As I said in my previous email accuracy required depends on what you want to
do. We have used GPS over several years for aerial photography for example
and that doesn't require a very accurate location at all.
Now I am mainly interested in updating our SMR in the field - identifying
new sites, and correcting existing but inaccurate SMR details. How useful
is a four figure grid ref for a planning or forestry enquiry? People want
to know where the archaeology is.
SMRs are collections of locations that have some significance to us. Global
Positioning Systems (GPS) are good for updating them. We have over 33,000
records and cover an area larger than Belgium (and rather less flat, built
up or covered with roads or motorways). The amount of past survey here has
been pitiful. Some pre-forestry surveys have identified 100 times the
number of previously recorded sites - there's a huge amount of unrecorded
archaeology out there. Even the OS have had difficulty in the past
re-locating sites they have identified in the hills. With a lack of handy
reference points old surveys can be very inaccurate too.
So I don't look at the moment for submetre accuracy - the system I am using
shows you close enough where you are on the map (although when I try it in
my garden it tends to tell me I am next door in the neighbours) You can
then work from the map itself if this is better. I don't take much notice of
the height OD I get - it seems very inaccurate and not very useful. I am
more interested in the horizontal location.
Much better accuracy all round is achieved using Differential correction -
signals transmitted from various beacons including lighthouses the OS etc.
to get over problems of atmospheric distortion, poor satellite geometry etc.
Seeing the Time Team using real-time differential GPS at Kinlochbervie to
2cm accuracy was an eye-opener. With that sort of equipment you could do
accurate, detailed archaeological survey anywhere, very fast - throw the old
survey manuals in the bin! Within a short time every archaeological
fieldwork project will be using this technology.
Real-time correction is best, but failing that it is possible to correct
your data afterwards using software like Trimble's GPS Pathfinder Office.
Cost? around £500
Current costs are:
1-2cm accuracy - currently around £25000 (but coming down fast)
1m accuracy (requires a bulky backpack) about £7000
5-10m accuracy - the Trimble Pathfinder Pocket unit is £500; the Compaq Ipaq
Pocket PC (which incidentally has lots and lots of other uses too) £400;
ArcPad (this is the pocket version of ArcView GIS) £500 per licence. With a
GPRS phone as well you can send / receive SMR, map and other information
wherever you are, anytime.
ArcPad is great as it integrates with ArcView and you can download datasets
to take out in the field. It really does put the GIS in your pocket. But
get version 6! Earlier versions are terrible.
John
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John Wood
Senior Archaeologist
Planning and Development Service
The Highland Council
Glenurquhart Road
Inverness IV3 5NX
Tel: 01463 702502 Fax: 01463 702298
Mobile: 07748 574471
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Web: <http://www.higharch.demon.co.uk>
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