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State Plane Coordinate question

  • 1.  State Plane Coordinate question

    Posted 03-08-2023 14:16

    I am having some trouble conceptualizing when to work in SPC and when not to.

    For example- I am doing bridge a job for NYSDOT, they've given me a list of control points with information on the state plane zone and what "combined scale factor" was used. 

    I input those point coordinates into TBC ensuring that the job is set to the proper State plane zone export them to siteworks which I also ensure is set to the proper state plane zone. I then go out with my robot (no GPS involved) and locate the control points and set some of my own- everything checks out pretty well ( within +/- .05')

    My question is when do I need to be concerned about grid vs ground if both siteworks and TBC are set to the same zone? If I stake-out a station/offset or a cogo point I created in TBC- the software is automatically going from grid to ground and vice-versa correct?

    The bridge is about 530' long so the scale factor of .99994.... wont neccessarily make or break me (though it could) but I just want to make sure that I'm not introducing more error right off the bat.


    Christopher Partridge

  • 2.  RE: State Plane Coordinate question

    Posted 03-09-2023 10:55

    It sounds to me like you have it set up correctly. You just need to drink the cool-aid and have your project settings correct and all the same and use real elevations. Also, you definitely don't want to use a SPC file on assumed coordinates or vice-versa.

    If you are using Trimble Access in the field it will convert your raw measurements to grid (assuming you have it set to do so), so that it will be pretty transparent to the users. I am a surveyor for California DOT. Everything we do is in grid, so I can't speak much to what tricks you might need to go between grid/ground. In my area, we haven't built any long pre-fab bridges high in the mountains since the 70's. That is the only situation in transportation work that I can imagine that it would matter in the field. The material quantities are well within our acceptable margin of error.

    Before we were using Trimble (prior to 2003), we'd have to enter the Combined Scale (Grid) Factor in our total stations to work our data collectors in grid. Trimble Survey Controller (now Access) took care of that, even the temperature and pressure corrections. At this point, I bet we have technicians in the field that really aren't even aware of what is going on behind the scenes.

    Oh and make sure you're using survey foot (if that is appropriate). I have heard of at least 1 DOT that uses SPC for control but ground for their projects. I'm glad we don't do that!!

    Dave Olander