NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Measuring (and Calculating) Dip
From: Paul Hirose
Date: 2013 Mar 11, 20:21 -0700
From: Paul Hirose
Date: 2013 Mar 11, 20:21 -0700
Bruce J. Pennino wrote: > > I honestly don't remember how my internal optical angle scale functions in the "reversed" mode ie, whether the sign of the angles will be greater or less than the previous mean value. Vertical circle graduations have so many variants, the Coast & Geodetic Survey used to require the circle be sketched on the observation log sheet so the staff in Washington could properly reduce the numbers. The Wild T3 with its "degrees" which are actually 2° is the most unusual I've seen. A common scheme is 0° to 360°, with 0° at the zenith. A near horizontal shot will read about 90° face left, 270° face right. Subtract the little number from the big one, subtract 180°, then divide by 2 to get elevation above the horizontal. The amount by which face left and right fail to sum to 360° is twice the index error, which should remain constant regardless of the angle observed. This can be used to detect blunders when reducing your data. If your instrument has extra hairlines for stadia measurements, you can take a reading with each hairline, reverse face, and repeat the process. The spacing of the hairlines is immaterial since the top hairline face left is on the bottom face right. In effect, you have a big index error, which is eliminated by combining readings on both faces. The Wild company recommends that method over multiple measurements with the same hairline. But it is a little more complicated, since the observing and recording routine must pair up the readings correctly (though I'm sure a goof is not hard to detect and fix). --