Need some advice on how to address and solve an issue.?ÿ
I'm interning for a large GC and have been tasked with completing the layout for a large residential building. When I arrived, the site had control already established via 4 reflective targets and two crosscuts which I have been using to setup my SPS930 station (SPS930 with T7 controller using SiteWorks). As the site has existing buildings and varying topography, I usually establish position with 3 targets and/or crosscuts which generally gives me HA error of less than 5 seconds, and VD and HD errors of less than 3mm.?ÿ
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We have now poured most of the foundation along with some small sections of walls and columns, and had a survey done to check locations, which revealed that the relative error of points is <25mm however every point is shifted horizontally by 50mm. I was hoping some more experienced users know the most common reasons for shifted coordinates and how to avoid this on future projects?
Prism offset versus reflectorless sighting of control.
Prism offset versus reflectorless sighting of control.
Could also be angle of incidence on the targets giving odd distance returns.
Or....whoever did the check survey might have had a problem with their setups or observations. Depending on the size of the building, 25mm relative accuracy is a little loose. For tightly laid-out columns or gridlines, I would expect more like 15mm. But it all depends.
If the whole figure is shifted, I would ask that whoever did the asbuilt check your primary control along with the asbuilt. It is possible that you primary control is not correct.
Also, I would suggest that you also set up, and using your standard technique, measure the relevant points, and a few of the other surveyor's control points. Doing so might allow you to have a bit more information.?ÿ
I would recommend that you also have an experienced technician observe you while you make these observations to determine if there is something in your workflow that is inducing error.
That certainly seems possible, given the distances named. Perhaps the targets were shot with 0mm offset, but it was never switched for layout.?ÿ
I'm interning for a large GC and have been tasked with completing the layout for a large residential building.
Lets have the intern do the surveying. What could possibly go wrong?
There is a chance that the problem is not in the surveying, but rather in the interpretation of what was surveyed. That is, that the point were layed out just fine, but the points you layed out were not the foundation corners. Bad calc points.
We have now poured most of the foundation along with some small sections of walls and columns, and had a survey done to check locations, which revealed that the relative error of points is <25mm however every point is shifted horizontally by 50mm.
What points are off by 50mm? Walls? Did you lay out centerline points? The thickness of walls is dependent on how well the form panels were laid out prior to concrete pouring.
@jonathan50 Two wall corners and three column corners are all off by 50mm, and for walls have been laying out the outside corners. Form panels have been laid out with some level of error which factors into the relative errors.
@rover83 yes, was a little disappointed with the relative accuracies, although have since determined that the foreman built the column forms with a 15mm skew on one side which has now been corrected, might explain those values being on the higher end. At first glance, the prism constant being used seems correct so I may need to do some more digging.
@dmyhill I appreciate the help, I have set up (reflectorless points) and checked three surveyed cross cuts which have been accurate horizontally to 5mm and vertically to 3mm. I have also had our more experienced supervisor watch a setup as well which went fine. I am starting to lean towards the possibility that our main control may be the issue.
@norman-oklahoma That could be the case. I am working off structural CAD on my T7 tablet that was produced for us by a third party and so have used their calc points. Not sure if there could be errors with those points
The thing to do now is to tie the foundation yourself and see if it corresponds to the calc points. If that is not possible with the software you have stake out the points and see where they fall relative to the foundation.