Therion Protractor

Cave surveying tool

Stacho Mudrak, Martin Budaj

July 20, 2000

This page is dedicated to cave surveyors, who:

Solution is very simple: Therion protractor. Put it once on your sketch and draw next shot in correct bearing with length reduced to horizontal plane in your prefered scale.

How does it work

Example: suppose you have measured following shot:

  1. put the protractor origin (point 0) on the last drawn station and rotate it to required bearing (like with ordinary protractor)
  2. find unreduced lengt from 0 (i.e. point A)
  3. imagine an arc with radius 10.5 m (starting at A) -- in the picture marked red
  4. imagine a segment with slope 58 grades from origin -- marked red
  5. find intersection (point B) of arc and segment
  6. using parallel vertical lines find reduced length (point C)
Reduced length is approximately 6.4 m
(exact formula gives 10.5*cos(58) = 6.436)

Short FAQ:

With a bit of practice about 0.5 mm / survey shot, as good as other commonly used methods. "Absolute" precision is not the purpose; precise maps (with closed loops) are generated by computer.
Is it not too slow?
It's much more faster then computation or graphical reduction
I need different scale or length/angle units.
Some standard combinations are in downloadable pdf file. In the future we will add customizable MetaPost source code (in terrible state, not commented yet).
How to make it?
Print it on transparency and laminate it.
Is it free?
How to get it?
Download here.

Updated: November 16, 2006