How accurate are OS (Ordnance Survey) Maps

Q. How accurate are OS (Ordnance Survey) Maps?

A friend (more an acquaintance really) told me that if there are two features that are close to each other - then the OS will just move one of them so that both can be accommodated on the map.

Is this true?

And if it is - by how much do they 'move' things?

Mark Kissipie

Trevor Armstrong, writing in the Autumn edition of the MBA newsletter states that about 10 years ago he had cause to write to the OS about the accurate positioning of trig points following a close shave on Ghlas Tulaichean, a Munro near Glen Shee.

In reply to his letter, in June 2002, the Ordnance Survey said:

" . . . .some features on OS mapping are shown at a slightly different geographical position to where they are situated on the land" The reason given was "it is necessary to be able to read the map clearly without features superimposed on one another"

Which could be a bit scary - and was for Mr Armstrong, according to his account.

Armstrong points out, quite rightly, that the best method of compensating for any inaccuracies (and there are many more causes than this one) - is to use "aiming-off" wherever possible - this helps to cancel out the effects of clumsy compass use, wandering magnetic variation, map inaccuracies and even strong winds blowing you off course, of course.

Seekers of trig points should also be aware that since the OS are not now maintaining the majority of these, that there's a chance that a trig won't even be there when you arrive, or may be lying on its side some distance down the hill.

Mike Knipe, Crook

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