laying out bridges using SPC grid coordiantes
I have a construction project that I am woking on, specifically a bridge that is over 1000' from abutment to abutment. The provided control is maryland SPC nad 83 and the control report states that a combined factor of .999943 is to be applied. when applying a scale factor in the data collector from my understanding evrything is scaled even stakeout. My concern is that if i stake out a point that is supposed to be 1000' away is it going to actually be 999.943' or 1000.057'. Are my concerns unfounded or am I on the right track. I know most folks would not worry about .06 but we have prefab steel and we want to shoot for perfect upfront knowing there will be some error introduced.
You need know the direction of your scale factor. Is that scale factor ground to grid? If yes, then your ground distance is longer than the grid distance. The Tappan Zee was built on grid coordinates, that was 3.2 miles long. The temperature correction for the prefab steel will likely be more than the combined scale factor.
Licensed Surveyor in NY
sUAS Remote Pilot
You need to set your cad drawing up so that you're working in the right zone.
How is the bridge situated on the grid. NJ is a transverse Mercator.
A north/south orientation will have a different at both ends vs an East/West.
One orientation may deal more with convergence.
Not sure about 1000 ft though. I'd work with grid to ground regardless. It makes good practice and you will be right.
I know of a large bridge replacement project in NJ where the contractor's surveyor could not deal with the grid to ground conversions and had a hard time design with the whole concept.
The contractor reneged on the contract and paid 10% back to the NJDOT for non-performance.
The 10% was about $9 million!!!!
On another the design engineer did not understand the grid to ground conversion and the first steel girder dropped between abutments!
I thought I saw Angels
But I could have been wrong!
Are you laying out points for the bridge with GPS? If so I would be more concerned with the GPS error(s) than the scale factor error. If it were me, I would use the ground values for the control and use only a total station and half of your head aches will go away.
Professional Surveyor - MO, AR, KS, KY
"Well that depends"
The fact of the matter is that on any bridge I have done over the years, it can be moved a few inches one way or the other and not cause any problems. I have never done any long bridges or any interchanges. Once you pin down where it will be, then use that method/control throughout the job.
I am not advocating just haphazardly marking the bridge location. I would mark the two centerline points (assuming it is a straight bridge) that are far enough back from the abutments to stay put for the duration of the job. Then I would set on one and shoot the other to get an accurate distance (NOT with the dc) and adjust the station of each point to fit the distance. All layout would be done by station and offset from those two points. If you wish to use the data collector, you can setup a separate job file with the North being the station and the East being left/right of centerline. No SPC, no scaling.
I would also set two intervisible points off location from the two centerline points as a backup plan. Preferably one close to each abutment.
If this is an interchange, then don't take advice from a country surveyor that hates even driving thru an interchange, much less staking one.
East Texas, Like a State of it's own, It's a State of Mind.
If I were in your position I would measure the grid distance between your two abutments in project design files (that I assume are state plane). If they are 1000.00' apart, that means the bridge was laid into the design without regard for state plane grid/ground. In that case, I'd pick a point from middle of the bridge and scale the coordinates for the abutments from there. Then I'd check the distance again. Most software should be able to show grid vs. ground, and you want ground to be 1000.00.
Don't over think, if you do, you are bound to create error. Just apply the combined scale factor in your data controller. It will apply the CSF to all EDM distance and reduce the COGO to grid coordinates. The DOT's control points and contract plans are on SPC (GRID), therefore you should stay on the grid. Don't mix apples and oranges.
You can verify by setting up on the existing control and shooting distances to other baseline points.
What controller and software are you using. Most software today will compute the CSF at each control point, and apply accordingly when setup to use a SPC projection. This goes for both EDM and GNSS.
On DOT projects you should never, never, never need to use a localization or calibration. Always use the SPC projection. If you think a localization is needed, then that is your first clue something is wrong.
Licensed Surveyor in NY
sUAS Remote Pilot
What ever you do; you need to have project coordinates on known, undisturbed points, and check a distance between them, using the total station and data collector designated for this project.
An ounce of prevention...
I never did a day's work in my life. It was all fun.--Thomas A. Edison
Citius, altius, fortius
The answer is simple and lies in understanding what you have in the control and in the design datum, then getting some settings right on your equipment. It's no big deal. But it is true that not every surveyor, even the licensed ones, understands how to work on the grid. I'd suggest that you bring in a professional who does to help you out with the set up. It will be money well spent.
"Convention is like the shell to the chick, a protection till he is strong enough to break it through." Learned Hand