Keyboard short-cut: Alt, E, A.
Mouse short-cut: 
Changes the function of the mouse so that it can set the scale of the map. Click the left button down in the Map Window. Hold it down and drag the mouse across the Map Window. When the left button is let up, there is a prompt asking for the distance the line represents. The scale value is used to calculate the watershed area, areas of catchments and lengths of river segments.