6th August 2007, 10:18 AM
It all depends the accuracy required is the answer.
These days most archaeological work is tied into OS with GPS surveying equipment â to an accuracy of about + 500mm. Without this equipment then the method is to survey the position of reference points such as bench marks and triangulation points.
Common practice is however to locate site grids relative to building or other OS detail. Detail on OS plans is usually accurate to + 2.5m in absolute terms. Even the 1:1250 plans are partly symbolic rather then being true plans. A good example of the use of symbols are buttresses of Churches. In addition the points may have moved â for example a fence since the area was mapped. In addition there are straight forward errors on the plan. The deliberate errors by the OS can be ignored.
On the scale of things these are however unlikely to matter.
In many respects today with total stations or even with offsets with tapes the easiest thing to do is to map OS detail ie building lines which can then be fitted to an OS plan. A check must be made that this detail is shown on a recent 1:1250 or 1:2500 plan. This will be far from perfect however comparing a site drawing to the OS mapping.
From this grid references for any point on the site can be derived.
In addition the site grid will have to be tied in to a bench of that the height of points above ordnance datum can be established. This is done by a traverse with a total station or a level.
I have never understood the logic that tieing in a grid was not good enough because the building would be demolished. Provided the buildings are shown on the OS then the plan is tied in.
If the grid is to be re-established for what ever reason at some later date then reference points should be established using for example ground anchors.
I would also note that the accuracy required will depend on what is being planned and where.
Peter Wardle
These days most archaeological work is tied into OS with GPS surveying equipment â to an accuracy of about + 500mm. Without this equipment then the method is to survey the position of reference points such as bench marks and triangulation points.
Common practice is however to locate site grids relative to building or other OS detail. Detail on OS plans is usually accurate to + 2.5m in absolute terms. Even the 1:1250 plans are partly symbolic rather then being true plans. A good example of the use of symbols are buttresses of Churches. In addition the points may have moved â for example a fence since the area was mapped. In addition there are straight forward errors on the plan. The deliberate errors by the OS can be ignored.
On the scale of things these are however unlikely to matter.
In many respects today with total stations or even with offsets with tapes the easiest thing to do is to map OS detail ie building lines which can then be fitted to an OS plan. A check must be made that this detail is shown on a recent 1:1250 or 1:2500 plan. This will be far from perfect however comparing a site drawing to the OS mapping.
From this grid references for any point on the site can be derived.
In addition the site grid will have to be tied in to a bench of that the height of points above ordnance datum can be established. This is done by a traverse with a total station or a level.
I have never understood the logic that tieing in a grid was not good enough because the building would be demolished. Provided the buildings are shown on the OS then the plan is tied in.
If the grid is to be re-established for what ever reason at some later date then reference points should be established using for example ground anchors.
I would also note that the accuracy required will depend on what is being planned and where.
Peter Wardle