In biological mapping location data is often binned to central points. For
example:
23 deg 01.50 mins N, 54 deg 07.30 mins E
would bin (using 6 minute bins) to -
23 deg 03 mins N, 54 deg 09 min E

As an added complication figures along a bin line would move into 2 bins, e.g.

23 deg 06 mins north would bin as 23 deg 03 mins N AND 23 deg 09 mins N

I assume a look-up table is required but does anyone have an existing
solution to this one, albeit that the data would sit in a true tabular form
not text like this.

Regards
Vic