28th June 2012, 04:57 PM
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?