Set Scale...
This function allows you to set a scale for the image.
Do not use it to see if the scale is already set. If you do so, you may erase the correct scale. It is better to check the scale using the properties of the image.
If you need to modify, or set the scale, you have to use it indeed. A pop-up window "Set Scale" will appear :
If you did not select a line just before choosing "Set Scale", there will be no default information (even if the scale was already set).
The different parameters asked in this window define the scale :
You provide a "distance in Pixels", and the corresponding "Real Distance" with a certain "Unit of Length". If your pixels are not isotropic, you must specify the "Width/Height Ratio".
Once you have set up those two distances, the software computes the scale, and write the result as the number of pixels/cm (if the unit is chosen to be centimeter...)
For exemple, if the image shows the Sun, you may measure the diameter of the Sun (using a rectiligne selection, and the tool "Measure"). Enter this number (in pixel since no scale was set earlier) in the first line. Then, enter the real diameter of the Sun (here, you need to put an average value, which may need to be accounted for when you discuss errors...) in the second line, and specify the unit (here, it would be km)
An easier way to proceed is to start by a straight line selection, "Measure" it ; and then open the "Set Scale" window. By doing so, the first line is filled with the Lenght of the selection. You only need to fill in the Real Distance (that is obviously unknown to the software !)
You need to click "ok" before doing any other operation on the image.
All those steps are shown in a podcast.
