Multiple geohashes decode to same lat/lon

[dasjo]

hi,

i'm having a problem with decoding geohashes:

• 'u26r' and 'u26q' both decode to lat 48, lon 15.

i can "reproduce" this here
http://geohash.org/u26r
http://geohash.org/u26q

on the other hand, these sites visualize those geohashes as different squares

u26r
latitude :47.724609
longitude:14.589844

u26q
latitude :47.548828
longitude:14.589844

http://geohash.2ch.to/u26r
http://geohash.2ch.to/u26q
http://openlocation.org/geohash/geohash-js/
http://geohash.gofreerange.com/

i'm confused, thanks for any clarification :)

[dasjo]

to me this seems like a rounding issue. here's my workaround
dasjo@fba595c

[mprins]

geoPHP conforms to the spec/reference of geohash.org (see also: https://en.wikipedia.org/wiki/Geohash)

The second list of examples that you provide is actually incorrect as these provide a sense of accuracy that is false, anything beyond the integer values (or at least the thenths fraction) is made up, an artifact of the conversion.

Also because you're using 4 chars the precision is different for lat and lon

That doesn't mean our rounding is perfect or couldn't be enhanced, however I think your change actually introduces more error/noise than it solves.

ps. if you want the squares/bounding boxes you should pass the $as_grid =TRUE to read().

[dasjo]

thanks for the quick feedback. i'm not sure, if the implemented rounding really conforms to the spec.

the wikipedia article states:

"Final rounding should be done carefully in a way that
min <= round(value) <= max
So if rounding 42.605 to 42.61 or 42.6 is correct, rounding to 43 it is not."

i didn't double check manually, but the example from http://geohash.2ch.to/u26r stated that u26r maps to
latitude min / max: 47.636719 / 47.812500
longitude min / max: 14.414063 / 14.765625

if those are correct, we should only round up to
lat: 47.7
lon: 14.6

[dasjo]

besides that, thanks for the pointer to pass the $as_grid =TRUE to read(). i should be able to use this workaround for calculating a center point within the application.

the reason why this bugs me, is because for http://drupal.org/project/geocluster, i decided to use geohash prefixes for spatial clustering. this obviously doesn't work when i want to visualize clusters based on their geohash prefix and sometimes different geohash prefixes map to equal lat/lon points.

[mprins]

I think you're correct on the rounding.

I think you can force higher precision hashes using the $precision = parameter on write(), i think this should be an integer..

[phayes]

@dasjo, would you be willing to create a pull request for this?