Hi guys
A local surveyor carried out a topographical survey on a plot of land 12 years ago. The project was to demolish the old farming buildings that were there and build some houses. The survey was carried out to a local grid and level datum.
The development didn't get off the ground at the time, but last week a project manager asked me to get involved as they're building on it soon. The size of the plot of land is 190metres by 70 metres.
I attended the site yesterday, found 4 of the original survey stations, performed a closed-loop traverse, and added in some extra stations. I also surveyed a few corners of neighbouring houses, and other boundary features on the original survey to ensure there is a good correlation between my survey and the original one. The stations I found were in tarmac/ashfalt.
The only two stations I could find to set up from were too close for me (28 metres) for a large site, but it was all I had so I got on with it. So the first back sight was 28 metres, the deltas were 0mm horizontal and 5mm for vertical.
I traversed round adding in my extra stations and finding and traversing the other 2 original stations which were both around 190 metres away from my original short baseline.
The traverse accuracy was great, 1 in 190,000. However, the co-ordinates I got for the 2 stations far away from my original baseline weren't amazing. I got them about 20mm away in x,y from the survey carried out 12 years ago. The levels were in good agreement, within 5mm.
My feeling is that I suffered from my short starting baseline. If I rotate the whole traverse around my first set on station I can get x,y agreement for all stations to those in the original survey of 6mm.
Is there any advantage to doing this? I don't have and don't fancy getting least squares software for this one project.
Kind regards, Andrew
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I agree that the short initial baseline is influencing the traverse coordinates on the further away points.?ÿ But it did its job by allowing you to find additional original control.
You said "If I rotate the whole traverse around my first set on station I can get x,y agreement for all stations to those in the original survey of 6mm."?ÿ I would go with that rotation if it puts you within acceptable tolerances on the other original control points.?ÿ I would consider 6mm acceptable for this initial control work on this project.
This is the stage of the game to get the project on a single datum.?ÿ As long as your future work is tight, I think you'll be fine.
Cheers!
If you are unlicensed and not working under the responsible charge of a licensed surveyor in a particular state within the boarders of the USA, I would tell your PM that you do not want to risk prosecution for fraud and the associated civil penalties.?ÿ
Since you're measuring in the easily converted (but unnaturally related) unit of the meter, I suspect that you are not in the USA.?ÿ I'll assume that you have verified that you are not practicing surveying without a license or breaking any other laws.?ÿ
One option would be to perform a resection after setting your instrument up at least 150' away from the existing traverse stations.?ÿ You seem to already know that least squares is the correct solution so if you're fine with not doing it correctly, in CAD just group or select all the project points and lines drawn around your traverse, translate and rotate them to the old control, then split the 20mm difference by moving all the points 10mm in the direction of the coordinate deltas.?ÿ
I hope this is not the case, but your post presents warning signs that you know just enough about surveying to create a serious problem.?ÿ Best of luck.
Question: Did you start your traverse using coordinates from the 12-year old survey?
Rotate your traverse onto the old stuff or rotate the old stuff onto your traverse.?ÿ Or maybe hit the stations with GPS and rotate both sets of work onto that.?ÿ?ÿ
It's topo, so I'd be fairly happy with the H & V checks.?ÿ Use your control for all boundary work.
As far as using topo from 12 years ago, that's your call.?ÿ At a minimum I'd be doing a lot of checks, if using it at all, since once I use it and/or stamp it, I own it.?ÿ Somehow we as licensed surveyors can't just un-own topo by writing a note the way engineers do, "....topo performed by XYZ Survey Company in 2009...."
But as Murph was getting at, maybe you're not a licensed surveyor, but working for an engineer or contractor.
By putting it all on SPC, it becomes generally more usefully into the future.
I'd suggest that it will add value to do so.
N
A regional grid may be appropriate, but as the distances were given in meters, I looked at some of his other posts and find he is in the UK so it won't be State Plane Coordinates.
Least squares corrects for random errors. It strikes me that a too-short baseline would create systematic errors.
I would be careful about believing that a least-squares adjustment would fix this one.
I wouldn't buy LS software for just one project. I have it and use it for every project.?ÿ Nevertheless you seem to have proven that the 12 year old coordinates are good. Why not just use it and propagate your new control off them??ÿ?ÿ
I really like to get every project on a common coordinate basis. It has a number of advantages. Coordination with GIS data is one. Last ditch, long term, recoverability is another. My state has a system of low distortion projections which facilitate the practice.?ÿ
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As far as using topo from 12 years ago, that's your call.?ÿ At a minimum I'd be doing a lot of checks, if using it at all, since once I use it and/or stamp it, I own it.?ÿ Somehow we as licensed surveyors can't just un-own topo by writing a note the way engineers do, "....topo performed by XYZ Survey Company in 2009...."
I don't read that he is using the 12 year old topo. He simply jumping on the old control to perform construction staking and possibly some boundary work.
I think LS would reduce the effect of a rotation if you included both old and new data, although that is probably not be the best method.
An even more rudimentary solution would be to redo the traverse starting with a long backsight now that you have found the old control.
@murphy?ÿ
given the mm measurements it is highly unlikely he is working in any state in the US
