Surveying Question

[Richard]

Hi all,
Would like to ask a question as I have confusion over this issue.

After setting up a baseline from which grids are to layed out from it, it must be surveyed in. This can be done by a series of offsets to a reference point for example a building. Once this is done it has therefore been surveyed into the current Ordnance Survey grid system.

Is this statment correct?

Thank you!

[leg11aug]

Hi Richard. er, not really. you've tied your grid to standing structure but unless you can assign actual OS grid references to at least 2 of your baseline points it's not really 'tied in'to the OS grid. A building can be knocked down/altered etc. sorry if I'm being picky, hope this helps. Leg11

"...Ok, so ten out of ten for style, but minus several million for good thinking..." Z.B

[Richard]

Thanks for the reply.

Rather than 'tie in' the two baseline points, is it okay if the buildings OSGB Grid Reference is known thus solving the problem?

Cheers!

[Bier Keller]

Best really to tie it into as many substantial corners as is practicable. I would use a minimum of three to get a good triangulation when you end up playing about with it in ACAD. I say substantial corners as smaller ones such as doorways are other pokey out bits do not show up on the OS mapping and therefore you cannot tie them in with accuracy.It's no good though when you are in the middle of a field or where the only buildings are circular. It could happen - you could be working in Uhlan Battor and be surrounded by Gyhurt's. Just my Pfennig's worth.

Just give me a cold Becks

[Richard]

Thanks for the reply.

Does anyone know of a good website where this and OS Grid References are explained?

Thanks!

[BAJR Host]

http://www.galaxypix.com/glowworms/maprefs.html

try that

"No job worth doing was ever done on time or under budget.."
Khufu

[kevin wooldridge]

Richard, the other problem that you will come across is one of OS accuracy i.e corners of buildings as shown on OS maps aren't exactly where they are in relation to 'real' co-ordinates. This even extends to digital maps. Some rumours suggest OS are deliberate in their inaccuracies so as to catch out 'map pirates'. This may or may not be a problem depending on the degree of accuracy you are seeking to achieve.

With hindsight it has to be said that before the advent of modern survey machinery probably 90%+ of archaeological sites were located by using a combination of triangulation from fixed map points (buildings, gate posts etc) using 30-50 metre tapes and 'best-fit' overlays. So console yourself if you find an error in fittin' your site, that at least you are continuing a long established UK archaeological tradition.

[John Walford]

quote:Originally posted by kevin wooldridge
Some rumours suggest OS are deliberate in their inaccuracies so as to catch out 'map pirates'.

I believe that this is more than just a rumour. A few years back there was legal dispute between the OS and the AA about plagarism, in which these deliberate errors formed a key part of the evidence. See the press report at http://www.telegraph.co.uk/htmlContent.j...naa08.html

John

"Hidden wisdom and buried treasure, what use is there in either?" (Ecclesiasticus ch20 v30)

[drpeterwardle]

It all depends the accuracy required is the answer.

These days most archaeological work is tied into OS with GPS surveying equipment – 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 – 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

[Sith]

quote:Originally posted by John Walford

quote:Originally posted by kevin wooldridge
Some rumours suggest OS are deliberate in their inaccuracies so as to catch out 'map pirates'.

I believe that this is more than just a rumour. A few years back there was legal dispute between the OS and the AA about plagarism, in which these deliberate errors formed a key part of the evidence. See the press report at http://www.telegraph.co.uk/htmlContent.j...naa08.html

John

I've always treated this as more of a factoid than absolute truth. There are lots of errors in OS maps and I've certainly been caught out in the past by things on the ground not being as depicted on the map.

However, even though the they may have relied on these 'inconsistencies' in court, I think that calling them deliberate is probably a*se covering from the OS. What has to be borne in mind is that a lot of the digitisation from the old paper maps was carried out by poorly-trained data input temps (the same way as other bodies such as utilities), often leading to errors. These are often not obvious unless you measure them on the ground and this is a level of checking even the OS couldn't afford.

PS. Richard - There is an excellent guide to OS grid references on... The OS Website! (http://www.ordnancesurvey.co.uk/)


D. Vader
Senior Consultant

Vader Maull & Palpatine
Archaeological Consultants

Your powers are weak, Curator