Decoding a GeoRef code

When we decode a GeoRef code, we have to separate the two parts: the four characters at the beginning and the numeric details at the end. Once we've split off the first four characters, we can divide the number of remaining characters in half. One half of the digits will be longitude and the rest will be latitude.

The first four characters must be looked up in our special GeoRef alphabet. We'll find each character's position in the ABCDEFGHJKLMNPQRSTUVWXYZ string to get a numeric value. An expression such as georef_uppercase.find('Q') gives us 14: the position of Q in that alphabet. We can then multiply one position by 15° and add the other position number to translate two characters to the degrees portion of GeoRef.

The remaining digits are simply minutes, which are 1/60 of a degree. During conversion there is a matter of creating a number and possibly dividing it by 10 or 100. The final step is to take out the offsets that were used to avoid signed arithmetic.

The whole...