I will add to this that a dual station GPS set up, which will get you down to centimetre accuracy, is comes in somewhere at around £40K... and is still a bargain compared to the commercial cost of Mastermap... and more accurate....
To that end, on can probably argue that using designated OS points based on a full GPS reading is in fact a more accurate way of identifying the geographic location of a site than the use of the OS maps at all (which are, after all, at best only accurate to about 2m). Or better yet, Lat-Long coords which no one has to pay copyright to use at all.
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From: Issues related to Historic Environment Records on behalf of Thomas Rees
Sent: Mon 30/10/2006 14:51
To: [log in to unmask]
Subject: Re: Base Maps for Client Reports
Having read Peter's post I agree with it all (as a consultant who previously maintained an SMR), except the passing comment on the expense of Differential GPS.
A DGPS system like the Leica GS50 (backpack mounted) is designed to capture GIS compatible data at a given accuracy (with the UK beacon correction (free)) of between 0.3 to 0.5m. Clearly not good enough for site detail work but in terms of mapping trench boundaries, site boundaries etc it is excellent. The DGPS system also translates the co-ordinates straight into the OS co-ordinate system using their (ie OS) modeling. Hence the accuracy (0.3 to 0.5m) remains constant with NGR co-ordinates recorded. These are downloaded in .shp files (ie straight into your GIS system).
Thus for about £4k (2nd hand) you get 1 person survey kit which, with ArcView (about £1.5k), allows you to map sites, trenches etc to OS grid and accurately. The plans we can spit out (or pass on in .shp format) can be to any sensible scale (ie 1:2500) and be overlain by an accurate OS grid without the need for expensive OS bases.
I would not accept that any archaeological company that can afford a total station and employees cannot also afford these (one-off) costs. Rather many companies seem to have focused on total stations and AutoCAD then wonder why map generation is difficult.
(I do acknowledge that there are some problems with UK Beacon reception in some areas and 'urban canyon' effects (ie satellite availability due to limited sky) but we find that this only affects occasional sites.)
Cheers
Tom
Thomas Rees
Director & Senior Archaeologist
Rathmell Archaeology Limited
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