How large of a project would be the limit for a single Combined Scale factor? A hypothetical would be a large diameter pipeline covering many miles with more that 3 survey firms involved.
> many miles with more that 3 survey firms involved.
That alone could get sticky, on the scale factors!
It depends on how far from the intersection of the ground with the grid you get. That is, AT the intersection point, the scale factor is the same. (Grid and Ground are the same scale) You need to learn a bit more about State Plane Coords. (I need to learn more too.) My point is, that often, you will wind up working with data, that was "Right" by the survey tech, but he may not know enough, to discern a blunder, by a tech. I am saying that the time has come, for you to really learn this stuff. IF I were in your shoes, I'd definitely have to bone up, and learn EXACTLY what each crew, and company did, in order to work with it, AND HAVE CONFIDENCE.
Go for it.
Nate
I'd recommend using Corpscon (free download from Corps of Engineers) and plugging in approximate lat-lon and elevation for points every few miles along the route to see how much the CSF varies. Then compare that to your error budget.
I'm/more concerned with the elevation part of the combined scale factor. Ie I'd be concerned with how/much elevation change there is/in the project more than the horizontal distance
I'm working on a large R/W engineering project in California. I was given the task to come up with the combined factors for a portion of the proposed route and where convenient breaking points would be. We went with maintaining a single combined factor throughout a township. The difference East-West, from township to township was neglegable. North-South seemed to differ by enough to calc a new one. I think it would depend on the SPC projection as to what you could get away with. I compared the calculated CF's of townships having a common line and they were different by +/- 1:300,000-1:100,000 until I got to within 12 miles of the North or South edge of a zone, then it got as bad as 1:50,000. The job is prety flat where we are working.
> How large of a project would be the limit for a single Combined Scale factor? A hypothetical would be a large diameter pipeline covering many miles with more that 3 survey firms involved.
Bill's advice to actually go ahead and compute the CSF at various points along the route was very good. It isn't the extent so much as it is the actual variation in CSF on the actual route of the pipeline that would be most relevant. In Texas, the change in CSF will naturally depend upon the direction of the pipeline. If it runs more or less East-West, the map projection scale factor will change very little. The elevation scale factor (or, more exactly the ellipsoid height scale factor for the Texas Coordinate System of 1983) will be the main cause of CSF variation in that case.
The other thing to consider is what the CSF is going to be used for. Best practice would be to survey the route making rigorous reductions to grid. That way, you know the actual coordinates in the Texas Coordinate System of 1983 (I would hope) more or less exactly. That will require knowing the ellipsoid heights of all points positioned so that those reductions can be made, not a biggie when GPS is used, but requires somehow accounting for height changes in conventional measurements.
I'd normally assume that the main use of the CSF will be for generating some "surface" mapping for pipeline design and for generating approximate surface distances for pipeline easement descriptions. Is that the case here?
> How large of a project would be the limit for a single Combined Scale factor? A hypothetical would be a large diameter pipeline covering many miles with more that 3 survey firms involved.
Your reason for wanting to work in Ground measurements?
Your rational for not working in State Plane Coordinates?
Larry P
With several firms involved, I'd say hold them pretty tight, just as Kent says here. Hopefully one person/firm could dictate this specification. If the rest are being asked to do it and organize the calcs that way from the start, it should be no big deal to do it.
Thanks for responses. Easements involved, so surface distance necessary. I am not setting up the project, just trying to work with it.
Is a custom low distortion projection (LDP) possible for this project?
The fact that this question is coming up means that it's poor planning on the part of the pipeline company.
The best way would be for the regs to require all of the data to be on the Texas Coordinate System of 1983, PICK a zone, NAD83 (2011) Epoch 2010.0000. All then that is required is for the end of one surveyor and the beginning of another surveyor to check the value for that end/beginning point, and the next one would roll on. Scale factor is fairly irrelevant as the companies that compete for this work HAVE GPS and can all get on the same base. It's even better if it's all on network. Then, EVERYTHING is on the grid and one is not concerned with applying one scale factor for a region. If a conventional line must be run, odds are you will have a pair of points at the end and the beginning of the conventional segment and the appropriate scale factors can be computed for that run. Carlson is great about reducing raw data and applying the appropriate factor at every set up correctly instead of meaning factors out.
If it's all E-W, really doesn't matter, but your elevation change does.
Mike
The answer might be a management question not maths. If the construction of the pipeline is segmented, you might adjust your factor so that no segment has multiple factors.
The maths portion of the decision I would suggest you determine the amount difference that would exist between the factor for a segment and the factor at control points along the route.
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