We have had several questions on the use of these lately, so here is a quick answer. It takes two of them to do the job. They look like this:
Problem: Draw a line through Point A in direction 057 T.
1. Choose any point B on the nearest meridian line to point A, and
orient triangle 1 (either one of them, they are the same) so that it
points in direction 057 T. This will have the center point of the
triangle on the meridian and then you rotate the triangle so the
meridian crosses the 057 line on the protractor printed on the triangle.
Double check by just looking at its orientation that it is about
right... ie that you did not just line up some numbers without thinking
of what the orientation was supposed to be. There are several scales on
the triangle and you may have grabbed the wrong one if you do not look
at the edge crossing the meridian to be sure it makes sense. You can do
this alignment very precisely, relatively easily within ± 0.5°.
you are always using the degree marks on the outside edge of the
triangle shown above. The inside scales are just there to show you the
different orientations and subsequent meanings of the outside scales.
Step 2. Holding triangle 1 firmly in place, align triangle 2 under it so you can slide 1 over to point A as shown below.
3. Slide 1 over to Point A and then draw in your line. If you have any
doubt that you slipped some, then slide 1 back to the meridian to check
the bearing of a line on the chart is just the opposite. Align one
triangle with the bearing line, then put another under it to slide it to
the nearest meridian to read the bearing.
Hope that helps for now. We have triangles for sale in our online Accessories Catalog. They cost $14.95 each.