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Staking out from another surveyor’s station co-ordinates or mine?
Posted by landmanjohn on June 7, 2019 at 7:27 pmI’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
landmanjohn replied 4 years, 9 months ago 14 Members · 18 Replies- 18 Replies
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.
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
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.
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.
It depends upon whether you are building a slab or setting bolt patterns.
Pick two of their points for alignment and use your numbers for staking purposes.
Either way, it is your liability and personally, I would use my numbers before using anyone else.
Your best way would be that your contract says that you use the other surveyor’s control and then you are exempted if it is wrong.
Measure it with a micrometer, mark it with a crayon, chop it with an axe. Pick the best fitting pair of points, hold that as your primary baseline then turn new control out from there as you need. Getting bent about 0.03′ in someone else’s control isn’t worth a single point in blood pressure IMHO. If someone called me 2 years after I did a design survey and said they were hitting my control that tight I would ask if they wanted to grab a beer and celebrate! I’ve seen a building move more than that throughout the course of a day.
Cheers fellas – I reckon the best thing to do is to tell the construction firm how the original control checked out and get them to confirm in writing what to do – original control or mine.
LandManJohn – your approach is legit.
Unless the earlier survey had a significant blunder you would be wise to use their values.
- All the design is based on the topo and boundary that is based on the original control values
- All features are designed relative to these coordinates – you only add error relative to the design by changing the coords
- All the building offsets (relative to boundaries) are based on the original coordinates
- You take on responsibility/liability for slop when you use your coordinates.
- You don’t have to defend the coords on the plans – just the coords that you develop.
If you don’t think things through, the differences can definitely create issues if the tolerances are tight – especially if you stake from multiple control points and they are not in harmony. The best way handle these differences depends on the size of the project, nature of the differences, and what you are building. If there are errors that you can’t work around – let the person stamping the survey control sheet/site plan deal with the problem. In this case, it doesn’t sound like there are differences that can’t be worked around.
Don’t make other peoples problems your own – you expose yourself to liability when you deviate from the plan values.
- All the design is based on the topo and boundary that is based on the original control values
- 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.
How much of a deviation from the original control co-ordinates would you start getting nervous about?
Sure it depends on what’s being built, how close to the boundaries etc.
In this case its a gas station with a car franchise showroom, forecourts etc with a decent amount of space.
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.
How much of a deviation from the original control co-ordinates would you start getting nervous about?
Short answer – 0.03′ is drawing my attention, 0.04′ is a concern, 0.05′ is clearly too much. Maybe.
Longer answer – a least squares analysis will yield error ellipses that are the tell of how much error is appropriate for the equipment and methods employed. It could also best fit your measurements to the points, instead of holding one and a bearing at random.
Always keeping in mind that any of the points might have been strategic points thrown off the main traverse using less stringent methods. Or one or more points may have been subject to some disturbance.
“…I reckon the best thing to do is to tell the construction firm how the original control checked out and get them to confirm in writing what to do ..”
I wish there was a “dislike” button. Don’t do that. Possibly you could report on your findings and recommend a course of action. But do not ask them how to do your job.
Very likely the construction firm has little to no idea about surveying matters and is relying on the professional help it hired to tell them if things are good-to-go or not, and how to proceed on that score. Do the electricians and plumbers ask the contractor to advise them, and sign off, on how to wire and plumb the place?
If it’s already under construction, stay with the original grid. ????
I see the missing 0.04 has shrunk. Time marches on.
In 45 years I have never chased 0.04′, I might take a closer look at 0.4′ but nothing less than that. ????
It is important to think about which tolerances need to be tight. Nobody is going to get excited if the foundation is .04 ft further from the boundary than the plans called for. They might scratch their heads if the foundation is 0.04 out of square. They will cuss a lot more if two nearby features are 0.04 off from each other.
Think about error propagation. If you lay out one side of the foundation from two control points and the other side of the foundation from two different control points, it is likely there will be a lot more error in the shape of the foundation. There are more error sources adding up than if you can do the layout from one setup and one backsight.
.that’s all great advice – many thanks its much appreciated ???
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