Mercator Projection

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Mercator Projection
The Mercator projection transfers the surface of the Earth onto a cylinder tangent at the Earth’s equator. In the Mercator projection, meridians and parallels appear as lines crossing at right angles. Meridians are parallel on the Mercator projection, unlike meridians on the Earth that meet at the poles. This results in increasingly exaggerated areas toward the poles. Scale, which is the relationship between distance on a chart and actual distance, changes with latitude.

The advantage of the Mercator is that a straight line on this projection crosses all meridians at the same angle. This allows the navigator to set a constant course from one point to another. A course crossing all meridians at a constant angle is known as a rhumb line. Figure 10-1 shows a rhumb line from San Francisco (KSFO) to London (EGLL). 

A rhumb line is not normally the shortest distance between two places on the surface of the Earth. The great-circle distance is always the shortest distance between points on the Earth. A great circle is an arc projected from the center of the Earth through any two points on the surface. A great circle, unlike the rhumb line, crosses meridians at different angles, except in two special cases: where the two points lie along the equator or the same meridian (in both cases, the rhumb and great circle coincide).

For practical purposes at low latitudes, rhumb and great-circle distances are nearly identical. As latitude and distance increase, differences become increasingly significant; however, the projection is conformal in that angles and shapes within any small area are essentially true.

Transverse Mercator projection
The transverse Mercator projection rotates the cylinder so that it becomes tangent to a meridian. These projections are used in high north and south latitudes (high-numbered latitudes toward the north and south) and where the north-south direction is greater than the east-west direction. All properties of the regular Mercator are preserved, except the straight rhumb line. Parallels are no longer straight, becoming curved lines; meridians become complex curves. The projection is conformal. The line of true scale is no longer the equator, but the central meridian of the projection, where the cylinder is tangent.

Distances are true only along the central meridian selected by the cartographer, or else along two lines parallel to it, but all distances, directions, shapes, and areas are reasonably accurate within 15° of the central meridian. Distortion of distances, directions, and size of areas increases rapidly outside the 15° band.

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