One of the most confusing aspects of bringing data together from different sources is the varying coordinate systems, or lack thereof. Fortunately, most designers, drafters, and engineers don’t have to dive too deep into the world of Geodesy–they just need to understand the basics of Civil 3D coordinate systems for their projects.
When you have GIS or CAD data, there are 3 general types of projects to consider:
Grid Projection (Grid Distance)
- Used to flatten out the curved earth
- Most GIS data uses a grid projection. This could be world, state, county or city, or other regional projections.
Ground Projection (Ground Distance)
- In order to get a more accurate and precise location for points, surveyors will typically have a modified projection using local ground control.
- Surveyors often start with a known grid projection and apply a modified scale factor or rotation.
No Projection (User Coordinate System)
- Occasionally, projects may not have been designed with any particular coordinate system. When this happens, data may need to be georeferenced (rubber-sheeted) to the correct location.
Do I Need to Set My Drawing to a Coordinate System?
The answer to this depends on what you need to do with your drawing files. If you want to bring in Bing Imagery as a background, or import Raster Imagery or external GIS data, then the answer will be YES.
Think of it this way: If you want to bring data into your drawing that already knows where it is in the world, you have to know where your current CAD data is in the world. Otherwise, Civil 3D doesn’t know how to bring the two data sets together.
What Coordinate System am I On?
In many cases, the project may have been designed with a grid coordinate system. You can verify this by setting the coordinate system to what you think it is. In this example, we’ll use a project that has simple centerlines and boundaries. The project is in NAD83 NEW MEXICO STATE PLANES, CENTRAL ZONE, US FOOT (NAD83-CF).
For a visual verification of the Coordinate System, do the following:
In Toolspace, select the Settings tab and Right click the DWG name. Select Edit Drawing Settings.
In the Units and Zone tab of the Drawing Settings dialog box, you will need to make sure the correct Coordinate System is set up.
Tip: if “No Datum, No Projection” is currently set, it does not mean that the project wasn’t drawn using a coordinate system, it just means the DWG file doesn’t know what Coordinate System the linework and points were drawn on.
Browse to the correct Zone in the Categories as follows:
Tip: If you know the Coordinate System Code (ie NM83-CF), you can just type it in.
For the next step, we’ll use the GEO command to bring in Bing background imagery to see if things line up correctly.
Type GEO. Then hit ENTER for the default <Map>. Select the Next button for the default marker and then select the Next button for the default NM83-CF Coordinate System. Finally, hit Enter twice to accept point and rotation defaults.
If the linework doesn’t match up with the imagery, then the DWG is either in another Coordinate System, is not tied to any Coordinate System, or has its own Ground Projection.
Stay tuned for my next blog on Local Ground Coordinate Systems and how to tie GIS data to it.