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dsigner
05-26-2009, 03:00 AM
Hello all,
I am trying to create a system which will enable me to input my useful collection of maps (mainly OS) and then enter a town. The system should tell me whether i have a map which shows that town. Simple idea but a few nasties in the woodwork. These mainly concern different reference systems. In particular
Lat/Long and National Grid. Everyone seems agreed that exact conversion is very complicated (spherical geometry and all that) but I will settle for something which works to about a Km or 1/2 mile.
I am thinking of a straight interpolation between the corners of the map and also the NG 100Km squares. Trouble is I need an origin in Lat/Long for each NG 100Km square. Does anyone know if such a listing exists? I could create it but it would not be very accurate and take quite a lot of work. Better I think to start from real datums at least.
Any comments would be really welcome especially since there may be a different approach which would give the required resul

Winwaed
06-02-2009, 05:17 PM
The conversion involves a bit of trig, but it is listed in a publication on the OS website. It isn't *that* bad. The hardest bit is error checking the geoid conversion because errors tend to be small. But the typical error (100m or so) is less than your stated tolerance.

Actually if your tolerance is so wide, why not get the values from an online mapping program - eg. the Ordnance Survey's own public map portal?

(another option is to buy a Landranger map for each corner!)

Richard