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RedEarth Wrote:how many times have I found myself attempting to relate something to a drawing I presumed accurate that was produced by an architect or similar? Even the OS mapping is sometimes remarkably dubious when compared to something actually surveyed. A superbly accurate trench plan, for example, isn't much use if the OS map it is going to be related to isn't very.
As for the point of the survey, I'm not sure, probably just something else to make us feel inadequate!
Humm I think your comments sort of make the point it is worth understanding that architects are not surveyors they draw what they think should be there. As for trying to fit a trench surveyed at 1 to 10 on an OS map surveyed at 1: 2500 I am not surprised you are having problems. If you want an accurate frame work go back to the OS control frame work which is accurate to mm or use GPS. OS digital map data is again only accurate to the scale at which it was surveyed it is no good blowing up 1:1250 mapping to another scale such as 1:50 and expecting it to have the same tolerances as mapping surveyed and plotted at 1:50. Understanding the origins of your base map or your control stations is crucial to accurate surveys.
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Wax Wrote:And what is the fascination with physically setting a grid out over a site with tape measures when you have access to a total station?
More to the point when you have access to a total station why are you bothering with a fixed grid at all....
With peace and consolation hath dismist, And calm of mind all passion spent...
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28th June 2012, 04:18 PM
(This post was last modified: 28th June 2012, 04:27 PM by Wax.)
kevin wooldridge Wrote:More to the point when you have access to a total station why are you bothering with a fixed grid at all....
Ah at last someone who understands that the grid does not have to have a physical presence on the ground beyond a couple of control stations (you would not believe the number of times I have been met with blank faces when trying to explain that one)
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Wax Wrote:Humm I think your comments sort of make the point it is worth understanding that architects are not surveyors they draw what they think should be there. As for trying to fit a trench surveyed at 1 to 10 on an OS map surveyed at 1: 2500 I am not surprised you are having problems. If you want an accurate frame work go back to the OS control frame work which is accurate to mm or use GPS. OS digital map data is again only accurate to the scale at which it was surveyed it is no good blowing up 1:1250 mapping to another scale such as 1:50 and expecting it to have the same tolerances as mapping surveyed and plotted at 1:50. Understanding the origins of your base map or your control stations is crucial to accurate surveys.
It's fair enough comment about architects, but it is still reasonable to presume that their drawings (of buildings I hasten to add) would be reasonably accurate. At least in terms of the more significant elements. I recall one example where a wall was 1m lower than it should have been!
As for OS mapping I've never been sure how it even is surveyed these days, or at what scale. I probably assumed it was all digital and therefore effectively at a scale (in digital form) of 1:1. Presumably it isn't?
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In my experience architects' drawings are frequently not what one could call accurate - a recent job for a small garage/workshop building was enlivened by the realisation that the two end elevations on the plans for the new-build were not exactly the same dimensions....apparently a respected local architect too, even if he clearly still works in ink with calligraphy...
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28th June 2012, 06:06 PM
(This post was last modified: 28th June 2012, 06:41 PM by CARTOON REALITY.)
Quote:As for OS mapping I've never been sure how it even is surveyed these days, or at what scale. I probably assumed it was all digital and therefore effectively at a scale (in digital form) of 1:1. Presumably it isn't?
Digital machines don't really record at any scale. They can print (etc.) the relationship between the points surveyed out at a scale of 1:1 if needs be - but it's just recording angles/points in abstract zeros and ones -
surveyed data does not exist at a scale - the maps and models it can produce exist at a scale. There's a difference there.
I hope you don't think I'm trying to be a hair splitting smart-arse about this (and I'm sure that's how it sounds) - but reality doesn't have a scale.
On the maps bit - my own experience is that Georgian maps can be a bit ropey but very illuminating, C.19th OS maps are extraordinarily accurate (compared against modern street patterns etc.) And there are surprising mistakes on the modern maps - I have heard (can't say whether this is true or not) that rectified aerial photography is now used in the mix.
Some of the worst drawings I've seen in my life, came from architects.
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28th June 2012, 07:17 PM
(This post was last modified: 28th June 2012, 07:24 PM by Wax.)
I repeat architects are not surveyors their remit is to design, someone else has to fit it together hence the problems that arise if they are asked to draw an existing structure. Much OS data is now captured using differential GPS with aerial photography. Some poor sod then ground checks it with a pad linked to GPS and plots in real time. Met an OS surveyor a month or so back and he was complaining that the programme was written so the surveyor on the ground could not use all the little ruses that they developed when plotting with a pencil and measuring with a tape. He said all the skill had been taken out of the job. He also showed me how using the GPS was high lighting all the little positional errors inherent in 1:2500 mapping.
What a lot of people do not think about is that a map is not a drawing of a flat surface it is a depiction of the curved surface of the earth plotted in two dimensions Grids such as the OS national grid were developed to factor this in and there are a variety in use. If you are mapping over distances you need to factor in the mathematical corrections to bring the curved surface to the grid you are using. This can be around 10cm in 300 m. Corrections may also be needed if you are levelling over any distance. GPS usually asks you what grid you are using and does the corrections for you. it is these little details that archaeologist are often not aware of that make me suggest that archaeologists need to understand some basic principals. Transverse Mercator any one?
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Quote:10cm in 300 m
As far as I remember, on average, it's undetectable under a distance of 10km.
Generally (there are exceptions,) archaeological sites are never so large that the surveyor need worry about such things. No harm to think about it though . . .
Try surveying at 1:20 on steep hills with only tapes - it can't be did.
Well it can, but not accurately.
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28th June 2012, 08:37 PM
(This post was last modified: 28th June 2012, 08:42 PM by Wax.)
CARTOON REALITY Wrote:As far as I remember, on average, it's undetectable under a distance of 10km.
Generally (there are exceptions,) archaeological sites are never so large that the surveyor need worry about such things. No harm to think about it though . . .
Try surveying at 1:20 on steep hills with only tapes - it can't be did.
Well it can, but not accurately.
The correction I am thinking about is something called the Scale Factor, this is a figure the OS use to apply to their measurements to bring things in to fit the National Grid. This would vary according to your location in the country but it was crucial to apply to your observations to enable you to fit your survey to the National Grid. It was not strictly a correction for curvature of the earth but a correction to fit your local survey to the National Grid. The grid (or projection) being developed to enable coverage of the whole country taking into account the curvature. If you measure between two OS co-ordinates stations over some distance you may notice that the distance measured may not be the distance that Maths would suggest should be between the two points. There are many different types of grid and before the National Grid the county mapping used its own local grids hence mis matches at the edges of county sheets. If you have ever tried to overlay large areas of old mapping over the current you will see these discrepancies quite clearly with some areas appearing slightly elongated when compared to the modern map.
My appologies to the techy surveying types amongst you if I have got this wrong but it is a long time since I did my basic survey training and I am not a mathematical type so I may have misunderstood some of the things I was taught. But I have seen the discrepancies caused by not applying the scale factor demonstrated and it can be quiet significant especially if your survey is a metric survey which by definition is tied accurately to the National Grid
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Wax Wrote:I repeat architects are not surveyors their remit is to design, someone else has to fit it together hence the problems that arise if they are asked to draw an existing structure. Much OS data is now captured using differential GPS with aerial photography. Some poor sod then ground checks it with a pad linked to GPS and plots in real time. Met an OS surveyor a month or so back and he was complaining that the programme was written so the surveyor on the ground could not use all the little ruses that they developed when plotting with a pencil and measuring with a tape. He said all the skill had been taken out of the job. He also showed me how using the GPS was high lighting all the little positional errors inherent in 1:2500 mapping.
What a lot of people do not think about is that a map is not a drawing of a flat surface it is a depiction of the curved surface of the earth plotted in two dimensions Grids such as the OS national grid were developed to factor this in and there are a variety in use. If you are mapping over distances you need to factor in the mathematical corrections to bring the curved surface to the grid you are using. This can be around 10cm in 300 m. Corrections may also be needed if you are levelling over any distance. GPS usually asks you what grid you are using and does the corrections for you. it is these little details that archaeologist are often not aware of that make me suggest that archaeologists need to understand some basic principals. Transverse Mercator any one?
Hang on a minute 'curved surface of the earth', you mean it's not flat!! That's where I've been going wrong all these years! Seriously though, I would consider the level of surveying knowledge you've been explaining way above the requirements of most archaeologists. And certainly above most architects. If you understand all that, good for you. Can you now perhaps think of a few other things that archaeologists might use everyday that you don't understand in such detail and let me know so I don't feel quite so useless! Cheers.
