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Staking out from another surveyor's station co-ordinates or mine?  

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landManJohn
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I'm doing a stakeout job next week which will be ongoing for a while. A topographical survey by another survey firm was carried out on the site 2 years ago and I've manage to find 5 of the 6 Mag nails used.

The construction firm got me to check the existing stations before I stake out any points. All the staking out drawings are done and are based on the local grid and station co-ordinates from that original survey.

So I set up over a station of known co-ordinates from the original survey and use an azimuth worked out from a backsight station also of know co-ordinates from the original survey. And then I works my way round the stations checking them all. My accuracy was really tight but I found 4 of the original station co-ordinates to differ from mine by 1/8 inch up to 3/8 inch.

So now I have checked the original stations as requested by the foreman. Who's co-ordinates do I use for setting out? ........ or is it not an issue with the magnitude of the differences I've discovered. The site is roughly 300 foot by 150 foot.

John

 

 

 

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Mark Mayer
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Posted by: landManJohn

 I've manage to find 5 of the 6 Mag nails used..... I found 4 of the original station co-ordinates to differ from mine by 1/8 inch up to 3/8 inch.....

First, I don't care for using Mag Nails for permanent control. I rank it very lucky to have only lost 1 in 2 years.

More properly than saying 4 of the 5 found don't match you should say that all 5 differ. The only reason the one matches is because you held that one fixed. Probably a best fit LS analysis would have the maximum difference between your results and that of the original surveyor as half of that 3/8" you quote. Which is definitely acceptable.  No two sets of measurements will be identical.

Use  the original surveyors coordinates.  Note that the control points will frequently be destroyed by the construction  process.  We have no idea where this control is in relation to the proposed works, but if it is within the construction zone - as control for topo usually is - you should spray some new control out from it to positions that are safe from the demolition.  Like the other side of the street. By the time the rubber meets the road you will probably be using your own control points with your own coordinates. 

Your statement that you held the coordinates of a point and the calculated  backsight bearing is a curious one. Every dc software allows you to name the backsight point and handles the BS bearing calculations for you. Also your quoting of deltas in inches instead of decimal feet.   

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MightyMoe
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Not enough info, it would need to be something very tight for me to worry about 1/8".

3/8" vertically would get a bit of attention from me, but probably not horizontally unless like I say above something very tight.

 

 

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JERRY ATTRICK
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Yeah, need more info.

I you are staking out limits of a temporary stockpile, probably OK. High-rise - maybe not.

Whats the job?

JA, PLS SoCal

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Tom Bushelman
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I rarely have control check that well, especially with any time lapse.  If I were staking a job out and it were started out on a local coordinate system based on recoverable monuments, I would stick with the local control.   An alternative to sitting on their control is to set out in the middle of the site and do a resection on all of the points, providing you with a least squares average of the original control.  Then re-shoot the control and call it "100-A", "101-A" etc.  An 1/8' would not get me excited enough to do that though.

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jim.cox
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How good is your total station?

The S6 I use is rated to 3mm for distances - that's your 1/8 right there.

Not to mention any plumbing or prism constant errors

The design was done using the original control - and I would be staking based on that.

Changing the numbers opens you up to a huge liability when something goes wrong.

 

 

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VA LS 2867
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Not to mention that the size of the site, he is dealing with relatively short distances.  Short backsights, centering errors, long foresight magnifies the error.

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