[postgis-devel] geohash is a strange beast

[Nicklas Avén]

Hello

First I thought that this was a bug in PostGIS. 
But now I am not sure. It is likely just a smelling brain fart.

First what made me start thinking:

The query:
SELECT ST_Box2dFromGeoHash('0');

I expect to get the western half of the world back like :
"BOX(-180 -90,0 90)"

from my understanding after reading for instance this post:
http://www.bigfastblog.com/geohash-intro

But it doesn't. It returns:
"BOX(-180 -90,-135 -45)"

The reason is that geohash is coded with base32

So the function have no way to understand if we on bit level mean 0 or
00 or 000 or 0000 or 00000

The way the function is written it iterates all 5 bit-positions and if
there is a 0 it divides the eastern and southern part.
lon goes through those divisions
starts with -180 to 180
-180 to 0
-180 to -90
-180 to -135

and lat through
-90 to 90
-90 to 0
-90 to -45

3 rounds for lon and 2 for lat.

That is what the 5 bits in the base32 representation of 0 does.

Worst is that I guess this happens on all geohashes that ends with 0.
How to know how many bits to use?

And in the PostGIS case I have a question about if we are doing this
right.

We are assuming the maximum precision from the inputed 0
Is it not more right to assume the worst precision?

I hope I am totally wrong in all this or geohash is far from that usable
that I thought.

Did I forget my brain at home today?

Best Regards

Nicklas Avén