Digest, Vol 16, Issue 29

michael michael at
Wed Feb 21 12:20:31 GMT 2007

Postcodes in central London are much much smaller than post codes in the
Scottish highlands.

So, if the coordinates are derived form postcode data,  you would expect 
that the Scottish coordinates would but much less accurate than the London 


Michael John Lush PhD			Tel:44-20-7679-5027
Nomenclature Bioinformatician		Fax:44-20-7387-3496
HUGO Gene Nomenclature Committee	Email:  nome at
The Galton Laboratory
University College London, UK

On Wed, 21 Feb 2007, at wrote:
> Message: 6
> Date: Wed, 21 Feb 2007 10:19:56 +0000
> From: Graham Seaman <graham at>
> Subject: geographical accuracy
> To: at
> Message-ID: <45DC1CCC.2090006 at>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Hi all,
> I'm trying to generate a set of latitude/longitude points for use with
> google maps (to avoid having to query google for the actual locations).
> What I have as source data is a set of northing/easting values with 6
> digits for northing, 6 digits for easting. I'm using
> Geography::NationalGrid::GB to convert these to lat/long pairs. The
> results are all approximately correct, but generally about 20 metres or
> so off in an apparently random way. The features I'm trying to locate
> are mostly quite large and clearly visible on google's satellite/hybrid
> view (but I have around 20,000 locations to deal with, so can't correct
> them all manually). I have seen the same features located accurately on
> another google map application, so I know the problem is not at the
> google end of things.
> I'm trying to pin down where the errors are coming from, and what I can
> do about them.  I believe that the northing/easting values have enough
> precision to give an accuracy of a metre (from a back-of-the envelope
> calculation...). However, it's possible that they were originally
> derived from post office data (ie. corresponding to postcodes) in which
> case I've read that the accuracy is limited to 10 metres, and the last
> digit is effectively noise. Then there's the question of the accuracy of
> the conversion - I assume there's some inevitable distortion in
> converting from a projection of a sphere onto a flat surface, followed
> by some loss from rounding in the calculations, but don't have an idea
> of how large this is likely to be.
> Can anyone with a GIS background tell me if my assumptions are wrong
> here, and more to the point if there's anything obvious I can do to
> improve my results?
> Cheers
> Graham

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