Edited by Rachel Duncan 4th Edition ISBN 0-907649-91-2 London ...

6.2 Compasses
Navigation 59
There are numerous compasses available. What you are looking for is a magnetic needle
on a pivot housed so that it is easy to read and interpret. With even the cheapest compass,
the needle will point magnetic north/south. Your accuracy of travel will rarely be better
than a few degrees. Look for a simple system, such as you would use in your home
country. A Silva Type 7NL, Suunto A1000, or Recta Type 110 are all inexpensive. Most
contain a liquid to dampen the needle to prevent it oscillating.
It is important to contact the manufacturers (or a reliable supplier) and state where
you are wishing to travel: the Arctic or Antarctic. Compasses are affected by ‘dip’ with the
needle being pulled down at the tip, and compensation must be made for specific areas of
the world. Silva provides compasses for five areas of the world.
If you wish to carry out sighting or accurate back-bearings of features, you may
have to purchase more expensive compasses with mirrors, cross wires or similar additions
for fine readings.
When near the magnetic poles—the North Magnetic Pole is north west of the
islands West of Ellesmere Island and the South Magnetic Pole is in the sea just off the
coast of Antarctica opposite Tasmania—compasses become unreliable and sluggish. In the
past some form of sun compass would be used.
A sun compass can be anything from a calibrated watch dial to a sophisticatedly
manufactured sun compasses such as those used in aircraft, or the traveller’s own shadow.
A chart can easily be constructed to determine the direction of your shadow relative to
your position for any time of the day. E.g. In the Arctic, at midday local time your shadow
will be pointing due geographic north. At 6pm local time, it will point due west.
The key to solar navigation is to obtain your longitude from a GPS then to adjust a
watch from GMT to the exact local time specific to your longitude, i.e. navigation time.
The Earth’s 360° rotation in 24 hours equates to one degree every 4 minutes. For example,
if your Longitude is 90°West, 90°x 4 = 360 minutes = 6 hours so move the time back six
hours from GMT to give navigation time. If your longitude is 170° West, navigation time
is GMT minus 11 hours and twenty minutes. On navigation time 0600, 1200 and 1800 hrs
cast shadows from due East, South and West respectively. A second watch is useful for
using this method.
Navigation near the magnetic poles in fog may be hampered or stopped until
landmarks or the sun reappears. Further from these poles, even if the compass is sluggish,
mist and whiteout should not be a worry unless you are near to objective dangers such as
open water, crevasses or ice cliffs.
Magnetic variation (the difference between where the compass needle points and
the true geographical north) in the polar regions can be as much as 180°. Beware of this
and check your charts. If you do not know the variation, you can make an approximation,
if you know your local time (sun time but not watch time). At midday local your shadow
should be (Arctic) due north. Measure the magnetic bearing and compare it to true north.
6.3 Altimeters
A reliable altimeter is essential for glacier and ice-cap travel where landmarks are minimal.
Only consider purchasing a model that can read accurately to 10 or 20m. The combination