[erlang-questions] Debugging float math in geohashing module

Nick Gerakines <>
Thu Jun 5 02:00:03 CEST 2008


Thanks! Your explanation and a little more reading gave me everything I need.

# Nick Gerakines

On Tue, Jun 3, 2008 at 8:44 PM, Gustavo Niemeyer <> wrote:
> Hi Nick,
> (...)
>> 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
> (...)
> Can you present an example of what you'd expect to happen, and what's
> actually happening?
> If I understand what you mean, there might be nothing wrong with your code.
> Note that due to the nature of the Geohash algorithm itself, in most cases
> you won't be able to decode the value into a precise decimal floating
> point.
> What is done by the software behind geohash.org during encoding is to
> compute how many bits you need to at least match the decimal precision
> given as input, and during decoding the inverse is done: it only shows
> the decimal digits it can correctly represent given the number of bits
> available.
> As a hint, if you have 8 bits for the longitude code, and a maximum value
> of 360, the maximum error is 360 / 2^8, and with log(maximum_error, 10)
> you get the decimal exponent of the maximum error, which is pretty close
> to what you want.
> --
> Gustavo Niemeyer
> http://niemeyer.net

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