Underway - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 128

not to identify the best sight in our series (impossible without GPS), but to have a method
that will reliably produce a good estimate of the true altitude value. Most importantly, it
will eliminate outliers such as sights 1 and 3. Had we only taken one sight, our LOP
would have been off by 1.8 NM (sight number 1) rather than 0.4 NM and - without know-
ledge of our GPS position - we would have had no way of judging the quality of that
single sight.
The example above also demonstrates that sights number 1 and 3 skew the mean (aver-
age) intercept (as opposed to the median intercept) upwards to 17.5 NM. Scenarios like
this, where the outliers are distributed asymmetrically around the true value, are quite
common with waves rolling the boat and obscuring the horizon. In these cases taking the
median sight will produce better results than using the mean of a series.
The complete fix from our example is shown in Figure 6, with the second LOP derived
from a five-sight series of Arcturus.
Figure 6: The completed fix