A Simplified Guide to Small Marine Craft Navigation.

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Chapter 22 (v.1) - The Navigational Chart.

Submitted: May 15, 2017

Reads: 144

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Submitted: May 15, 2017

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The Navigational Chart.

 

As previously stated the most important of all navigational instruments is the chart, a map of the sea upon which a navigator plots a position and plans courses. Charts are in essence maps, and are used by a navigator on which is depicted all the essential information necessary to assist the mariner to navigate his craft.

Charts are virtually contour maps of the seabed and its surrounding coastline except that whereas a land map gives the height of the land contours above sea level, a chart gives the depth of the bottom below sea level.

It is said, that nothing on land or sea ever truly remains the same for all time, and so mariners down through the ages have depended upon the experience and reports of those who sailed before them. In this way an ever-increasing amount of information has, and is, accumulated. Such evolving information is constantly being set down in detail on Charts and Sailing Directions as corrections to navigation.

Although there are now no undiscovered lands or seas, and most coasts have, to a greater or lesser degree, been surveyed and mapped, nevertheless the accuracy of modern charts and sailing directions depend as much as ever on reports from mariners at sea, and from others responsible for inshore surveys, lights and other aids to navigation. Without the supply of information from these sources, it would not be possible to keep the charts and publications corrected for new and changed conditions.

Maritime nations publish Navigational Charts by using the services of their Hydrographic Office. However, many of the smaller nations publish only charts of their own coastline and adjacent waters, relying on the charts of the major maritime nations for coverage of the rest of the world.

The Hydrographic Office of the United States, established in 1866 and now termed the Naval Oceanographic Office, prepares and publishes maps, charts, and nautical books required in navigation, as does the British Hydrographic Office of the Admiralty, instituted in 1795 solely for the production of navigational charts.

Today, both of these great offices produce many thousands of different charts covering all the navigable waters of the world, and a great deal of labor and care goes into the production of navigational charts, especially from the Hydrographic surveyors who survey a sea area or coastline that requires charting.

Therefore, a navigational chart in addition to being a navigational instrument is also a scientific instrument and must be regarded and treated just as any other instrument carefully made and precisely calibrated would be, for a sea chart differs greatly from a land map. This difference is because a land map can be useful even when sketched without much regard to scale and relative positions, whereas a sea chart must be extremely exact in every detail.

Normally features on the land are there to be seen and recognized, but on the sea the greater part of the features which are of  vital importance to the navigator are hidden beneath the surface of the water. This means the navigator must have faith in a chart because, unlike the user of a land map, there is no visual means of checking all of the underwater information it gives and on which the safety of his vessel depends.

Resulting from an inability to check every detail, a rigid and minute accuracy is the first essential of every chart produced. For the mariner must be given everything on the chart which is necessary for safe navigation, but this must not be confused by the inclusion of details which are not useful to the seaman and which would needlessly crowd the space available.

Therefore, everything and anything requiring displaying on a chart requires doing so with the utmost clarity, and must be entirely unambiguous. A chart embodies a great deal of the most careful selection and a very elaborate system of symbolic representation in addition to its precise accuracy of construction.

Every navigational chart is prepared with one consideration only in view, and that is to render the maximum possible service to the navigator in the clearest possible manner. To match the responsibility, care and accuracy with which charts are produced the navigator has an equal responsibility to understand how his charts are constructed and how to interpret the wealth of data which they contain and use it to the best advantage.

 

 

 


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