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Originally Posted by mr darcy
I'm using this pattern:-
http://www.sphere.bc.ca/test/build/yokota-loglog.pdf
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Originally Posted by Puster Bill
Here is how it works:
You take observation 1, say, 7300 yards at bearing 220
Then, a few minutes later, observation 2, 6100 yards at bearing 213.
Subtract 213 from 220, giving 7 degrees.
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that makes sense.
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Originally Posted by Puster Bill
Fiddle with the S scale on the slide against the A scale until the distance between 6100 and 7300 on the A scale matches 7 degrees. In this case, it comes to about 52 degrees matching with 6100 (actually, 6.1 on the scale). That is the AOB at the second observation.
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you say the distance between 6100 and 7300, do you mean 1200? (1.2 on the A scale) |
No. You are sliding the 'S' scale so that the number of degrees between 6100 and 7300 on the A scale is about 7 degrees worth on the 'S' scale. In other words, you will align the slide so that 52 degrees on the 'S' scale is directly below the SECOND 6.1 on the A scale, and 7 more degrees up on the 'S' scale (right about 59 degrees, although it won't be *EXACT*, I get about 59.4 or so, close enough for our purposes) should fall right under the second 7.3 on the 'A' scale.
Then, you take the cursor and slide it down to the 7 degree mark on the 'S' scale. You can then read the distance off of the A scale.
In effect, what you are doing is solving for the third side of a triangle: Given the length of two sides of a triangle (in our example, 6100 and 7300 yards), and the angle they form (7 degrees), find the length of the third side.
This time when I did it, I got about 1,463 yards. However, I put a new cursor on my rule over the weekend, and it (or the previous one) might be a little off. Still, it is within a small margin of error (in this case, less than 17 yards off).
You can also do it with the D scale and the S scale. It works the same. But this time it works out so that when you move the slide so that there are 7 degrees between 6.1 and 7.3 on the D scale, 30 degrees on the S scale is above 6.1 on the D scale, and 37 degrees on the S scale is above 7.3 on the D scale. Then, you slide the cursor down to 7 degrees on the S scale, and read the distance travelled on the D scale. Checking mine, I get 1.485, or 1,485 yards. Using the D scale is more accurate, but in my case maybe not quite, as I said I may have a misaligned cursor. In any case, the ship itself is larger than the 22 yard difference (I hope!).
By the way, if the angle change had been *REALLY* small (less than 6 degrees), you would have to use the 'ST' scale. You would also use it for really large (over 80 degrees) angles, but if that is the case the target is moving *VERY* fast, or you just weren't paying attention for a long time.
It might be easier to manufacture one of the circular slide rules at the site. All you need to do is print out the base on regular paper (I use cardstock myself), then use inkjet transparency for the cursor/scale overlay. Pin them together using a thumbtack to a piece of cardboard or a piece of wood. I'm planning on doing that when I can get some transparencies. It would also be more accurate, because the scale length on an 8 inch circular slide rule is just over 25 inches, compared to 10 inches at best on the rule we are both using. That means greater accuracy.
If you get serious about it, check some local antique shops for a real slide rule. You can find them cheap quite often. You could also check EBay. They have *TONS* of slide rules for sale, some of them brand new. Just make sure that one you buy has at least A, B, S, C, and D scales on it (anything with 'Trig' in the name will have it, the 'S' scale is the sine function). A circular rule would only need the C, D, and S scales (The A/B scales essentially duplicate the C/D scales, but they are doubled. This is necessary on linear slide rules, but not on circular).
I hope that clears it up for you.