Celestial Navigation Net - ZetaTalk

How David Thompson Navigated
'common watches' (pocket watches) with second hands.
Today, navigators can compute both latitude and longitude from a pair of
observations. In Thompson's time, due to the lack of accurate watches,
latitude and longitude observations were done separately.
The accuracy of any given observation depends upon what type it was.
Thompson uses three different techniques for observing latitudes. The first is
called a double meridian altitude observation, usually of the sun. This means
that he is observing the height of the sun when it is at high noon (crossing
the meridian), and he is doing it using his parallel glasses, a reflecting artificial
horizon made by placing a glass cover over a bowl of mercury. ('Double'
refers to the fact that the altitude measured with an artificial horizon is twice
what would have been measured using a sea horizon.) Using the same
techniques today, I generally compute latitudes to an accuracy better than
half of a nautical mile. (1 nm = 1852 meters). On good days my accuracy is
within 300 meters. Based on my comparisons with David Thompson's data
from Rocky Mountain House, as well as the data in the case study, I feel that
for any of Thompson's double meridian altitude observations it is reasonable
to assume that his observation would place him within 1½ nautical miles of
his true latitude.
Thompson sometimes mentions a second type of latitude observation which
he calls a meridian altitude. This is an ambiguous term. In most cases, it means
that he made the observation using his artificial horizon, but in some cases it
means that he has used a local body of water as an artificial horizon. This
technique involves using the far shore of a lake or long river as a level, and
estimating the distance to the far shore. The navigator then applies a
correction to the observation based on this estimate. This technique is more
properly known as the dip short method. It is far less reliable than using an
artificial horizon because of the difficulty of accurately estimating the
distance to the far shore. Thompson did not make any dip short observations
during the case study period.
The third method of observing for latitude is called a double altitude
http://www.northwestjournal.ca/dtnav.html (15 of 55) [9/6/2004 12:52:58 PM]