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Having trouble importing survey with correct co-ordinates


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I'm having real trouble importing a survey in DWG format.

Initially I assumed I had imported it correctly because the building measured off correctly and it fit the page, however when it came to producing co-ordinate drawings I noticed that the co-ordinates are out by a margin of 3 decimal places (e.g. E:533440 N:183990).

So if I then import the survey at a unit setting and scale such that the decimal point is in the correct position (i.e. E:533.440), then the building doesn't scale off correctly.

I phoned the surveyor and we agreed that he should try scaling the drawing and resending but this didn't resolve the matter. I still don't seem to be able to import the file so that both the building and co-ordinates scale off correctly.

Any ideas?

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It seems I've found the problem. At an earlier stage we asked the surveyor to co-ordinate their survey with the Ordnance Survey map and it appears, shockingly, that the OS Map file is incorrect.

I've attached it (you'll need to change the .txt suffix back to .dxf)

Have a look tell me what you think. It's disconcerting that an OS Map (via Promap) is incorrect. Or am I importing incorrectly I wonder?

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Yes, of course, that's the first thing I thought of and I tested it over and over but clearly I was doing something horribly wrong because you're correct, bringing it in as millimetres works. I'm sure we usually bring our OS Maps in as metres. F**k knows what I was doing wrong. And it must simply be coincidence that the surveyor made the exact same mistake! Anyway I'll have another look in the morning to try and make sense of it all. Thanks for making me look at the obvious over again Petri.

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Four times out of five, it is the obvious... I must have used the wrong unit hundreds of times at the Lab, but never missed it when troubleshooting someone else's procedure.

Curiously enough, the DXF "standard" has (I've been told) always (?) had the option to embed unit information, but few programs - most notably AutoCAD - do it.

Speaking of curiosity: I wonder how AutoCAD users handle this.

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I've just realised what it is you're after. It's doing exactly what I would expect it to do when I import into VW or ACAD. The coords are not wrong, they're just in mm now instead of the m that you expected. You'll either have to manually insert the point where you need it or change your building units to m.

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I think i do. Sorry, Christiaan, I did misunderstand the problem. OS coords are in the 100000s in metres. But usually we leave off the first 3 digits when coordinating a drawing, because, unless the site is abso-bloody-lutely ginormous, those 3 digits are superfluous - they never (well, almost never) change. Sorry for the confusion.

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Ahh - finally the mystery of Land's End is solved!

As a schoolboy, that was as fascinating a place name to me as Tananarive, Sulawesi or Mahagonny.

For the benefit of VW users in geographically larger countries than the fly-speck of the United Kingdom of Prydain Fawr (England, Wales, Scotland, Cornwall and various isles, I gather) and Northern Ireland: there may be Zones.

When the spherical world is projected to Cartesian coordinates, only a limited area can be dealt with in any reasonable accuracy. Eg. in Awstraya, one cannot have an accurate and meaningful Facilities Management CAD-map of one's home in Toorak, Melbourne Victoria, the beach house in the North Coast of New South Wales, the crocodile hunting lodge near Darwin, Northern Territory of South Australia and the Festival Pad in Adelaide, South Australia.

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The OS origin is in Newlyn which is I guess is 533km away from London.

So, two questions:

1) Why do you disregard the first 3 figures?

2) And how would I create a co-ordinate drawing with the plug-ins are currently available? None of them are designed to automatically disregard this these first three figures as far as I know.

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If you include the first 3 digits, you are providing the full OS coordinate. Although this is correct it is also unnecessary. It is almost certain that, on your site, those first 3 digits will be the same for every easting (and northing respectively). So, we disregard them, and so will the setting out engineer - he doesn't need them once he's set up his stations.

Unfortunately, the available plugins won't knock off those first 3 digits. So what I do is move the OS sheet (and the building) or create a new origin.....

In your case you'd move it 533000m west and 183000m south. Then all your coords will read correctly, but without those first 3 digits.

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I do appreciate your concerns and if you're not comfortable then don't do it. And of course there's nothing wrong with leaving those numbers in as long as you've got space for them on the drawing. I've done it so many times for so long now that it's second nature. No ups in 25+ years suggests that it is safe, though.

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Just spoke to the surveyor and he said that the figures are correct and under no circumstances would you knock of the first 3 figures, because the figures need to be in metres and if it's 533000 metres from the origin then that's what it is.

What caused me to ask the question in the first place is that it appears to be relatively knew thing to be using OS co-ordinates and this threw some of the old-timers, which forced me to ask the question.

Still, learnt quite a bit today, including that we've got a dodgy survey because it's graphically co-ordinated with the OS map rather than being started in the first place with a GPS device.

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