Civil 3D and Coordinate Datums

The reason is to allow the XY coordinate to convert easily in CAD to LAT, LONG. If you change the projection but insert the same XY coordinate, it should show a different L,L #. Depending on the change it could be a very small number change or a huge one if you're changing zones. So by importing the same XY value, all you're doing is moving the position in the real world. N1,000,000 E1,000,000 In Wyoming state plane will be in a way different location than the same number in a Colorado zone., but it will still be 1,000,000x1,000,000.

Check the lat long on each point, they should change with the different datum.

If your drawings are all simple and isolated you can just use "No Datum, No Projection" and get along just fine. But if you start mixing and matching external references or aerial imagery that is in this projection and that projection - as you might if employing GIS data - you will want to pay close attention to those settings.

All,

Thank you for that feedback. I will test the recommendations.

Concerning lat-long locations, I thought they were mostly used to measure on a 3D geographic datum, but I see this also in the Civil 3D prospector as point location data. So, the lat-long must be interpolated on the projected grid surface. Is my understanding correct?

@cturlington Yes, you are correct

Latitude and Longitude of a given point tends not to change - other than physical movement - it takes a change of spheroid model

Northing and Easting coordinates depend on the projection - change the projection and you will see those numbers change, sometimes quite significantly

All,

Thanks a million for all your feedback.

I have experienced the change in lat-long values but have been unable to force a change in the Northing-Easting values by changing the datums/projections and even units. My guess is that because the coordinate systems are grid-based in Civil 3D, the units from the origin are already fixed. So, changing the datum will not change the distance to the origin unless I move the points in the file. However, I would think exporting the points with an odd datum should create a CSV point file with distances relative to the odd datum. Does that sound correct?

Most cad systems store their data as XYZ, not Lat/Long.

What you are experiencing is a side effect of this.

When the projection gets changed, the coordinate XYZ's do not. This is lazy behaviour in my opinion, tends to introduce errors, but not necessarily wrong.

Then any lat / long is calculated in terms of the projection in use at that particular time - so they DO change.

GIS software is often better behaved - if you really want to play in this area I would recommend QGis

Best practice is to

- know and understand your software

- get your coord system/projection etc set right from the very start

- write down / record what it is

- and DONT go changing it

And pay special attention to the transformation algorithms in C3D. We have encountered that the transformation of coordinates from rectangular coordinate system to geographic (or any other) coordinate system in C3D differs from the transformation algorithm in (TBC) Trimble Business Center, for example. In (TBC) there is a separate utility for updating coordinate system parameters, frankly speaking, I trust Trimble more.

Thank you all for sharing your expertise with me.

Does anyone know if there is a certification program or similar geared toward engineers and CAD people that teaches these concepts? I am familiar with NGS, NOAA, etc., but there is always some fundamental lack of knowledge, like what we have discussed here, which leaves the question about confident transformations, real software understanding, etc.

I assumed the main purpose of setting the coordinate system in Civil 3D was to get the google earth background to come in on the right spot. heh

This makes me ask if it is even necessary to set the coordinates in a CAD file for just importing and exporting points. I can imagine that coordinates would be necessary for transformations but not for exporting or importing point data.

If all you're doing is connecting some dots then who cares where in the world they're sitting. 😎

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?