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.
Some image formats, such as GeoTIFF, GeoSPOT, and any image with a World file for correlation source, have density units that are in real-world coordinates. For example, a satellite photo can be based on dots-per-mile. In this case, select Mile as the density unit.
The density unit that you specify can vary depending on what type of image you are inserting. The following two examples explain the difference between inserting an image that was scanned using dots per inch, and an aerial photo/satellite photo that was saved in real-world units.
The following example shows the relationship between paper scale, density units, and insertion scale for an image that is not georeferenced (an image that was not saved with real-world unit data). Any image that is drawn using a scale (such as a floor plan) and captured with a scanner will likely fall into this category.
The image in the following example was drawn at a paper scale of 1" = 48", scanned at 300 dots per inch, then inserted into a drawing using 300 as the density value and Inches as the density unit. Then it was scaled based on its paper scale of 1" = 48".
The density units that you select when you insert an image that is not georeferenced should match the units at which the image was scanned. In this case, the units are inches because the image was scanned in dots per inch.
The following example shows the relationship between density units and a georeferenced image. Because there is no paper scale to consider, you can insert the image using the units that the image represents, such as feet or miles, and you do not have to scale the image when you insert it.