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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Bowditch Table 15**

**From:**Bill B

**Date:**2005 Jan 24, 16:57 -0500

Jim wrote > To further confuse matters, see the difference in definition for the > entering value height of object in the NGIA online calculator at > http://164.214.12.145/NAV_PUBS/APN/Tables/T-15.pdf. I emailed NGIA to > obtain their underlying formula for that calculator. If I understand, we are discussing the table (15 in my copy) that determines distance from an object by using the vertical angle between its top and the horizon. In this case the base is below the horizon (or, I can see the mast top but not the hull of a sailboat). First, the sextant reading is adjusted for index error and dip, but nothing else. Imagine the dip correction is what may yield negative values for the angle. As a non math-major I'll do my best to represent the formula from Bowditch in ASCII: D = (square root of (tan angle/0.0002419)squared)) + H-h/0.7349 - tan angle/0.0002419 The constants 0.0002419 and 0.7349 account for standard terrestrial refraction. H is height of object above sea level in feet h is height of observer's eye above sea level in feet If I read the explanation correctly, the distance (D) is from the observer to the object, not distance from object to observer's horizon. (Perhaps the dip correction helps out here as well?) At any rate, will let those with appropriate skill sets chew on the formula). Hope this helps. Bill