This page is dedicated to cave surveyors, who:

• are not satisfied with the precision of free-hand sketches of a polygon drawn directly in the cave
• have not enough time to compute reduced shot length (or survey stations coordinates) by calculator/computer in the cave or reduce this length graphically in the margin of the sketch sheet

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:

• length: 10.5 m
• bearing: 20 grades
• inclination: 58 grades

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)

Short FAQ:

Precision?

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?

Yes.

How to get it?

Download here.