The dialogue box should now look something like the one shown above. When the Select Image File dialogue box appears, navigate to your work folder and select the Map Image.jpg file. Next, start the Attach Image command from the Reference toolbar. Use the Layers command to create an new layer called "Map Image" and set it to be current. Start AutoCAD if you have not already done so and create a new drawing. However, this is a special case and you may not always know the true width of an image. We could, therefore, work out that in order to correctly scale this sample image in metres, all we need to do is set the scale factor to 3000, the width of the image being 3000 metres. AutoCAD always inserts images in such a way that their width is one drawing unit in length when the scale is set to 1. Using this information, you could easily work out the scale factor to display the raster map with drawing units set to metres. The physical size of the image is 600 X 600 pixels. The sample image is a 3km X 3km extract from an Ordnance Survey 1:50,000 scale map.
To download the file, right-click on the document icon below and select "Save Target As…" from the menu ("Save Link As…" in Netscape Navigator). If you would like to follow this exercise step by step, you may like to download the map image, shown on the right. If you have experience of working with images in AutoCAD but you need a reminder of some of the finer points, you can use the QuickFind toolbar (above) at any time to get information about a particular command. If you have little or no experience of working with images in AutoCAD and you would like to learn more, it is recommended that you work through the All About Images tutorial and then have a go at the corresponding Using Images exercise. The length of a wall, the width of a road or the distance between grid lines can all be used to scale a raster image providing you can accurately pick each end of the measured length. The only piece of information that you need is a known length that is easily identifiable on the plan. It works even if you do not know the image resolution. The technique described below can be used to scale any raster image in such a way that units on the scanned map or plan correspond to AutoCAD drawing units. This is more likely to be a special case rather than a general rule. AutoCAD can help to scale some images if the file format contains information about the image resolution.
However, the only way to create an accurate drawing from a raster image is to accurately scale it. In the past, this would have been done using a digitising tablet and although AutoCAD still supports the use of tablets, the new image support options have, in many respects, made them redundant. For example, you may want to use a map base to draw the line of a road or to draw some contours.
From time to time, you may need to use a raster image to trace off information.