1

Personally I would use a 32 bit decimal fixed point representation, dividing by 1,000,000 as per Evan's answer and my comments.

However, if space is truly at a premium, here are some additional ideas:

• You could use a 26 bit fixed point representation on the wire. This will require marshalling and unmarshalling the latitude and longitude into a large array of bytes, but will save you 12 bits for each location over the 32 bit value representation - almost a 19% saving, so it might well be worthwhile.

• You could take advantage of the fact that longitude values need less precision as you get closer to the poles - they only need 26 bits worth at the equator. So you could write a scheme where the number of bits used to encode the longitude depends on the value of the latitude.

• If your data has other compressible attributes - say, all the points are usually quite close together - you could take specific advantage of those, like using a delta coding scheme (where each point other than the first can be encoded as a delta from the last point).