A Simplified Guide to Small Marine Craft Navigation.

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Chapter 25 (v.1) - Chart Scales, Graduations and the Measurement.

Submitted: May 18, 2017

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Submitted: May 18, 2017

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Chart Scales, Graduations and the Measurement.

 

 

The distance on the planet Earth's surface between any two Meridians is greatest at the Equator and decreases uniformly until it is zero at the poles, where all the Meridians meet.

For instance, the distance on planet Earth's surface representing 30° of Longitude at lat. 60°N. is 902 miles, whereas 30° of Longitude at the Equator is represented by 1,800 miles. The linear distance of a degree of Longitude on planet Earth's surface therefore varies with the Latitude and cannot be taken as a standard measure of length.

However, the distance on the surface of planet Earth representing a degree of Latitude is for practical purposes the same no matter at what part of the planet Earth it is measured. For instance, the actual distance between 0° and 20°N., is the same as that between 20°N., and 40°N., or between 40°N., and 60°N.

The standard international unit for measuring distance at sea is the Nautical Mile, which may be defined as the distance, measured along a Meridian between two places whose Latitude differs by one minute, 1/6oth part of a degree.  The linear value of a nautical mile is 1852 meters (6076.1 feet).

Because one nautical mile equals one minute of Latitude there is no need for a distance scale on a chart, although some very large-scale harbor plans do have a separate distance scale. However, distance can be measured using the Latitude scale at either side of the chart.

Since a budding navigator will be measuring distances frequently on charts, this must be done carefully and accurately and they should note the different ways in which Latitude scales can be divided on charts.

On large-scale charts or harbor plans, each single division represents 6 seconds, 6”, of Latitude or 0.1 mile, as fractions are not used in navigation so do not refer to ½’ of Latitude;  so always quote the angular measurement 30”; similarly do not refer to ½ mile - always use the decimal measurement 0.5 mile.

A chart representing a small area is termed a large-scale chart and one that covers a large area is termed a small-scale chart. These terms, large-scale and small-scale, refer to the ratio between the size of details shown on the chart and their actual size.

The natural scale of a chart is quoted under its title, and if it is shown as -1 over 20000 this means that the details on the chart have been reduced 20,000 times, or that one-inch on the chart represents 20,000 inches on the planet Earth's surface.

On a small-scale chart the scale might be shown as l:l,000,000, where the details would have been reduced a million times. For ocean and off-shore coastal navigation, small-scale charts are used, but when approaching land larger-scale coastal charts are used; and finally, when entering port, the largest-scale charts, called plans, are used. A prudent navigator always uses the largest-scale chart available.

The important thing to remember about the peculiarity of the Mercator Projection mentioned in the preceding paragraph, is that although one minute, l', of Latitude always represents one nautical mile of distance.

The length of 1' of Latitude on a Mercator Chart increases progressively from the Equator to the Poles and therefore from the bottom to the top of any chart in the northern hemisphere. This is not very apparent on a large-scale chart, but is quite noticeable on a small-scale chart.

The Parallels of Latitude are spaced at increasing intervals towards the Pole on a Mercator Chart. Thus, one can say that on a Mercator Chart the Latitude scale measures correct distances only for places in that Latitude.

 


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