For some images, the density value and units are already stored in the image file or correlation source. When you use the Insert An Image command to add an image that does not have correlation information, AutoCAD Map 3D uses the settings from the Image Defaults tab in the Raster Extension Options dialog box.
To change the density unit when inserting an image
In the Density box, type a default density for images. This should be the scanned resolution. For example, if the majority of your images were scanned at 300 dots per inch, type 300.
In the Units box, select the default unit for the insertion point and density of images. For example, if the majority of your images were scanned at 300 dots per inch, then select Inch.