A Simplified Guide to Small Marine Craft Navigation.

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Chapter 26 (v.1) - A General Description of Charts.

Submitted: May 22, 2017

Reads: 194

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Submitted: May 22, 2017

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A General Description of Charts.

 

Charts are printed on quality tough paper, but although this makes them durable, nevertheless they should be handled with care, avoiding folding other than in the original folds and keeping them as dry as possible. However, some waterproof charts are available

On standard printed charts, dampness will cause distortion, but any suspected distortion can be checked with an accurate ruler, comparing the measured dimensions of the chart with the actual plate dimensions printed outside the border in the lower right-hand corner of the chart expressed in either millimeters or inches. The dimensions are those of the inner rectangle of the chart excluding the chart border.

The Title of a chart is displayed at some convenient position where it does not obscure any details of navigational significance. It defines the general sea area and geographical limits of the chart, and usually contains important information which should be studied carefully before using the chart.

This information may include details of the survey on which the chart is based. Such as, the level below which depths and above which heights are expressed, the units of depth sounding used on the chart, either in fathoms, feet, or meters, the Natural Scale of the chart, notes on Tides and Currents, the dates of the Magnetic Variation Curves, and if any, Cautionary Notes about prohibited or dangerous areas. In addition, warnings of various descriptions, and either a paragraph headed as Authorities, or an insert headed Source Data, which tells the user the dates of survey and hence the probable reliability of the information on various parts of the chart.

The Margin of a chart contains additional useful information. In the top left-hand and bottom right-hand corner can be found the charts Catalogue Number, and the date of the original engraving, the month and year when the printing plates were made, in which case you might see A8-85 printed in both black and magenta. This means that the aluminum printing plate for each color was prepared in August 1985.

On the back of the chart, in a position where it will be easily seen, when the folded chart is in the drawer or folio, is the chart number and title together with the month and year of printing.

Every chart has what is termed a compass rose printed on it, and most charts have two or more. Their purpose is to enable courses and bearings to be measured and or plotted on the chart with the aid of a parallel or roller rule. Most coastal charts and harbor plans have two concentric rings of graduations on a compass rose, in which case the outer ring represents the True Compass and the inner ring represents the Magnetic Compass.

The 0°-180° line of the true outer rose is always on the Meridian of the center of the compass, in example it points due North and South between the True Poles of the planet Earth. However, the ordinary Magnetic Compass, does not seek this True North and South direction, instead it seeks the Magnetic Poles which are some considerable distance from the Geographical Poles.


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