I??m about to start a new construction staking project that we did not do the original design survey???ÿ
I??ve only been out and calibrated to the control at this time, but after doing so, it seems to be on our city system (essentially state plane) not moved to ground or anything. ?ÿThat being said, the site is relatively large so I do see a little difference between my grid and ground distances (about 0.2?? across the site). For most of the project, it??s negligible and I??ll roll with the calibration as is for all the civil work, however, there is a large building with a lot of structural steel and a pool inside that will require certification. I??m thinking of setting some additional points to create a scale factor only system for just the building layout after a majority of the site work is done since I have some tolerances that are pretty tight on the building and pool. ?ÿ
anyone had experiences like this with projects??ÿ
?ÿCheck the existing control monuments. I??ve been on jobs like that where another company did the prelim and design but messed up the control then the next poor guy doing the layout has to figure out what they did. And usually the contractors want you to give them some layout the first day your on site. ?????ÿ
I recommend two datums on large projects. This keeps GPS outside the building, and EDM inside the building.
We use SPC coordinates for site work and BIM coordinates for vertical construction (building layout). Most often, these are the coordinate systems already established in Architect's and Engineer's contract drawings (CAD files). Also, this prevents GPS hackers (or contractors) from trying to layout BIM data with their magic stick. In short, all site work is done with GNSS on SPC coordinates (grid), while building construction is done with EDM on BIM coordinates (ground).?ÿ
Today's large construction projects should require a single Professional Surveyor as site coordinator. What I do is verify and or set permanent control monuments near the project perimeter on SPC to create a Static GNSS network. These coordinates are published for site contractors to use with their GNSS. As with the other thread, most under-educated contractors will likely localize or calibrate to the control monuments when it is completely not necessary. All they need to do set up the projection and verify. For very long buildings (800' or more) we establish an LDP on the BIM datum that is only used by myself or qualified professionals.
My preference is for LDP. Been on projects where multiple scale factors were used for different parts of the job
I like @leegreen 's solution for inside vs outside layout. I would probably set up the job with an LDP, and but have an internal Grid (Local) transformation to relate the inside layout (BIM coordinates) to the outside. That way it's possible to do outside or inside layout in the same job.
Today's large construction projects should require a single Professional Surveyor as site coordinator.
It's probably the best solution, but there are a lot of licensees whose knowledge of geodesy and map projections is about the same as the builders/subcontractors.
That way it's possible to do outside or inside layout in the same job.
Yes, it is always a pain to keep switching between jobs. It much easier to have everything on one job.?ÿ
Note 3 names for 3 coordinate systems on one project: Internal, Shared, Project. The meanings are something less than intuitive, at least to me, and each must be some transformation of the others. I would never make it in the real world.
Understanding Coordinate Systems | BIM 360 2023 | Autodesk Knowledge Network
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Yeah, this is another reason why LDPs - that can keep project coordinates under the arbitrary limit that Revit imposes - are extremely helpful. Map, design, and build in the real world.
the site is relatively large so I do see a little difference between my grid and ground distances (about 0.2?? across the site)
So if the building site is a tenth of the size of the whole that is 1/4" of error across the entire building site due to scaling.?ÿ
@norm the building is pretty large on the site...in the length of the pool its just under 0.04' between them...the tolerances of a pool being certified are -0.00' to +0.1'. Staking the pool on grid would essentially put me in the place to have a short pool (if the contractor built everything exactly to what was staked, and things were staked to dimension without compensation for length).?ÿ There is quite a bit of wiggle room to be on the long side and within spec, but can't be short at all.?ÿ
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that is exactly my thought on it as well.?ÿ I'm going to roll with the 2 datums/scale factor only system inside the building given the circumstances.?ÿ
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Thank you to everyone taking the time to read and reply. Much appreciated. Have a Merry Christmas.?ÿ
in the length of the pool its just under 0.04' between them
That??s quite a difference between grid and ground??or that??s a very long pool ???ý?ÿ
Here I??ve seen a difference of 0.1?? per 1000?? between SPCS and ??project coordinates? on some sites ?ÿdepending the site elevation relative to the ellipsoid.
