This popup converts latitide and longitude coordinates in the WGS84 datum to a national grid reference in the OSGB36 datum. It is useful for people who like to keep their GPS set to a WGS84 datum (eg some geocachers).
You can enter decimals under any of the above boxes, to cope with DDD.DDDDD° or DDD° MM.MMM' or DDD° MM' SS.S" type coordinates.