[postgis-devel] geohash is a strange beast
nicklas.aven at jordogskog.no
Wed Feb 24 05:54:48 PST 2016
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:
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:
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
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?
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