How to Create a Better Workflow for Coordinate Systems from Grid to Ground


Here’s a common scenario: You have a project that was designed on a local coordinate system. You try to bring in imagery or GIS data into the project, but they aren’t inserting in at the correct location. So you move, scale and rotate the imagery to line up correctly.

Stop. There are better Civil 3D workflows that do this for you.

Let’s take a look at one of these workflows by creating a Local Coordinate System in Civil 3D’s library database that is based off a Grid Coordinate System.

Local Ground Projection

In this example, the Surveyor used NAD83 NEW MEXICO STATE PLANES, CENTRAL ZONE, US FOOT and modified it by a scale factor to get a higher level of accuracy. They noted the Combined Scale Factor of 1.0003672940 on the cover sheet of the plans.

combined scale factor

Tip: This is an example of a project that used a Combined Scale Factor, which may not be the case for your project.  Verify with the Surveyor or the source of the original Survey data

Since this project is designed using a Local Ground projection, we will create a custom Coordinate System, using NAD83 NEW MEXICO STATE PLANES, CENTRAL ZONE, US FOOT (NAD83-CF) as our starting point.

  • In Toolspace, select Settings Tab
  • Right click the DWG name
  • Select Edit Drawing Settings

toolspace settings

Since we know the Coordinate System to start with, browse to the NM83-CF zone as follows:

NM83-CF zone

Tip: You can also just type in NM83-CF in the Coordinate System Code.

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
  • Hit ENTER for the default <Map>
  • Select Next button for the default marker
  • Select Next button for the default NM83-CF Coordinate System
  • Hit Enter twice to accept point and rotation defaults

geo command

As expected, our linework doesn’t match the imagery.  Let’s fix that by adding our own Ground Coordinate System.

Create a Custom Coordinate system Definition

  • In the “Planning and Analysis” Workspace, select Map Setup Tab
  • Under the Coordinate System panel, select the “Create Coordinate System Definition”

Create Coordinate System Definition

  • Select “Start with a coordinate system”
  • Select Next

Start with a coordinate system

  • Click the radial button for “Create a new coordinate system from an existing coordinate system”
  • Click the “Select” button

Create a new coordinate system from an existing coordinate system

  • Double click the correct NM83-CF Coordinate System

NM83-CF Coordinate System

Tip: Use the search bar to type in keywords or the Coordinate System Code.

 

  • Click Next

specify coordinate system

  • Modify the Code name and description so that it doesn’t conflict with the standard Coordinate System.

modify coordinate system

Tip: A good practice is to add the project number as a suffix, since this coordinate system will be used by this particular project.  Also, it’s not a bad idea to put it in a different category than the original Coordinate System you copied from (In this case, I put it in the Arbitrary X-Y Coordinate System Category).

  • Multiply both the False Easting and the Scale Reduction by the Combined Scale Factor. Here are the default numbers prior to any multiplication:

specify projection

Tip: Avoid truncating any of the numbers. Use all the decimal places your calculator and dialog box will allow.

Here are the revised numbers after multiplying by the Combined Scale Factor, which you will need to type into the correct fields below:

False easting calculation: (1640416.66666667) * (1.0003672940) = 1641019.18186583666789098

Scale reduction calculation: (0.9999) * (1.0003672940) = 1.0002672572706

false easting calculation scale reduction calculation

  • Select Next and Close until you are out of the dialog box.

Select the Custom Coordinate System

  • In Toolspace, select the Settings Tab
  • Right click the DWG name
  • Select Edit Drawing Settings

toolspace

  • This time, select the Local Zone you created above.

local time zone

If done correctly (matching the Surveyor’s Combined Scale Factor), your linework should align with the Bing Imagery.

big imagery

Tip: You may need to pan or zoom to get the imagery to refresh.

This makes Civil 3D do the moving, scaling and rotating automatically by re-projecting the data for you.

Rob is a GIS specialist, civil designer, project manager, and CAD manager. He is uniquely skilled in CAD and GIS products, networking, security, database, cloud services, and computer technical support. Rob is a Certified Geographic Information Systems Professional. He understands a customer’s project from the initial conception to the final stages of approval, consistently meeting their needs, budgets and timelines. As a CAD manager, he helps customers achieve their goals of implementing reasonable solutions, more efficient methods, user-friendly standards and procedures, documentation and libraries, cloud storage solutions, and ways to use database objects to their full potential.