Measure Scale. This option allows you to set the image's resolution through the scaling process. In other words, it sets the number of dot/inch or dots/centimeter in the image. This is useful for standardizing your images to a consistent scale for merging and, ultimately tracing.
Before you can use the tool to measure the scale of your image, the image must be marked in some way to identify an inch or a centimeter. This can be a set of marks on the paper or grid lines, but it must be visible in the image. To be clear, this is Paper-Scale; we are defining the number of pixel per unit of paper, not the Cave-Scale, which we will handle later.)
When the Measure-Scale option is enabled, you can draw a line on the image by clicking and dragging the mouse. The program will measure the length of the line in pixels, If you draw the line so it is an inch long on paper, the resulting measurement will tell you the number of pixels per inch.
For example, in the image to the right, horizontal grid lines are spaced 1/8 inch apart. The red line covers 8 grid lines, so it is one inch long on paper. The line measures 300 pixels, so the image was scanned at 300 pixels per inch.