Importing Points - Assumed Datum

[Iceberg]

What is the purpose/or value of shooting survey points in an assumed datum, importing them into Civil3D in an assumed datum, and then moving and rotating the points to the correct coordinates, such as NAD83?

I have been getting some concerns about this and need to get some information so that they can be addressed appropriately.

Thank you for your input on this matter.

[cadtag]

an opportunity to accumulate errors? I am not a surveyor, rather a 'consumer' of survey data. I would much prefer the surveyor take the responsibility for shooting their points in the correct coordinate system. It's their job IMHO to correctly and properly understand coordinate systems and mapping, not that of the junior tech assigned to jump in and help/

[Iceberg]

I agree with you cadtag. Was exactly why I was asking. For good or worse I'm missing something I think.....

[cadtag]

quoting rkmcswain on another forum:

Don't forget about the grid to surface (or surface to grid) scale factor that may need to be applied after you have transformed from assumed coordinates to state plane.

Ref:
http://forums.autodesk.com/t5/AutoCAD-Civil-3D-General/Points-Convert-Grid-to-Surface/td-p/2442843
The first 12 or so pages of: http://www.ucs.iastate.edu/mnet/_repository/2009/ascesurveying/pdf/Norman%20Miller.pdf
http://onlinemanuals.txdot.gov/txdotmanuals/ess/geodetic_surveying.htm#i1028861

[trevor.hatch]

This is something that should be a non issue to a decent surveyor. Sadly, I've seen a lot of PLSs that don't understand scale factors or calibrations. We use state plane coordinates almost exclusively, but for any terrestrial data that our field crew collects, they used assumed coordinates and I do the adjustment in the office. Just make sure that they get sufficient control to make several checks. It is very common practice to use assumed coordinates.

[ironmlh]

If you need your data on <insert datum here>, make sure it's in your subcontract (if your subbing out survey work) and hold them to it. If this is an internal problem then you need to address it with the surveyor.

I haven't used an assumed vertical datum in 6 years, everything we do is tied to both a horizontal and vertical datum. Asssumed datums are like two man survey crews....non existent.

[Iceberg]

If you need your data on <insert datum here>, make sure it's in your subcontract (if your subbing out survey work) and hold them to it. If this is an internal problem then you need to address it with the surveyor.

I haven't used an assumed vertical datum in 6 years, everything we do is tied to both a horizontal and vertical datum. Asssumed datums are like two man survey crews....non existent.

Actually all we use are 2 man survey crews and everything is assumed. The reason I have been told is that coordinates systems like state planes are a trajectory on a curved surface, where as plans are design and constructed on a flat surface. The curved surface is not accurate there for the points are shot in an assumed datum.

[cadtag]

Uhm, well, no. at least not completely, and only even partially correct unless your job is big enough to see from space. The state plane coordinate systems are a transformation of a curved surface to a flat plane. the errors induced by curvature of the earth vs a plane surface are minimized, and frankly pretty much not relevant to typical civil work. Not measurable on a site

The point of doing that transformation is to have a consistent planar surface to work with. Assumed coordinates are going to be no more 'accurate' or precise, but since they are assumed and arbitrary, it's much more difficult to bring in additional information. with a known state plane coordinate system, suddenly GIS maps, aerial photographs, etc are useful.

Now it's easier on the survey people to just ignore geographic location, and pull a 0,0 point out of their hat than do the work to geographically locate the project, but it's a much, much less useful way to do things.

[Iceberg]

Thanks CADTAG... That's what I have been use too from previous companies but here it is different...

But like I've been told, I'm not a surveyor so what do I know..... lol

[cadtag]

well, when a surveyor hands me a job not in state plane, he doesn't get hired again.... he's paid for that one job, and that firm goes on the 'not worth hiring' list

[rkmcswain]

well, when a surveyor ..... he doesn't get hired again....

Impossible when the client hires the surveyor directly...
We have to take what we get. :-|

[barshnizza494659]

We shoot all of our total station work on an assumed datum as well, but then run through everything with a gps unit and then rotate everything to match the gps points. Or do a transformation... I would, however, definitely be annoyed if I hired a surveyor to put everything on state plane and they did not.