A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ken Muldrew
Date: 2016 May 3, 18:38 -0700
Using calculated altitudes was the standard method for land-based lunars ca. 1800. The aforementioned David Thompson always used calculated altitudes as did his instructor, Philip Turnor, who himself was instructed by William Wales and was also one of Maskelyne's calculators (i.e. this wasn't just a matter of taste among fur traders but was the preferred method of the lunar cogniscenti). The method did not, however, render the use of the artificial horizon superfluous as one would have to take a time sight at some point (within the accuracy of the watch that was being used to carry the time). The advantage of taking a time sight rather than a co-incident altitude was that one could wait for the sun, or a star, to be rising or setting (if necessary).
Before going West the two explorers were given a quick cel-nav course by Robert Patterson and Andrew Ellicott. Patterson had developed a method of Lunars where you do not need to measure the altitude of the moon and the star or sun. His method #4 calculated the altitudes from an asumed lat & long. He thought this was easier for land explorers because they would not have to set up the mecury artifical horizon when doing a luner.