It is possible for a Globalhash to be in exactly the same place as a Geohash on the same day. Making it there satisfies the requirements for the time-traveller achievement. To calculate the coordinates of this colocation for a specific graticule, use the following pseudocode formula (or non-pseudo equivalent, left as an exercise for the reader):
Latitude Offset: (graticule_latitude + 90) / (180 - sgn(graticule_latitude))
Longitude Offset: (graticule_longitude + 180) / (360 - sgn(graticule_longitude))
where sgn(x) is +1 if x is positive (including +0) and -1 if x is negative (including -0)
Example for 52,9:
Latitude Offset: (52 + 90) / (180 - 1) ≈ 0.793296089385474
Longitude Offset: (9 + 180) / (360 - 1) ≈ 0.526462395543175
So, if a colocation were to fall in 52,9, the coordinates would be be at 52.793296089385474,9.526462395543175.
On weekdays, West of 30W, the Globalhash uses a different Dow opening to the Geohashes, so a colocation can happen anywhere in a graticule.
Colocations have never occurred in recorded history. As of 2018-09-06, the record holders for approximations are:
- The closest approximation ever occurred on 1946-07-15, in the North Pacific Ocean, with a distance of 210 meters.
- The closest approximation on land occurred on 1998-01-20, in Ghana, with a distance of 430 meters.
- The closest approximation after the invention of the algorithm occurred on 2009-01-10, in Foxe Basin, with a distance of 730 meters.
- The closest approximation on land and after the invention of the algorithm occurred on 2009-04-12, in Australia, with a distance of 2120 meters.
About the methodology:
- The approximations only check the distance between the Globalhash and the Geohash in the same graticule. If a Globalhash were on the edge of a graticule and a Geohash on the other side of the graticule border, that currently would not part of the above records.
- The approximations are the closest coordinates, which is not necessarily the closest distance, since the distance between longitudes is shorter the further away from the equator they are.
- The distances were taken from Distance.to.
