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HSH Yacht Articles

Navigational charts

By

Jeremy R. Hood

All charts (as maps relating to marine navigation are more properly called) are attempts to represent the curved surface of the earth on a flat sheet of paper. Different methods of achieving this are called different projections and some of these still used today were first invented by the ancient Greeks such as the Gnomonic projection. However the projection used for the majority of navigational charts was not invented until 1569 by the Flemish geographer Gerardus Mercator.

Each type of projection has its advantages and disadvantages and so tends to be used for a particular purpose. Gnononic charts are usually used for planning ocean passages because a great circle route (the shortest distance on the surface of the earth) appears as a straight line. Other charts used for navigation include the Conic projection which is principally employed for sailing near the poles, and a derivative of this the Lambert projection which, with a small error, allows great circles to be plotted as straight lines and because of this it is often used to plot radio bearings as radio waves follow great circle routes.

Mercator charts are almost universally used for navigation as they have two special features. Firstly, bearings (directions) plotted on the chart are true bearings and can be used directly after allowing for compass errors (these will be covered in a later article). Secondly, distances are true distances and can be measured directly so long as certain rules are followed.

Though in reality lines of longitude start at the North pole and travel along the earths surface to the south pole, on a Mercator chart (Fig 1) they are drawn exactly parallel to each other and equally spaced. At the equator this is fine but as the area of the chart moves further North or South a distortion occurs. To compensate for this the lines of latitude are spaced further apart as they move away from the equator. The actual spacing of these lines is crucial to the accuracy of the chart and it is calculated mathematically using a table of meridional parts. The chart of Galveston bay (11326) is a typical Mercator projection chart as indicated on the title page (Fig 2).

For all these projections, allowance has to be made for ellipticity as the earth is not an exact sphere, and there a number of standard ways of achieving this mathematically. The latest edition of the Galveston Bay chart tells us that it conforms to the North American Datum of 1993 which is for practical purposes equivalent to the World Geodetic System 1984. Readers familiar with electronic navigation devices such as Loran or GPS will know that so long as the chart and the electronic navigation unit use the same system then positions will be equivalent on both. The WGS 1984 system is fast becoming the standard used for both navigation electronics and newly issued charts.

The title page of a chart also gives other important navigational information. For instance the Galveston bay chart tells us that the soundings (depths) are shown in feet. Other charts may use fathoms (1 fathom = 6 feet), fathoms and feet which may be shown as, for example, 104 meaning 10 fathoms and 4 feet, or meters. Many charts are now being converted to comply with international agreements and so meters will be seen more and more frequently even on U. S. charts.

On the Galveston chart the soundings (in feet) are at mean lower low water. This is called the chart datum. Mean lower low water (MLLW) is an average of the low water levels taken during a 19 year period. Rarely will the depths be less than MLLW and so for practical purposes the soundings will be minimum depths. The chart also shows us that heights are in feet above mean high water. Thus any heights, such as the clearance height for the Kemah (Hwy 146) bridge of 73 feet, will be minimum heights. Rarely will there be less clearance than that shown on the chart.

To understand fully all the features and symbols of nautical charts you will need Chart No. 1 which is published jointly by NOAA ( whose charts cover the coastal and inland waters of the U.S.) and the DMA (who producecharts which cover offshore areas and foreign countries). At a price of $2.50 this must be one of the best value official publications available and it is really essential for navigation. In twenty-four sections, Chart No. 1 covers such items as Rocks, wrecks and obstructions, Lights, Landmarks and a whole lot more. If you cannot identify the symbols shown in Fig 3 go out and get yourself a copy!


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