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+--- Thread: How accurate are we? (/showthread.php?tid=178)
How accurate are we? - Talisien - 3rd April 2009
Quote:quote:Originally posted by Oxbeast
I reckon if a feture is within 2m it is good enough
This question is of significance where I work as a difference of 2m can mean hitting or missing the archaeology - we're getting pretty accurate on predicting where it is, but we also need accuracy down to a metre to ensure the 360, when cutting the trench, is holding the correct line; a subject and one that is not always covered sufficiently at university level or even in site training.
We need to understand the pros and cons of each method and use a combination approaches. Still need to teach people how to triangulate and analogue techniques - always surprises me (why, I don't know) the number of people who look blankly at you when you ask them where the point of origin is for the site grid. And batteries do run-out and EDMs fails. Oh, happy days B)
How accurate are we? - kevin wooldridge - 3rd April 2009
Quote:quote:Originally posted by Talisien
Still need to teach people how to triangulate and analogue techniques - always surprises me (why, I don't know) the number of people who look blankly at you when you ask them where the point of origin is for the site grid. And batteries do run-out and EDMs fails. Oh, happy days B)
I am in two minds regarding the need to teach triangulation/analogue techniques. If it is a case of training a surveyor- definitely. If it is a case of teaching an excavator how to use the total station - not so important. (I remember when I was learning archaeology being tutored on how to use a plane table. Interesting at the time, and I was assured loads of archaeologists used such equipment, but in nearly 30 years I have never come across one used in anger). I would recommend however that folk try and obtain some good referenc literature explaining survey, triangulation and the mathematics of geometry. I have an Indian army manual dated to the 1920s that I often refer to and has shown me some interesting techniques and/or mathematical formula for use in large landscape survey, free-point survey and for example obtaining polar co-ordinates of difficult to access areas.
Batteries running out, EDM failure....Not so much with modern equipment. My total station (Leica 1205 series) gives me plenty of warning of the batteries getting low and has a Bluetooth function that enables me to back up survey data fro the data logger to my mobile phone or my laptop at the touch of a key. My worst nightmares are mainly down to my own stress related failings (forgetting to reset the TST after moving locations happens at least twice a season(!!), but even that 'failure' is generally recoverable)
With peace and consolation hath dismist, And calm of mind all passion spent...
How accurate are we? - Talisien - 3rd April 2009
Quote:quote:Originally posted by kevin wooldridge
I am in two minds regarding the need to teach triangulation/analogue techniques.
Generally agree, but you still need to know where you are on site and where to plan your features, and a knowledge of the basics helps you make informed decisions as to the best strategy for dealing with site recording etc. I think we are essentially all in agreement and that the semantics are worthy of a discussion over a pint or two.
How accurate are we? - Dirty Dave Lincoln - 3rd April 2009
Compasses are wonderful gadgets-but wont there be a slight error, as they allign to magnetic north and the plan/map is to grid north? and as for when the poles flip/reverse.....[:p]
How accurate are we? - Bier Keller - 5th April 2009
In my opinion, so long as you have maybe a couple of solid OS co-ords from CAD then tapes are fine. Yes, on windy days tapes are real pain the C?lus and sure they can stretch, but within the constraints that are encountered on WB's, I think they are perfectly adequate. If archaeology of sufficient quantity and "quality" are discovered then surely another form of mitigation is required, even if this just means a few more hours and maybe a bit less pressure from the client - then yep, go for the TST option. But for common-or-graden jobs, just stick with tapes. In any event, how many different jobs do units run at the same time? Do they generally have a TST for each of the half a dozen they have on the go? I have only limited experience of GPS on site and found them to be only good to within a few metres (maybe this was just a cheapy version?) and was more confident using tapes and OS co-ords - although trying to tie in OS co-ords when one is on a windswept WB on Haltwhistle common (for example) would be difficult. I can see GPS being useful then. Just my opinion. Perhaps in 100 years time archaeologists will loo at us and say "Fools! What were they thinking?"
Beer is your friend
How accurate are we? - kevin wooldridge - 5th April 2009
I think that the Brave New World is probably a bit closer than next century!! There are already units in the UK using recording systems that require TST planning of features etc which kind of implies that a TST needs to be on the site at all times when recording is going on.
I think that once the cost and time advanatages of such systems are realised (Intrasis being the big 'sell' at present and maybe the Oxford Archaeology in house 'freeware' system when available) the shackles of old style archaeological recording will soon fall away.
(Although I have said that many times before on this forum and it always follows that lots of folk reply 'Not in our lifetimes'.....well we will see. Maybe the lean green archaeological machine that develops from this current recession will be more amenable to such technologies....)
With peace and consolation hath dismist, And calm of mind all passion spent...
How accurate are we? - BAJR Host - 5th April 2009
I was impressed with the New Lecia GPS system.. the TC 1200 .. it was hired by the week (at around 450 ish +VAT) but was accurate to 15mm in 3D so did the job nicely.
its all about horses for courses? a watching brief does not need top notch kit... only the next stage (if there is one) will have a cost benefit to having super accurate kit.. surely?
"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
How accurate are we? - kevin wooldridge - 5th April 2009
I too use a Leica 1200 kit and have done for 3 years now. I love it... We use it on all types of project from small excavations to large scale infra-structure projects, (although to be fair we have a slightly different legal basis for archaeology here in Norway that makes watching briefs effectively redundant).
The hireout rate that you mention David, seems to me to be met by the savings in staff costs, as the 1200 is a single-person operation. So basically for the saving of the cost of a single post, every site could afford to have a GPS/TST....seems like a bargain. It is also a very user friendly machine so training other staff in its effective operation is not a considerable problem either.
I know that EH are using the same make of machine to implement their Intrasis system and I guess the user-friendly nature of the 1200 is a factor they have taken into account. (Although I have also heard one of the EH team using swear-words I didn't even realise existed when he had problems finding enough sattelites to get a GPS reading on one occassion!!)
With peace and consolation hath dismist, And calm of mind all passion spent...
How accurate are we? - BAJR Host - 5th April 2009
Yup indeed to all that... it is so easy to use (well if you have a manual!) However I did find some words as well, when not only did I have trouble getting sat coverage but I could not get a mobile phone signal.. so the correction went off the scale... of course, being in the shadow of Lindisfarne Castle would not help...
I love my new avatar pic ... in fact.. I will change it to the GPS pic
"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
How accurate are we? - BAJR - 23rd December 2009
[ATTACH=CONFIG]5[/ATTACH] Well when you are in the desert accuracy is... relative