The Measure tool displays the distance and bearing between two points in your image.
To measure, click and drag using Button1 between two points. Each point can be dragged to a new position.
Measure Tool Settings
The Use Info Window checkbox will toggle the sending of information to the Info Window Dialog .
Ctrl is used to constrain the end points to a horizontal line.
Alt is used to constrain the end points to a vertical line.
Ctrl-Alt is used to constrain the endpoints to a 45 degree diagonal line.