You correlate an image by positioning it within the AutoCAD coordinate system with correct scale, rotation, and coordinates. You can specify correlation details for an image at the time of insertion or later. You can also use options such as Scale, Displace, and Rubbersheet to correct distortions in the image.
One way to correlate an image on insertion is to use an existing correlation source. Depending on the type of image you are inserting and whether or not a resource file or a world file exists for it, you may see the following sources listed:
- Image File: For certain types of images, correlation data can be saved as part of an image file. These file types include RLC, IG4, IGS, GeoTags in GeoTIFF, or HDR File in SPOT.
- Resource File: Created by the AutoCAD Raster Design toolset Export command or previous versions of AutoCAD Raster Design toolset. Resource files have a .res file extension.
- World File: Created by the AutoCAD Raster Design toolset Export command for a variety of image formats. World file extensions vary according to file type, but all end in the letter w. Examples: BMP - .bpw, TIFF - .tfw.
- Image Defaults: The values you set in the AutoCAD Raster Design Options dialog box.
The information displayed on the Pick Correlation Source dialog box reflects the values stored in the correlation source you have chosen.
With AutoCAD Raster Design toolset, you can insert several images at the same time. When you select more than one image to insert, the default correlation information is used to automatically position the images. This option is very useful when you have a group of images, such as tiled quadrant sheets, that have correlation data stored with the images.