Inserting text notes and dimensions	Notes, labels and images
How to calibrate the background image
Concepts     Procedure     Commands

Using the Format menu Background Image  Calibrate background Image    it is possible  to calibrate the background image. 

1. The user is required to specify two points defining a known length (calibration segment).
2. Enter the length of the calibration segment (now displayed by a red line) using the proposed unit of measurement. Press ENTER.
   The raster image is now calibrated.
   

  Menu :  Format Background Image  Calibrate background Image

Also see:

How to insert a background image in the drawing

How to change the line color of the background image

How to remove the background image