Maps have been developed because they are more convenient to work with than globes. However, maps do not give a completely true picture of the surface of the Earth, because it is impossible to flatten.
Map projection is a method of flattening the surface of the Earth so that it can be printed on a flat sheet of paper. Many different versions of projections have been developed, three of which are of importance to the navigator. This projection can be thought of as having been made by wrapping a sheet of paper like a cylinder around a glass globe. A light is placed inside the globe, and the outline of continents, etc., is projected on the sheet of paper.
A. Mercator Projection: developed by Gerhard Kramer, whose Latin name was Geradus Mercator (400 years ago, Gerard the merchant).
This projection distorts the shape of the continents and oceans.
On the map:
- The meridians appear as parallel lines equally distant from each other. It stretches the Earth horizontally more and more as the poles are approached.
- The parallels of latitude appear as parallel straight lines crossing the meridians at right angles.
Disadvantages:
- The same scale of miles cannot be used over large portions of the Mercator projection. By not permitting the meridians to converge toward the poles, the Mercator projection stretches the distance between all places except those near the equator.
- Areas are badly distorted. (Greenland looks larger than South America, while its area is only one-ninth that of South America.)
- Straight lines on the map are generally not the shortest distances between two points.
Advantages: Any straight line drawn on a Mercator projection crosses each meridian at the same angle. This is of the greatest importance to the sailor and to the naval pilot.
B. Lambert Projection: Most frequently used in air navigation is the Lambert conformal conic projection, briefly called the Lambert projection.
- A cone is considered to be placed on the Earth so that it cuts the surface at two parallels of latitude. For maps of the US, these are usually the 33 and 45-degree parallels. Points on the Earth’s surface are then transferred to the cone.
- When the cone is sliced and opened out, a map is created. The meridians are straight lines that converge and would all meet at a common point (at the pole) if extended far enough. The parallels of latitude are arcs of concentric circles.
Advantages: The Lambert projection distorts distances so little that we can use the same scale of miles at any place on the map. A straight line on a Lambert is approximately the shortest distance between two points.
Disadvantages: A straight line drawn on the Lambert projection does not cross every meridian at the same angle, thus making it difficult to measure directions.
However, it is outweighed by the advantage of a constant scale of miles.
C. Polar projections: Neither
the Lambert nor the Mercator projection is suitable for showing the polar regions.
- A plane is placed tangent to the Earth at the North Pole, and the surface of the Earth is projected upon the plane. It has a very limited use for air navigation.
- Advantages: It is the best projection for showing the polar areas. Every straight line that is drawn through the North Pole represents the shortest distance between any two points. This feature can be used when planning routes for the transcontinental airlines of tomorrow.
- Disadvantages: Distances are distorted very greatly, being stretched as the distance from the pole is increased. A different scale
- When using any map, it's important to keep the following points in mind:
- 1. When a small portion of the Earth’s surface, such as the area of a small state, is shown on a map, it does not make too much of a difference.
- 2. It is not the edges of the page that determine the North, South, East, and West, but the position of the poles and the lines of longitude and latitude
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