Re: GPS to grid reference

Roger wrote:

I think we discussed this some years ago and the conclusion was that
with a gps the co-ordinates were of a point rather than the SE corner of
a square which the OS apparently thinks is the way to go. That is that
the gps averages the numbers rather than truncating them. I know it is a
bit of a moot point given that the resolution exceeds the discrimination
but I still think it matters.

That the resolution exceeds the discrimination is why I can switch freely between the two assumptions and not have it affect my navigation at all, so as you say, very much a moot point. In fact, I can do worse than that and still get away with it... if something is just inside a square I'll typically treat it differently at either side of the square, giving 1s rather than technically correct 0s on one side, and giving consistently proper 9s on the other side.

I still haven't found out why the OS switched to the square system from
the point system previously in use by the military who had a national
grid a decade or more before the revised version appeared on civilian
maps.

My guess is OS use a square system as the accuracy and resolution go hand in hand. A 4 figure ref gives you a 1km square, and you know it's that because it's a 4 figure reference. If you give a co-ordinate then it's always a point and you can't tell from a point what sort of accuracy or resolution you're talking about.
Once you get down to 10 figures it's effectively the same thing in any case.

Or indeed why they go to such great lengths to keep the general
public in ignorance of their meaning of a grid reference.

It's described on every map, so it's not /that/ secret. And people manage to use the system because of the nature of mapped data and what they give grid refs for. If I say "the summit at xxx yyy" folk will go to more or less xxx yyy and look around intelligently for summits, rather than take out the verniers and ascertain that actually there is no summit at that exact location. While this is done on the fly by human data processing, whether or not you consciously think of squares or points, it actually fits the square model better: go to an /area/ and look inside it. It works with a map because it's a highly selective presentation of data, and often where the symbols take up far more space than the feature really does on the ground.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net p.j.clinch@xxxxxxxxxxxx http://www.dundee.ac.uk/~pjclinch/
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Relevant Pages

  • Re: GPS to grid reference
    ... a square which the OS apparently thinks is the way to go. ... You can tell the accuracy by the length of the co-ordinates given. ... public in ignorance of their meaning of a grid reference. ...
    (uk.rec.walking)
  • Re: UK grid location / GPS positioning
    ... I found the setting in my GPS. ... >>How can I relate the indication found on UK OS maps with my positioning on ... > Where AA is the alpha code for the 100km grid square in which your ... > the south west corner of that square in metres. ...
    (sci.geo.satellite-nav)