I'm in the process of making two backstaffs for a museum in Denmark.
I've posted some photos of my construction of these backstaffs. This entails the use of a few special purpose jigs, and lots of M&T joinery - none at right angles. The first photo shows how one is used.
Backstaffs is a device for measuring angles. But the Backstaff represents a quantum leap in accuracy, user safety, and technology over the Cross-Staff, or Quadrant. It is not only easier to use; the Backstaff saved the eyesight of the navigator because he did not have to look directly at the sun to get a sun sight.
The major technological improvement of the Backstaff was that it allowed sun sights without the user looking directly at the sun, but, rather, a shadow cast from the shadow vane blocking the suns rays. That shadow is lined up with the horizon, so there is no parallax error. Also, the Backstaff could be used at all latitudes. Additionally, the backstaff used the diagonal scale for precise angle measurement.
At noon, the altitude of the sun was taken and this was used to determine your latitude.
All comments and criticisms welcome.