Charts &
Sailing Directions
The initial planning and the end results
of navigation are plotted on flat surfaces called maps on land and Charts
at sea. The quasi-spherical surface of the earth is represented
on a plane flat surface of paper upon which the desired
features of surface and underwater topography are superimposed with the
areas coordinates of Latitude
(vertical lines) and Longitude
(horizontal lines). Distance in nautical miles is alwAY
measured on the latitude scale (up the side of the chart)
where one minute of latitude is equal to one Nautical
Mile. The vertical lines on the chart all relate to true
north.
Coastal Charts depict the
shorelines significant geographical features, radio beacons, and guidance
features such as railwAY and highwAY on land and show significant navigation
aids, obstructions and indicate water depths along the coastline. Navigation
charts are divided into Small
Scale Charts which cover large areas of the coastline
with much of the detailing omitted. They are compilations of the Large-Scale
Charts which cover relatively small areas (Estuaries
& Harbors) in great detail and are the most useful for navigators
operating in crowded waters.
To a navigator charts accentuate the determination
of position, direction, and distance, and stress points of interes. Every
chart is full of information, most of it given by means of a wide range
of symbols. They are all listed in Chart
5011 (which is actually a booklet).
To keep charts updated with changes in lights, buoys or corrections to
other important features use the weekly Notices
to Mariners or quarterly Small Craft Edition published
by the Admiralty
Most of the navigable waters of the sea,
rivers, harbors, or lake bottoms in the world have been surveyed accurately
by the Hydrographic Services
of the principal maritime nations so that reliable charts of the
waters are usually available to the navigator.To ensure safe
inshore navigation, depths are done by hand soundings and located by observations
to or from control points on shore. Farther out from the shore where less
accuracy of location suffices, sonar soundings with simultaneous Electronic
Location of the sounding vessel permits rapid and exact charting.
Vessels furnished with modern equipment like Loran and satellite-navigation
devices are used for this purpose resulting in very accurate offshore
positioning.
The hydrographic services of various countries
also publish almanacs and sailing directions to assist
the navigator. Sailing Directions
are descriptive books containing detailed information on coastal waters,
harbor facilities, navigation aids, winds, tides, currents, dangers to
navigation, directions for approaching and entering restricted waters,
and other data that cannot be shown on the chart of the area.
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Chart Projection
Because no part of a sphere
can be spread out flat without distortion, several projections
of charts have been developed to accommodate the navigator. Each
projection has its own advantages and limitations and
fulfills a specific need of the navigator.
The most universally used projection
in navigation charts is the Mercator,
named after the Flemish mathematician and geographer Gerardus
Mercator, who devised it. These charts portray the earth's sphere
projected on a cylinder tangent to the surface of the earth at the equator.
When this cylinder is flattened out, the meridians, or longitudes,
appear as equally spaced vertical lines and the parallels of latitude
appear as parallel horizontal lines. The parallels of latitude
are drawn farther apart toward the poles to compensate
for the greater distortion in that direction for the meridians. The Mercator
projection, despite its great distortion, is popular because lines, directions,
and distances appear as straight lines and can be measured directly.
A navigator usually attempts
to find the shortest route between two points and this is accomplished
by following the course known as Great
Circle. The great circle is a theoretical circle,
such as the equator, formed by the intersection of the earth’s surface
and an imaginary plane that passes through the center of the earth and
divides it into two equal parts. Navigators use great circles to find
the shortest distance between any two points on the earth’s surface.
Great-circle courses can be determined directly from great-circle charts,
but because it is impracticable for a yacht to travel in an ever-changing
course, the usual practice is to follow a series of chords that approximate
the great circle. These chords are normally plotted on a Mercator
chart.
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