Georeferencing is the process of spatially adjusting a CAD drawing without actually changing the original source data. In ArcGIS Desktop this is performed by registering arbitrary points in the CAD drawing to known geographic coordinates. Once you have georeferenced a CAD dataset, subsequent ArcMap sessions transform the dataset in memory on the fly while the source data remains unchanged on disk.
Limitations of CAD datasets
DGN and DWG formats do not natively support the ArcGIS georeferencing system. To get around this limitation, ArcGIS Desktop extends the properties of a CAD dataset to include georeferencing control points as ancillary information. This information is defined as from-to coordinates and stored in an auxiliary world (.wld) file or saved in the map (.mxd) document.
Georeferencing a CAD dataset is limited to one- and two-point transformations using the similarity transformation method. This adjustment method maintains the aspect ratio of the CAD drawing and prevents skewing to the x- and y- axes. When you georeference a CAD dataset, it is for the purpose of overlaying the CAD drawing with existing spatial data as accurately as possible without skewing the geometry. If a more complex transformation method is required, you can load the data into a geodatabase and use the Spatial Adjustment toolbar.
Control points
Control points define displacement vectors called links and form the basis of georeferencing. A maximum of two links are possible with CAD datasets. A one-point transformation comprises one link and moves the dataset. A two-point transformation comprises two links and moves, rotates, and scales the dataset uniformly.
You can create control points manually with the pointer in a from-to direction, or load them from a world file.
World files
Using a world file to store control points is the best practice for sharing and reusing control points in other map documents. ArcGIS Desktop uses the file name and its location to link the file to a particular CAD dataset.
To link a world file to a CAD dataset, the following criteria must be met:
- The world file and the CAD drawing must have the same name (prefix). For example MyDrawing.dwg and MyDrawing.wld.
- The world file must exist in the same folder as the CAD drawing.
When these two conditions are met, the world file travels with the CAD drawing in a Catalog window as a hidden file. ArcGIS loads the control points into memory when you add the dataset to a map document. If you copy, delete, or rename the CAD dataset, its world file is also modified by these operations and maintained as a property of the dataset.
Universal world files
A universal world file defines control points for all CAD datasets that are stored in the same folder and have not been georeferenced.
To define a universal world file, the following criteria must be met:
- The world file must be named esri_cad.wld.
- The world file must exist in the same folder as the CAD drawing.
Storing control points in a map (.mxd) document
You can also store control points in a map document without using a world file. Control points that are stored in a map document persist in the document until they are overwritten in one of the following ways:
- Loading control points manually from a world file.
- Re-adding the CAD dataset that has since become linked to a world file.