Thursday, September 18, 2008

Lesson 4: Mercator Charts

No chart or map of the earth is accurate. As mentioned above, the very attempt to portray a portion of a sphere (such as the earth) on a flat surface is difficult at best. All charts attempt to be correct in some detail or at some points but are in error at others. The chart most used for coastal and inland marine navigation is the Mercator chart. Mercator's projection has some evident distortions as to shape, area and scale but it has one feature mariners prize well above all others - a line of bearing on the chart is equal to the compass course between two points. Most charts are "projections" in that they are supposed to represent on a flat plane a projection of a point on the surface of the earth.

Charts attempt to emphasize one of the following qualities:

- Preservation of area
- Preservation of shape
- Preservation of scale
- Preservation of bearing

To achieve any one of these qualities the rest must be, to some degree, sacrificed.

In the Mercator projection, preservation of bearing is all important and scale, shape and area are progressively distorted as the latitude increases. Therefore, charts using Mercator projections, all Great Circles are arcs with the exception of the equator and lines of longitude.

Mercator projections are "cylindrical" projections insofar as they are derived by imagining a large sheet of paper wrapped about the earth in the form of a cylinder. For this example we will imagine this sheet touching the earth only at the equator. Then, from the center of the earth, we will draw lines through features on the surface of the globe and continue those lines until they strike the paper cylinder.

It is easy to see that the poles can never be represented on such a chart and that the distortions inherent at higher latitudes make the area and shapes of such features as Greenland wholly untenable. Mercator's projection is not as simple as described above but a mathematically developed projection so as to preserve a specific ratio between the latitude and longitude.
As mentioned previously the Mercator chart allows a loxodrome - or line of constant bearing - to be drawn on it as a straight line between two points. Such a line cuts every line of longitude at the same angle.

Another nautical chart used by mariners is the Gnomonic projection. This chart allows Great Circles to be drawn as straight lines. It is even more complicated where bearings must be calculated for every position of the chart as no compass rose can be shown on such a chart - except for one particular point.

Gnomonic projections, or variants thereof, are frequently used to depict the earth’s polar areas as these areas lend themselves well to such projections.

Need To Know

A nautical chart is a mariner’s most important tool.
Select a chart (projection, scope, and scale) which meets your safe navigational and sailing needs.

Sailing Chart - scales 1:600,000 and smaller, are for use in fixing the vessel’s position as she approaches the coast from the open ocean, or for sailing between distant coastwise ports.
General Chart - scales 1:150,000 to 1:600,000 are for inshore navigation leading to bays and harbors of considerable width and for navigating large inland waterways.
Coast Chart - scales 1:50,000 to 1:150,000 used for inshore coastwise navigation. Shows shoals and reef areas, large inland waterways, and larger bays and harbor entrances.
Harbor Chart - scales larger than 1:50,000 are for harbors, anchorage areas, and the smaller waterways.
Special Chart - various scales, covers the Intra-Coastal Waterway (ICW) and miscellaneous Small Craft (SC) areas.

Charts must be studied and understood before using. NOAA Chart No. 1 is a must.

Vertical printing on charts which refer to features which are dry at high water.

Slanted printing on charts which refer to aids to navigation and features which are submerged at high water.

Chart legends which indicate a conspicuous landmark is printed in CAPITAL LETTERS.

The term RACON besides an illustration on a chart would mean a Radar transponder beacon

The Coast Pilot (published by National Ocean Service - NOS and NOAA) (Sailing Directions in foreign waters) and The Light List (List of Lights in foreign waters) are to be used with charts.
Each publication contains a wealth of important information for both navigators and mariners. Each publication starts with general information such as aids to navigation, landmarks, weather, radiotelephone, navigation regulations, VTS, and emergency procedures. Generally information that cannot be shown on a chart.

Notice to Mariners – The USCG on a weekly basis provides updates and corrections to navigational charts and related publications.

Pilot Charts – Ocean chart presenting historical weather, wind, current (ocean currents, not tidal currents), and sea conditions by month allowing a navigator to quickly determine the shortest and safest route.

Tide and Current Tables – Publications predicting regional and local tide and currents (tide, time, height, direction, and velocity).

Chart Correction System Background Information The Chart/Publication Correction Record Card System is used to conserve nautical charts and publications and to reduce the amount of chart correction work aboard ship. The Notice to Mariners, Local Notice to Mariners, Summary of Corrections, NAVTEX and SafetyNet are considered component parts of the system. A record must be maintained for Notice to Mariners corrections to all charts and publications carried aboard, with actual corrections being made on all charts and publications before they are used for navigational purposes. Never use an uncorrected chart for navigation purposes.

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