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