Archaeology Underwater: The NAS Guide to Principles and Practice

POSITION-FIXING 89

relationship to potential sources of deviation often
changes each time it is used, making correction factors very
difficult to calculate.
It is important to remember that the bearings taken are
relative to magnetic and not true north. The difference
between the two slowly fluctuates over time and also
varies with geographical location. Charts and maps usu-
ally have the relevant information printed on them and
it is normally possible to calculate the difference between
true north, magnetic north, and the north alignment of

the reference grid of the map or chart being used (if that

does not coincide with lines of longitude running true

north–south between the earth’s poles).

Transits

The visual alignment of two charted features establishes

a line of sight that can be drawn on a chart; a second pair

of aligned features, at approximately 90 degrees to the first

and visible from the same position, will give an excellent

50 °

1

2

50 ° 45 °

45 °

A

B

C

50 °

50 ° 45 °

45 °

A

B

C

Figure 11.6 Sextant angles can be plotted geometrically from baselines between charted features. (Based on original
artwork by Ben Ferrari)