NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Instrument Corrections
From: Fred Hebard
Date: 2003 Mar 9, 17:38 -0500
From: Fred Hebard
Date: 2003 Mar 9, 17:38 -0500
Bruce, Thank you. I have been researching my own question and came to the same conclusion. The relevant passage from the 1962 edition of Bowditch was: Usually, it is preferable to make a single correction for all three errors, called INSTRUMENT ERROR. Customarily, such a table is determined by the manufacturer and attached to the inside cover of the sextant case. The sign of the error is reversed, so that the values given are for INSTRUMENT CORRECTION (I). (The three errors are prismatic error, graduation errors and centering error). Elsewhere Bowditch makes the distinction between index error and index correction, where you reverse the sign of the error to yield the correction. I did not find any passages in the 2002 edition of Bowditch that contradicted the above, but also found little supporting it. My other two nav books, Turner and Toghill, also are relatively silent on the subject. The offending passage from Bauer was: So the instrument error taken from the certificate in the sextant box top is applied with reversed sign to sextant altitude. This clearly is incorrect. All the certificates illustrated by Bauer indicate "correction" not "error," although only the old Husun certificate indicates error existed in its instrument. Making this change sure has straightened out my data! I've been close to pulling my hair out for several months trying to get my accuracy close to my precision, even contemplating purchasing another sextant to try to narrow things down. Now I feel confident enough to try the instrument out on the water (but still close to coast)! It would help if we lived closer than 350 miles to salt water. By the way; John Luykx at Navtrak Nautical indicated that it might take about 500 observations to calibrate my sextant; I probably still need to get my standard deviation consistently under 0.2' before starting. Fred