graham at theseamans.net
Wed Feb 21 10:19:56 GMT 2007
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?
More information about the london.pm