Re: Why doesn't GPSmap 76C convert to British Grid?

From: David Lee (davidlee_malvern_at_dont.use.this.bit.hotmail.com)
Date: 11/29/04 

Date: Mon, 29 Nov 2004 12:30:12 -0000

Annapress wrote...
> >The British National Grid was very
> >specifically designed to cover Britain's particular size and shape,
> >and wasn't used anywhare else.
>
> Any particular advantage of the British National Grid over the more
universal
> UTM system? They both seem identical in concept, with variation only in
the
> details. Which came first?
>
> Steve

Two big advantages

The National Grid covers the whole of the UK with a single coordinate
system. Whilst the majority of the UK lies within UTM zones U & V 30 (since
30 is centred on the Greenwich meridian) the western and eastern parts of
the UK overlap into zones 29 and 31, which would lead to needless problems
with change of coordinate system if we were to base our mapping on UTM.
(Norway took a different route and simply extended zone V32 westwards to
cover the entire country)

Secondly: UTM is based upon the International 1924 ellipsoid (AKA Hayford
1909) - and so requires conversion from the GPS standard WGS84 anyway.
Being a global average it isn't a good fit to the Geoid in any particular
place. In contrast the Ordnance Survey National Grid is based on Airy 1830
which was derived to be a good fit to the area occupied by the United
Kingdom.

Regarding dates - whilst the Ordnance Survey mapping dates back to the 18th
century I believe that the National Grid itself may have been adopted in
1945 - if this is true then OSGB and UTM would have arisen at about the same
time, since UTM was developed by the US Army Corps of Engineers in the
1940s, however I would expect that the OSGB concept was probably developed
much earlier.

David