To create and edit measurement Rulers & Protractors:
- In the Tool Shelf > Grease Pencil,
Press Ruler/Protractor to activate Ruler/Protractor mode.
- LMB Click & Drag the 3D Viewport to create a measuring Ruler.
* Hold Ctrl while creating the Ruler to snap its start and end points to 3D elements.
* If a one or more measuring Rulers already exist, Ctrl must be held anyway to create a new Ruler.
- LMB Click & Drag along an existing Ruler line to turn it into a Protractor for angle measurement.
* Hold Ctrl while creating the Protractor to snap its apex to 3D elements.
- LMB Click & Drag a point in an existing Ruler or Protractor to change its location.
- LMB Click an existing Ruler or Protractor and Press Delete to delete it.
- When finished creating measurement Rulers & Protractors,
Press Enter to save them for later use of the Ruler/Protractor mode,
Or Press Escape to discard them (without discarding Rulers / Protractors that were previously saved).
To Delete all Ruler / Protractor data:
In the Properties Panel > Grease Pencil Layers,
Delete the RulerData3D Layer.