A Simplified Guide to Small Marine Craft Navigation.

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Chapter 23 (v.1) - The Measurement of Latitude and Longitude.

Submitted: May 16, 2017

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Submitted: May 16, 2017

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The Measurement of Latitude and Longitude.

 

When dealing with anything concerning navigation, and especially with navigational charts, it is essential to obtain a perfectly clear picture at the outset of what is meant by Latitude and Longitude.

This involves an understanding of the connection between the size and shape of the planet Earth and its representation on the flat surface of a chart.

A sphere is a surface every point on which is equidistant from one and the same point, called the center. The planet Earth is an oblate spheroid, which is a sphere slightly fattened, and in the Earths case that slight flattening is at the North and South Polar Regions.

However, this flattening is so small that for the purpose of practical navigation, the planet Earth is considered as a perfect sphere, and errors arising from this assumption are negligible.

If a sphere is sliced exactly in half, the flat surface of each section, where it intersects with the sphere's surface, always forms a circle. This flat surface, or plane of the circle must pass through the center of the sphere, as it was cut exactly in half, and such a circle is called a great circle.

Therefore, a great circle is defined as being a circle drawn on a sphere whose plane passes through the center of the sphere, and it may be drawn in any direction.

A small circle is a circle drawn on a sphere whose plane does not pass through the center of the sphere and may be drawn in any direction.

For navigation the planet Earth is represented on a small scale as a sphere with the oceans and continents mapped on its surface, and then in order to be able to state simply, and at the same time quite precisely, the position of any place on the planet Earth’s surface a system of lines are added to the sphere.

The lines run at right angles to each other enabling any position on the sphere to be described as so many units in one direction, and so many units in the other direction, from some fixed starting point. Two main reference lines are used for this system of lines, which are the Equator, and the Meridian, which passes through Greenwich, London, England.

The planet Earth, as is well known, rotates on its axis, and the extremities of the axis of rotation are termed as the Poles.

The Equator is a particular great circle on the Earth's axis of rotation, the line joining the Poles. It divides the planet Earth into the Northern and Southern hemispheres, and is the reference line from which latitude is measured.

The Greenwich Meridian is a semi-great circle terminating at the poles, which goes through the site of the original Greenwich Observatory in London, England, and is the main reference line from which longitude is measured. It is sometimes termed as the Prime Meridian.

Parallels of Latitude are all small circles parallel to the Equator, named North or South according to the hemisphere in which they are situated.

Meridians of Longitude are all semi-great circles, terminating at the Poles, and named East or West according to whether they lie East or West of the Greenwich Meridian. The continuation of the Greenwich Meridian beyond the poles on the opposite side of the planet Earth is the Meridian of 180 longitude.

Latitude of a certain place is the arc of a Meridian between the Equator and the Parallel of latitude on which the place lies.

The Latitude of the pole is 90°. Latitude is named as either North or South depending on whether the place described is North or South of the Equator.

Longitude of a certain place is the arc of the equator between the Greenwich Meridian and the Meridian on which the place lies.

All places which are situated between the Greenwich Meridian and the Meridian which lies opposite to the Greenwich Meridian , 180°, are said to lie in East or West Longitude depending on whether they are East or West of the Greenwich Meridian respectively. A place on the Greenwich Meridian has zero, 0°, Longitude.

The Latitude of a place is the angular measurement between the Equator and the Parallel of Latitude through the place. The angle is measured at the center of the planet Earth in the Plane of the Meridian of the place, and is expressed in degrees, minutes and seconds from 0° to 90° North or South of the equator.

The longitude of a place is the angular measurement between the Greenwich Meridian and the Meridian through the place. The angle is measured at the center of the planet Earth in the Plane of the Equator and is expressed in degrees, minutes and seconds from 0° to 180° East or West of the Greenwich Meridian.

Thus, the position of any point on the planet Earth's surface can be defined uniquely in terms of its Latitude and Longitude.

In navigation the word Latitude is frequently abbreviated to “Lat.” and the word Longitude to “Long.” and sometimes the words omitted altogether so that there must be a convention as to which term is expressed first.

Therefore, Latitude is always expressed first and before Longitude, and the suffix N or S for Latitude, and E or W for longitude, must be added to the appropriate term in order to avoid any ambiguity.

Just as the Equator divides the planet Earth into North and South hemispheres, the Greenwich Meridian when continued round the opposite side of the planet Earth completes the Great Circle and divides the sphere into East and West hemispheres.

An observer facing North on the Greenwich Meridian has the Eastern Hemisphere on his right and the Western Hemisphere on his left, and Meridians are named East or West according to the hemispheres in which they lie. Therefore, Longitudes  reach a maximum value on the Meridian which lies 180° from Greenwich. These two Meridians, Greenwich and the 18oth, are not named as East or West, but simply Long.0° and Long.180° respectively.


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