I have assisted Phil Newman on a few surveys using the equipment that he
uses, which has an accuracy of about ONE CENTIMETRE. Now if you think that a
GPS can produce anything near that, then I think you're wrong. From my
understanding a GPS survey can produce a basic survey, which will do if you
are trying to create a basic drawing, but if you require a very detailed
drawing then a GPS is NOT accurate enough to record everything in detail. I
can't understand why anybody using a GPS would try and plot their work on a
1:2500 scale, I would have thought for a reasonable end product a scale of
at least 1:10000 would be more beneficial. If the person using a GPS is only
taking a few points i.e. mine building, chimney, etc then I suppose plotting
it on a 1:25000 would be ok.
I used a GPS whilst working in the field with the Dartmoor Tinworking
Research Group following a leat which is still in use. I set it to take a
reading every 2 seconds and held it over the centre of the leat whilst
walking its course. The end result of over-laying it on Memory Map to be
fair,was not bad, with the greatest distance from the leat marked on the map
being 4 metres out. Now if I needed complete accuracy for whatever reason,
then that GPS survey would not have been up to standard in my personal
opinion. Anyway the discussion will no doubt continue as each individual has
their own preferences.
Chris Kelland.
----- Original Message -----
From: "Martin Roe" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, April 14, 2005 4:02 PM
Subject: Re: Mining in the South West of England
Phil Newman describes the unpredictable accuracy of hand held gps and
suggests that it cannot be compared to an accurately measured survey. Wrong.
The accuracy is not unpredictable. In upland areas with a clear view of the
sky an accuracy of 5-6m is normal. When that accuracy is examined for
example by recording the same point on several different days and comparing
the results it can be demonstrated that although the position does change it
is by a very small amount and when averaged 5m accuracy often equates to
+/-2.5m or better.
When I first started using GPS some 5 years ago I was very sceptical about
how accurate the results were and what the accuracy quoted by the instrument
really meant. By experiment and by comparing my surveys with others I am
confident that it is a valid and reliable method of survey. It is therefore
an accurately measured survey within the limitations of the accuracy of the
instrument just as any other accurately measured survey is. As I am aiming
to plot at a scale of 1:2500 a position error of 5m in the real world is 2mm
on the plan. Just like transcribing air photos this technique produces a
plot that shows a feature in approximately the right place, approximately
the right size, and approximately the right shape. It is not the same as a
1:500 hachured earthwork survey but it is not trying to be. See Lead Mining
Archaeology in the Yorkshire Dales, Landscapes vol 4 no1 Spring 2003, and
The Greenhow Hill Lead Mines Survey, British Mining 73, 2003 for examples of
the application of GPS to landscapes survey and A Survey of the Area Around
Jamie Mine, Sunside Allotment, Appletreewick, British Mining 71, 2002 to see
that I do also use detailed earthwork surveys.
Just to stir things up a bit in 1995 Paul Everson, who was at that time a
surveyor for RCHME and therefore may now be one of Phil's colleagues at
English Heritage, wrote that over detailed field survey rather than
providing more answers, can "produce unhelpfully undigested heaps of
information".
Mike suggested that producing plans in GIS is difficult. Not so as he well
knows and has seen. Importing the GPS data as points and simply joining the
dots is not difficult and talking about pencil drafts of plans is just
being, pedantic. No doubt the miserable old git will have further comments
to make so can I suggest that now that he has had a go at the archaeologist
he might want to find another victim so that I can get on with some work.
Martin Roe
President, Northen Mine Research Society,nmrs.co.uk
Conservation Officer NAMHO, National Association of Mining History
Organisations, http://www.namho.org
Lead Mining in the Yorkshire Dales,
http://www.martinroe.pwp.blueyonder.co.uk
Meerstone Archaeological Consultancy
http://www.martinroe.pwp.blueyonder.co.uk/meerstone.htm
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