Piloting is the navigation process required to navigate a vessel through
restricted waters using proximate landmarks and land-bound fixpoints as direction reference.
Frequent determination of the vessel's position relative to the available
geographic and hydrographic features must be performed in order to verify
the vessel is on the planned sailing track keeping a safe distance to potential hazards.
Mariners use position-fixing navigation, to obtain a "position fix" or "fix"
by measuring the bearing of the navigator's current position from known points of reference.
A visual fix of position can be made by using any sighting device with a bearing indicator
to obtain position lines from the navigator's current position to each point of reference.
Two or more objects of known position are sighted as points of reference, and the bearings
Bearing lines or transits are then plotted on a chart through the locations of the sighted items.
The intersection of these lines is then the current position of the navigator.
Usually, a fix is where two or more position lines intersect at any given time.
If three position lines can be obtained, the resulting "cocked hat", where the 3 lines do not
intersect at the same point, but create a triangle where the vessel is inside,
gives the navigator an indication of the accuracy in the three separate position lines.
If two geographic features are visually aligned (the edge of an island aligned with the edge
of an island behind, a flag pole and a building, etc.), the extension of the line joining
the features is called a "transit". A transit is not affected by compass accuracy,
and is in fact often used to check a compass for errors.
The most accurate fixes occur when the position lines are at right angles to each other.
The line connecting fixes is the track over the ground or sea bottom.
The navigator compares the ground track with the navigational course for that leg
of the intended route, in order to make a correction in "heading", the direction in which
the craft is pointed to maintain its course in compensation for cross-currents of wind
or water that may carry the craft off course.
Where a channel is narrow, as in some harbour entrances and on some rivers,
a system of beacons allows mariners to align pairs of daymarks, called "range markers",
to form a "leading line", along which to navigate safely.
When lighted, these markers are called "leading lights" or "range lights".
The relative positions of the marks and the vessel affect the accuracy of perceiving
the leading line.
Instruments used for piloting:
On shipboard, navigators may use a pelorus to obtain bearings, relative to the vessel,
from charted objects.
A hand bearing compass provides magnetic bearings.