Measuring a Distance
To measure a linear distance in a 2D or a 3D viewer:
Click Measure on the menu bar.
Click 
Distance. Alternatively, press [M].
Click the first point.
Notes
Right-click to cancel.
For a measurement on a 2D image that has not been calibrated, the calibration starts.
Click the second point.
The point-to-point measurement is shown in the viewer and added to the smart panel 
Visibility tab. To edit the distance measurement, drag the control points. The value is updated accordingly.