proper/best type for storing latitude and longitude

Question

In a system level programming language like C, C++ or D, what is the best type/encoding for storing latitude and longitude?

The options I see are:

• IEEE

Answer 1

After coming across this question after searching for an answer myself, here is another possible scheme based on some precedent.

The Network Working Group, RFC 3825 proposed a coordinate-based geographic location option for DHCP (ie the system that hands out IP addresses on a network). See https://tools.ietf.org/rfc/rfc3825.txt

In this scheme, latitude and longitude are encoded in degrees with fixed-point values where the first 9 bits are the signed degrees, 25 bits are fractional degrees, and 6 bits are used for the accuracy. The value of the accuracy bits indicates the number of the 25 fractional bits that are considered to be accurate (e.g. coordinates collected via a consumer GPS vs a high-precision surveyor's GPS). Using WGS84, the accuracy is 8 decimal digits which is good to about a millimeter regardless of where you are on the globe.

As a couple of others have posted, floating point encoding really isn't good for this type of thing. Yes, it can represent a very large number of decimal places but the accuracy is either ignored or has to be dealt with somewhere else. For example, printing a float or a double with full floating-point precision results in a number with decimal digits very very unlikely to be remotely accurate. Likewise, simply outputting a float or a double with 8 or 10 decimal digits of precision many not be a true representation of the source values based on how floating point numbers are computed (e.g. why 1.2-1.0 does not equal 0.2 using floating point arithmetic).

For for a humorous example of why you should care about coordinate-system precision, see https://xkcd.com/2170/.

Granted, the 40-bit encoding used in RFC 3825 is hardly convenient in a 32 or 64-bit world but this style can be easily extended to a 64-bit number where 9 bits are used for the signed degree, 6 bits are used for the accuracy, leaving 49 bits for the decimal portion. This results in 15 decimal digits of precision which is more than basically anyone will ever need (see humorous example).