[erlang-questions] Debugging float math in geohashing module

Nick Gerakines <>
Tue Jun 3 07:52:04 CEST 2008

I've taken an interest in the concept of the geohash [1] and wrote a simple
module [2] to encode/decode latitude/longitude pairs into these geohashes.
The modules is loosely based on Geo::Hash [3] but with a number of obvious

So far encoding works perfectly and the same test cases from the Geo::Hash
perl module pass in my module for encode operations.

The decode functionality is different however. When I attempt to decode a
geohash into a latitude/longitude pair the end result is slightly* off. When
I say slightly off I mean that the float precision is incorrect from the
latitude/longitude that was initially given. I think it has something to do
with the way the floats are being divided and rounded in the process. I'm
really having a rough time working around this, any help is welcome. I've
tried (unsuccessfully) to work around it using big int tuples ({N, E}) but
that didn't go so well.

[1] http://en.wikipedia.org/wiki/Geohash
[2] http://github.com/ngerakines/erlang_geohash/tree/master/geohash.erl
[3] http://search.cpan.org/~andya/Geo-Hash/lib/Geo/Hash.pm

# Nick Gerakines

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