How accurate are we?

[Cautionary Tale]

It has become relatively normal for excavations and evaluations to have fantastically accurate EDMs etc on site, but what of the watching brief? How do you measure in your results (whether positive or negative?). Ranging from bringing your own gps or EDM (or even borrowing a friendly site surveyors kit - done it many times), down to pacing (twitching eye...).

Of the Clan Sutton

[BAJR Host]

nowt wrong with a plus/minus 20cm for a watching brief... it only needs to get accurate after you find something...

"Entrepreneurs are simply those who understand that there is little difference between obstacle and opportunity and are able to turn both to their advantage."
Niccolo Machiavelli

[monty]

the old way... i.e triangulation with tapes.. will certainly suffice for most things...if anyone can remember how to use them correctly !!

[Sparky]

I would use one of the first three, depending on what is available or significance of the archaeology. Consequently, I have been disenfranchised because that voting option isn't availble [8D]

[kevin wooldridge]

Yeah and where is the option: Locate after site appears on Google Earth update.....



With peace and consolation hath dismist, And calm of mind all passion spent...

[afarensis]

Nothing wrong with tape measures tied into a 1:1250 site plan. What is accuracy anyway, everything is based on tolerance.

I have an issue with these so called fantastically accurate GPS EDMs. How accurate are they anyway, I have had so many people expound on their millimetre accuracy which is based on defining your position in relation to a fixed point in space, x,y,z on the long lat grid.

The long lat grid was last updated in 1984 (WGS 84) and defines a set 3d spatial grid. Unfortunately our planet wasn?t consulted when they did this and not only changes shape as it spins though the solar system and wobbles unpredictably, its major land masses are not fixed in the relative positions to each other on the surface. 2cm continental drift a year in a roughly NE direction times 25 years, how accurate is the GPS system now? The point it is defining is slowly moving across the face of the planet. Same as 12 figure grid references based on the OS system of 1936. Is the Scilly Islands 0m E and 0m N still the same distance from us now as it was when OS was defined.

Doesn?t make all that much difference when tying in with tapes on a site map but is an issue when you are measuring things in relation to a fixed point in time and space when the item you are measuring isn?t fixed, doesn?t it?

Especially when you think that the archive we create is suppose to last for all of time. These fantastically accurate readings from modern GPS total stations are going to point to an area of open ocean in a million years time whereas the feature they would have recorded would have moved to the other side of Canada by then. What are we actually trying to achieve?

Rant over, for now


ten years on and still no bottom

[Oxbeast]

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.

[Illuminated]

quote:Originally posted by afarensis

but is an issue when you are measuring things in relation to a fixed point in time and space when the item you are measuring isn?t fixed, doesn?t it?

So long as the fixed point remains fixed while the rest of the survey is conducted, the survey itself will be accurate. I was never the greatest at understanding the arcane depths of surveying, but surely all you need to do is measure in your field boundaries / nearby buildings, scale them and overlay with the latest accurate map and you have an accurate survey in the right place?
Adding the date of the map used for fixing the survey should be included in the data file so it can be seen if it needs updating to future maps.


To answer the original post, I've been known to use a single tape and a compass on a watching brief in a pinch...
ie - this point on the plan is 2.5 metres due north of the right hand gate post.. stick a nail in, 2 metres due north another nail and, voila! A baseline.

[kevin wooldridge]

I think that there are actually two queries here and they throw up a multitude of answers.

If you are measuring/surveying archaeology with a view to creating hand-drawn plans or maps then there is a degree of tolerance built into the operation that will allow for a greater degree of innaccuracy on site (scale of drawing, thickness of pencil line, thickness of pen line, scale of finished/published drawing etc). And as a result a greater degree of tolerance off site. And thats OK we have been doing it for years with fairly consistent and useful results.

If however you are surveying with a view to applying your data to digital media (raster or vector created maps for example) then your degree of accuracy should be at least equivalant to the degree of accuracy of the intended media. Accurate survey data can always be fitted to the requirements of an archaeological project. Innacurate survey data can't. And there are some examples where a strecthed 20 or 30 metre tape isn't good enough (particular with multi layer 3-D finds assembalages for example or understanding spatial relationships derived from complicated overlaying stratigraphy)

I find the largest number of queries I receive regarding my survey data and its translation into finished drawings are regarding what I call 'mixed media' e.g site field leaders asking why don't their post-ex digitised hand drawn sites plans seem as accurate as my TST site survey or grid locations.

As for GPS. It is as accurate as you are willing to pay for. If you think that a hand-held GPS is accurate enough, you probably have good reason for believing that to be the case. If you (or your employer or sponsor) can afford better you soon become accustomed to using better.




With peace and consolation hath dismist, And calm of mind all passion spent...

[Wibblehead]

The hand-held gps can be a reasonably good tool. We use it under tree cover (in winter) for marking archaeology in woodland; the more expensive equipment usually freaks out under trees, and tape measures is are a knotty nightmare. We tested the accuracy by walking along a number of known points, which we could match up to aerial photos and other mapping. That gave us an approximate accuracy range for the device in those conditions.

I would ideally have ticked more boxes; generally we use whatever is available. This usually involves a gps point (or points) if needed, and then tape measures etc etc. If we can possibly get hold of a total station we'll gladly use that, but that comes down to money/cake diplomacy.

I think i probably adhere to the old fashioned idea that if you know what you're doing with tape measures you can't go that far wrong. Good ol' fashioned techniques worked for the ancient egyptian tomb builders after all.

~~~~~
Thunder rolled. ... It rolled a six.