Measuring procedure
To measure the length of a segment, click the first point that defines the distance you want to measure, move your mouse to the endpoint and click on it.
In image drawings, the button ‘Polar track
lock’ 
is
activated by default, in order to foster the line drawing during the measurement
procedure.
Notice that
during the track of the pick, a dashed line appears in the continuation of the
straight part of the line which highlights the «locked» angle-position.
Notice also
that the current length of the respective drawing part is displayed in a
rectangle near the pick, in order to be aware of the length at any time and
facilitate the measurement procedure.
If you wish to measure curved lines, click first then right-click on the point where the curve starts; from the shortcut menu that appears, select the ‘Drawing arc’ option, which can be enabled or disabled during a measurement. Alternatively, you can use the F10 key.
Measuring Discontinuous Segments
Undoing a measurement (Ctrl+Z)