Civil 3D and Coordinate Datums
Are you importing the points through the survey database? (IMO the only way to go) Your drawing and database need to be on the same system or it will change the coordinates quite drastically as it tries to convert from one to the other.
If you're just inserting using the <points> and <import points> command then your points will come in exactly as they are in the csv regardless of drawing coordinate system settings.
We always set our database and drawings to match. There is zero benefit to do otherwise, as we work exclusively in those systems.
Just a quick note on C3D, points and projections.
First, point can be entered as local N / E or Grid N / Grid E. A point can have different local and grid coordinate pairs but only 'grid' coordinate will be reprojected. This is handy if you have project scaled to ground by a common C.F. but would like to reference data on the actual grid. That can all be set up in the 'Transformation' tab of the drawing settings box.
Second, as Jim mentioned earlier, CAD programs work on an arbitrary cartesian coordinate system. Geodetic values are calculated from the cartesian system based on the projection and datum you specified in the drawing settings. C3D will not reproject on the fly. Unlike most survey or GIS software, it will only reproject data during import or export. For point data you must specify 'grid' coordinates at import or export.
I can understand the way Autodesk and MicroStation deal with coordinate systems. What I cannot understand is why AutoCAD is so lacking when dealing with units. It should be a simple scaling of display issue to go from US Ft to meters to Inches. All you really need to know is the units used when importing data. MicroStation has done this since day one. There is no reason that an architect should have to scale engineering plans by 12.
I am only using basic point functionality, I have not tried the survey database, but will look into it. Sounds very interesting. My main objective is to understand the software behavior as it relates to the subject of datums/coordiante systems. Which seems like a black hole of technical stuff, and seems to be a moving target. Just when I think I have it figured out...
Ok, I see what you are saying. Yes, I agree that the survey database does perform the transformation to a new datum if one changes the drawing coordinate settings, and the survey database follows this update, which surprised me. I did not realize that C3d did re-projections. I always thought it was just doing grid transformations, not so much of the projection then reprojection for the more accurate datum changes. Very cool, thank you for that update!
Does changing the datum in the survey database by changing the coordinate settings to a different datum, provide a proper transformation? If so, what is the purpose of the Transformation tab in the prospector settings?
The survey databased, is just that, a database. Whatever coordinate system you designate in the survey database is the coordinate system it thinks your data is in. If your Cad coordinate system is different than your survey database it will 'convert' the coordinates to the Drawing coordinate system when the points from the database are imported in the drawing.
We have never found this to be a workflow we use in over 10 years of Civil3d use. We set our coordinate system in TBC and it never changes. We never have to convert from one system to another. If we did, i'd do it within TBC as that is our database.
Hopefully that answers your question.
