1st April 2009, 02:37 PM
WGS 84 has now been updated to OSTN02, afarensis. Probably the handheld type of GPS still uses the older system.
Modern GPS can update in real time; your station can recieve signals from a network of OS base stations which can fix your position to the grid on that paritcular day. The stations also talk to each other, to work out how geology and tidal forces are affecting the grid. This is particularly useful in places like Stoke, parts of which are subsiding at up to 10mm a year.
Differental GPS is excellent, though in most cases is more accurate than archaeologists really require it to be. Most of the stuff I plan, I reckon if a feture is within 2m it is good enough, but then I tend to work at 1:10,000.
I do remember working with someone on site who insisted on replanning areas because "that tape stretches". I stretched it and it deformed by about 1cm in 10m. Cue discussion about what acceptable magins of error are. I lost.
Modern GPS can update in real time; your station can recieve signals from a network of OS base stations which can fix your position to the grid on that paritcular day. The stations also talk to each other, to work out how geology and tidal forces are affecting the grid. This is particularly useful in places like Stoke, parts of which are subsiding at up to 10mm a year.
Differental GPS is excellent, though in most cases is more accurate than archaeologists really require it to be. Most of the stuff I plan, I reckon if a feture is within 2m it is good enough, but then I tend to work at 1:10,000.
I do remember working with someone on site who insisted on replanning areas because "that tape stretches". I stretched it and it deformed by about 1cm in 10m. Cue discussion about what acceptable magins of error are. I lost.