Sketching question, inclination

[jharman2]

You dont really need this chart- use the +2/-2 rule-
For example- your forward sight is 132---> +2 to the first number, -2 to the second number, third remains the same--312.
It reverses if you are in the 200s or 300s, ie front sight 313--->-2 the first number, +2 the second number=133.

Sure the math is simple but the fact remains that you are doing math in the cave. I have a rule that I will do NO math in the cave. That is why I use the Therion protractor. The Therion protractor allows you to GRAPHICALLY check fore and back sights. It also lets you GRAPHICALLY determine the plan projection of a high angle shot. The Therion protractor is cave man simple, accurate and fast. Both charts that driggs has in his book are on the protractor.

[rchrds]

Yea, I find it harder to see the numbers on the protractor than to add 2. I guess we'll agree to disagree.

[driggs]

Yea, I find it harder to see the numbers on the protractor than to add 2.

And they really let you fly aircraft with that vision? The military must have some great tricks if they've taught you to sketch accurately without using a protractor or ruler at all!

I'll use a Therion protractor the day that it's good for the one thing that I need a protractor for... sketching! If someone sold one on thick, solid plastic that I could actually use as a straightedge when sketching, I'd buy it on the spot. Printing them on transparency sheets makes them too thin to sketch with, means that mud takes the print right off of them, and means that when I drop them they're gone. Laminating doesn't help much. Anyone know where I can get a real modified Therion protractor?

[Rick Brinkman]

Actually, I only use a protractor as "backup" now. I use a pda with Auriga installed.

[Pat Kambesis]

The easiest way to compensate for high angle shots on the sketch is to plot the profile sketch first with protractor and scale i.e. plot out the distance and vertical angle for making the profile sketch (see image). On the sketch page at the "TO" station draw a perpendicular line down. At the "FROM" station draw a horizontal line that is perpendicular to the vertical line that you just drew - these lines will intersect and form a right triangle. The "measured distance" (what you measured in the cave) will be the hypotenuse of that triangle. The "true" distance, the one you want for the plan view, will be the base of the triangle. Use your ruler to measure the base of the triangle and then plot that distance in the plan sketch. This is quick, easy and doesn't require any in-cave trig.

Vertical angles that are less than 18 degrees won't cause noticeable foreshortening of the survey line so you don't really need to worry about plotting those in profile first. For angles greater than 18 degrees the foreshoretening will be significant enough to distort the plan view sketch if you don't take that into account.

pk