Building layout issues
Hello from Canada! I have a question for the professionals in here.
I work for a general contracting company in Canada. We have an in house position within the company doing foundation related layout, anchor bolt placement, quantity takeoffs but mostly as another form of QA/QC around our projects. We still do the same ol' thing and have a professional land surveyor come in and establish the placement of the building, leave us our usual building offsets, and establish the main floor elevation. Within the CAD software we use I align the structural file and everything else to the control pins that have been established for us and from there dependant on the day or task I'll occupy one of them and backsight some others or resection the total station maintaining a nice triangle to the points given to us.
Now to the issue. We are doing a larger scale project than I have been involved with. The distances between points are fairly significant and I've noticed a pretty big error amongst the pins. Since the company we use to establish everything is always exceptional at what they do I can't say there is a problem with the controls. I've had time to play around and try to tighten everything up with no luck. I'm wondering if the total station automatically accounts for a grid to ground scale factor on its own relative to how far away the points are from its occupied point, or if I need to use the scale factor they provided and input that in the coordinate settings.
I suppose one thing that I've done differently this time around is aligned the building on CAD to UTM coordinates and set the projection/datum/geoid for use of the network rover.
We have a Topcon GT-1203 TS
You didn't say how big the error is, but it sounds like the old grid/ground problem.
The control is in grid (UTM) and your total station is in ground (its usual desired state).
If this has been the case on smaller jobs, the difference was probably small and acceptable, but now you are on a bigger job with greater distances, it is noticeable.
No total station will apply a scale factor on it's own unless you tell it too. My guess is that if you applied it you would start agreeing with the control - but then you would be in grid and you probably don't want to be laying out concrete and steel using grid distances - I assume the plans are showing ground distances for the structures (though it's possible they are grid..you would have to check)
I have worked on large scale projects in Canada where the control had both UTM and local ground coordinate on them. The earth works contractors were working in UTM with GPS and the building construction was local ground.
I would ask the survey company to suggest a way forward.
Oops! I'm not a professional - just a survey tech 😎
You seem to be answering your own question. Working in UTM - when done correctly - would introduce scale effects on the order of 1 in 2500, or about 0.04' per 100'. Done incorrectly the scale factors could be dramatically greater.
I cannot imagine doing any sort of construction of the type you describe in UTM because of this scaling issue. UTM is appropriate for state or province wide mapping, not for detailed engineering works. Using UTM in the scenario you describe is incongruous. Switch to just using your coordinates as "local" or "no projection" or whatever your particular dc uses and see if your issues don't disappear. Better yet ask your surveyor what the appropriate projection zone is.
aligned the building on CAD to UTM coordinates and set the projection/datum/geoid for use of the network rover.
We have a Topcon GT-1203 TS
If you used the same projection settings as you described above with your TS, then the DC would apply the appropriate scale factors. But as Mark mentioned the scale factor in UTM would not be beneficial in concrete construction work or anchor bolts of a building a few hundred feet long. What are the dimensions of your building?
Best to have survey stake the building then change to a BIM datum and you work with ground dimensions not grid.
Yes this all just kind of lined up in my brain while checking some measurements on the data collector. The distance between two points is 191.951m under ground results and if I multiply that by the scale factor I have input already that distance becomes 191.857 and thats the number listed under Grid results.
So if I was in the middle of the building on a setup this explains the 2 inches each way that the pins are out?
Ground is best. That is what everything else in the pile of construction materials is measured in. 😉 😉
Really long projects (think airport landing strip) will highlight discrepancies. Heck, level isn't even level.
Long buildings can present similar problems.
The surveyor you are using for setting out the building offsets is always going to give you points in ground coordinates at a scale factor of 1.00000000. That is the way you should check the control. I find it easier to initially resect from a minimum of 4 pins (building corners) then read my stickers that I put on buildings and poles around the site. I only use the "surveyor pins" once, after that my stickers become my control. I had great results using this method and good feedback from the original surveyor who is usually tasked to do the as built at the end of the job. I know the stickers are not always a feasible way of setting control when you are in the middle of nowhere. A general contractor we worked with last year set leica MPR360 prisms around the site on steel posts that were affixed to concrete bases. This method worked ok for the most part. The settlement, or heavy machinery in the proximity of the prisms' bases create a shift of these control points, that's why they need to be constantly monitored and their coordinates updated.
@ramses cool. Please share some photos of the sticker placements, the good, the bad, and the ugly. I imagine putting them way up high reaching up to avoid troublemakers from disturbing them, etc…. Are they usually across the street, etc?
There's absolutely no reason to use grid for building layout. Site work, sure.
I'd pick one point, the one that's most critical for setbacks, etc, hold that one, BS the other one for orientation, shoot it as a sideshot, and hold that from then on. Layout everything from there, including additional control points.
The surveyor you are using for setting out the building offsets is always going to give you points in ground coordinates at a scale factor of 1.00000000. That is the way you should check the control.
Be cautious about this assumption. The projects I've done on Army Posts, Air Force Bases, and Missile Fields have mostly been in state plane coordinates from start to finish.
@frozennorth I'm curious. If the building is supposed to be 200' from one corner to the other, do the state plane coords for those corners inverse at 200' or 199.74' or whatever the reduction from ground to grid would be? If they inverse at 199.74 do you then put points on the ground at 199.74 apart or do you apply the scale factor so that the points on the ground are 200' apart. If the project is designed in state plane coords is everything a miniature version or does it somehow get built full size?
If the project is designed in state plane coords is everything a miniature version or does it somehow get built full size?
I use two coordinate systems.
SPC coordinates (grid) for the site work and grade
BIM coordinates (ground) for the building layout. This is the architect's datum with building grid lines or building faces aligned east to west.
This works well as it often means the site guys will not have any building layout for use with their GNSS. It should be done with EDM.
@lurker This is a very important point. In my experience, a 200' building is meant to be a 200' real-life (ground distance) building, and all the architectural, structural, and mechanical design will rely on that fact. However, the site work, drainage, etc. for the same site will often be done in state plane coordinates. So it would not be uncommon for the foundation corners to be called out with state plane coordinates that are 200' apart in grid distance, when they really should be 200' apart in ground distance.
For most of the sites I have been on, the scale factor has not been substantial enough to warrant chasing down the difference (say, 199.97 vs. 200.00). But if your scale factor is such that the difference is 199.74 vs. 200.00, then I would recommend elevating this question and getting an answer in writing about "what holds."
Yes! I am in this situation all the time. The plans are in UTM including work points and structures but the intent is obviously that the distances given or divined from coordinates are the actual true distances required.
The issue is never understood by the designer. The answer is always "Of course if it says 200' then I want 200'. I don't understand this discrepancy you're talking about."
So the surveyor chooses a point to hold to transition into ground and off they go. The problem is the next guy could choose a different origin and somewhere you meet and there is a couple inch discrepancy.
Things get built and life goes on but it seems like a problem that never goes away