ArcGIS uses a spatial reference to project features on the fly. It also uses the linear units defined by the map projection to measure distances and areas. You can add CAD data to a map without a spatial reference, but the data will not be projected, nor will you be able to perform measurements against the data. Also, geoprocessing tools that use linear units to perform calculations, such as the Import CAD Annotation tool, will default to meters if the spatial reference is unknown or undefined.
Limitations of CAD datasets
DGN and DWG formats do not natively support the ArcGIS spatial reference system. To get around this limitation, ArcGIS Desktop extends the properties of a CAD dataset to include a spatial reference as ancillary information. This information is defined as a text string in the well-known text (WKT) format and stored in an auxiliary projection (.prj) file or embedded in the CAD drawing.
Defining a spatial reference does not adjust the spatial characteristics of the geometry nor does it shift the CAD file's origin coordinates (0,0) to a known geographic location. When you define a spatial reference for a CAD dataset it is solely for the purpose of projecting the data in ArcMap and enabling the features to be measured. If the dataset does not align correctly with your spatial data, you'll need to georeference it.
Choosing a map projection
CAD data is drawn flat using a localized coordinate system. You can think of the x-axis as an easting direction and the y-axis as a northing direction, but they do not necessarily translate to grid directions in your spatial data. Typically, features are measured relative to other features at local ground elevations. Although it is possible to create CAD data that corresponds to the x,y coordinates of a projected grid zone, most CAD data is authored using surface distances.
When you define a spatial reference, choose a coordinate system that contains a projection that is likely to produce the least ground-to-grid scaling discrepancies.
Projection files
Projection files serve as the primary method for defining a CAD dataset's spatial reference. ArcGIS Desktop uses the file name and its location to associate it to a particular CAD drawing. Using the Spatial Reference Properties dialog box correctly names and saves the file automatically.
To link a projection file to a CAD dataset, the following criteria must be met:
- The projection file and the CAD drawing must have the same name (prefix). For example, MyDrawing.dwg and MyDrawing.prj.
- The projection file must exist in the same folder as the CAD drawing.
When these two conditions are met, the spatial reference travels with the CAD drawing in a Catalog window as a hidden file. If you copy, delete, or rename the CAD dataset, its projection file is also modified by these operations and maintained as a property of the dataset.
Universal projection files
A universal projection file defines a spatial reference for all CAD datasets that are stored in the same folder and do not already have a projection file defined for it. You can use the Save As command in the Spatial Reference Properties dialog box to create them.
To define a universal projection file, the following criteria must be met:
- The projection file must be named esri_cad.prj.
- The projection file must exist in the same folder as the CAD drawing.
Spatial references embedded in DWG files
A spatial reference embedded in a DWG file provides a portable alternative to an auxiliary projection file. The information is identical to a projection file—the only difference is that it is stored as metadata in the AutoCAD drawing.
The following tools in ArcGIS Desktop embed the spatial reference in DWG files (version 2007 and higher):
- The Spatial Reference Properties dialog box
- The Export To CAD tool
CAD professionals can, in turn, use this spatial reference in an AutoCAD-based application with the Esri plug-in ArcGIS for AutoCAD.