It is a common conundrum. One gets a legacy drawing on record with the council, and it turns out to be a raster image that has been scanned or photographed, and now the scale is out and/or the aspect ratio distorted. A raster image or a raster PDF has no embedded vector information, and so the software is unable to apportion starting and ending points to the entities that are contained in the image/PDF. While there are tools available to try to convert existing raster images (AutoCAD Raster Tools), the most common method is to trace over the image.
The process of scaling the image is important. In this case, we look at a PDF in Revit LT, although it might just as well be an image or PDF in other packages with the same kind of properties and tools.
Link in the image. Measure the longest X dimension then scale the image so that it has the same distance in the drawing as on the image.
In this case the scale factor is 50 000 / 22 937.6 = 2.1798
Now measure the distance along the Y dimension (if the aspect ratio has been distorted, they will be different). Record the actual distance against the distance indicated by the dimension and calculate the scaling factor that the image scaling factor needs to be multiplied with.
In this case, the aspect ratio is off. We need to scale the y axis by 30 000 / 32 967 = 0.91. Note we need to uncheck the Lock Proportions property and enter a new value of 217.982701 * 0.91 = 198.3644563
Unlock the aspect ratio and multiply the Y scaling factor by the aspect radio scaling factor. The image should now be to scale as is shown below.
If you need help adopting Autodesk products in your practice, please contact Micrographics so we may be of assistance.