How To: Create a custom projection file in ArcMap to align CAD data
Instructions provided describe how to create a custom projection file for CAD data, and how to align the CAD file with other data in a projected coordinate system with units of feet, meters or other linear units.
Note: These instructions should not be used to align CAD data with data in a Geographic Coordinate System, with units of decimal degrees.
In ArcMap, follow these steps to align a CAD file with other data by creating a custom projection file.
Open ArcMap with a new empty map, and add data the CAD file is supposed to line up with. These data must have a projection defined, and the units should match the units (feet or meters) used when creating the CAD file.
Add only the polyline layer from the CAD file to the map.
Click the full extent button, and measure the distance the CAD polylines are away from the data in the real-world coordinate system, in the east-west direction only. Do not measure on the diagonal, just a straight line east to west.
Note: It is helpful to write down these measurements on a piece of paper with a direction arrow indicating the direction the CAD data needs to move to align with the other data.
Go to View > Data Frame Properties > Coordinate System tab, and click Modify.
Type a new name in the top box for the custom projection file. The new name cannot contain special characters or spaces, but underscores can be used.
Under Parameters, there is a value for the False Easting.
Note: Making the False Easting value larger moves the CAD file west. Making the False Easting value smaller moves the CAD data east.
Add or subtract the measured distance from step 3 above from the existing False Easting value, and enter the new value in the field.
Note: ArcMap calculates coordinate position to 16 significant digits, so the zeros to the right of the decimal must be retained.
Click OK on the Projected Coordinate System Properties dialog box. Click Apply on the Data Frame Properties.
The CAD data moves east or west to more closely align with the reference data.
Click the Add to Favorites button. This writes a copy of the custom projection file to disk. Click OK in the Data Frame Properties dialog box.
Use the Measure Tool again, this time measuring the offset between the CAD data and the reference data in a direct line north to south. It may be helpful to write down these values, with a direction arrow.
Once again, click View > Data Frame Properties > Coordinate System tab > Modify.
The North-South adjustment is made by changing the False Northing value.
Note: Making the False Northing larger moves the data south. Making the False Northing smaller moves the data north.
Adjust the False Northing value by the offset measured in step 9. If the CAD data is too far south, subtract to make the False Northing value smaller. If the CAD data is too far north, add to make the False Northing value larger.
Click OK, Apply, Add to Favorites, and OK.
Repeat these steps as needed, making incremental adjustments to the False Easting and False Northing, until the alignment of the CAD polylines to the reference data is the best that can be achieved.
Note: Keep in mind that the CAD data may not align exactly with the available reference data, because of inaccuracies in one or both sets of data.
When the final version of the projection file has been achieved, click Add to Favorites one more time. Click Apply. Click OK, and save the MXD.
For Version 9.3.1 and earlier, the custom projection file is saved at the following locations:
Windows 2000 and Windows XP:
C:\Documents and Settings\<user_name>\Application Data\ESRI\ArcMap\Coordinate Systems
Windows Vista, Windows 7, and Windows 2003/2008 Server:
C:\Users\<user profile>\AppData\Roaming\ESRI\ArcMap\Coordinate Systems
For ArcGIS version 10 the path is:
C:\Users\<user profile>\AppData\Roaming\ESRI\Desktop10.x\ArcMap\Coordinate Systems
Note: If the AppData folder is not visible, make sure ‘Show hidden files, folders, and drives’ is checked in Folder Options.
a) To display hidden files and folders, open the Control Panel and click Folder Options. b) On the View tab, under Advanced settings, click Show hidden files, folders, and drives, and click OK.
In Windows Explorer, copy the custom PRJ file, then go to one of the following locations:
ArcGIS Desktop 8.x, 9.x:
C:\Program Files\ArcGIS\Coordinate Systems
ArcGIS Desktop 10.0:
C:\Program Files\ArcGIS\Desktop10.0\Coordinate Systems
On 64-bit systems:
Make a new folder in Coordinate Systems, and paste the custom projection file into the folder. An example of a folder name is 'Custom PRJ Files,' but any name can be used.
Note: This step does not work if the CAD file has spaces in the name.
Paste a copy of the PRJ file into the same folder with the CAD file. Rename the PRJ file with exactly the same name as the CAD file. This is case-sensitive. For example, if the CAD file name is Parcels022007-G.dwg, name the PRJ file Parcels022007-G.prj.
Note: If multiple CAD files in the same local coordinate system are stored in the folder, the custom projection file can be named 'esri_CAD.prj'. ArcMap recognizes this projection definition and applies the custom projection to all CAD files in the directory.
The next time the CAD file is added to ArcMap, the software recognizes the projection definition, and is able to project the data on-the-fly, along with any other data in the map.
If the CAD file name contains spaces, set the projection of the ArcMap Data Frame to the custom projection. Any other data added to the map that has the projection defined is now projected on-the-fly to line up with the CAD data.
Note: Three chapters in the book titled 'Lining Up Data in ArcGIS: a guide to map projections' are devoted to detailed instructions for this process, and address such questions as identifying custom units, or creating a custom projection file for rotated CAD data, which are too technical to address in this article. For additional assistance, contact Esri Support Services on the Esri Support site, or call 888-377-4575.