 The Measure tool displays the distance and
      bearing between two points in your image.
      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 .
Key modifiers:
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.