The Mercator projection is a cylindrical map projection presented by the Belgian geographer and cartographer Gerardus Mercator, in 1569. It became the standard map projection for nautical purposes because of its ability to represent lines of constant course, known as rhumb lines or loxodromes, as straight segments. While the linear scale is constant in all directions around any point, thus preserving the angles and the shapes of small objects (which makes the projection conformal), the Mercator projection distorts the size and shape of large objects, as the scale increases from the Equator to the poles, where it becomes infinite.
Mercator world map Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendata (1569)
Description of the Transverse Mercator Projection
The Transverse Mercator projection is a conformal projection for which the point scale factor equals one along the central meridian. The line Y = 0 is the projection of the equator, and the line X = 0 is the projection of the central meridian, as shown in the figure below.
Both the central meridian and the equator are represented as straight lines. No other meridian or parallel is projected onto a straight line. Since the point scale factor is one along the central meridian, this projection is most useful near the central meridian. Scale distortion increases away from this meridian.
Meridians and Parallels in the Transverse Mercator Projection
(0 is the central meridian)
The Transverse Mercator equations for X and Y, and for latitude and longitude, are approximations. Within 4° of the central meridian, the equations for X, Y, latitude, and longitude have an error of less than 1 centimeter.
UTM coordinates are based on a family of projections based on the Transverse Mercator projection, in which the ellipsoid is divided into 60 longitudinal zones of 6° each. The X value, called the Easting, has a value of 500,000m at the central meridian of each zone. The Y value, called the Northing, has a value of 0m at the equator for the northern hemisphere, increasing toward the north pole, and a value of 10,000,000m at the equator for the southern hemisphere, decreasing toward the south pole. The point scale factor along the central meridian is 0.9996.
For the UTM grid system, the ellipsoid is divided into 60 longitudinal zones of 6° each. Zone number one lies between 180° E and 186° E. The zone numbers increase consecutively in the eastward direction. It is intended that a UTM projection should be used only in one of the 6-degree zones, plus the overlap area.
Meridians and Parallels (dashed) on a UTM Grid
The area of coverage for UTM coordinates is defined by zone limits, latitude limits, and overlap.
Zone limits:
6° zones, extending 3° to each side of the central meridian. Note: there are several exceptions to this general rule.
Latitude limits:
North: 84° N
South: 80°N
Zone overlap:
40 km on either side of the zone limits.
Polar overlap:
30' toward the poles
North: 84° 30'N
South: 80° 30'S
Description of Oblique Mercator ProjectionThe Oblique Mercator projection is an oblique, cylindrical, conformal projection. As shown in the figure below, there are two meridians which are straight lines 180° apart. Other meridians and parallels are complex curves. The poles are points that do not lie on the central line. The projection is symmetrical about any straight meridian.
Source : http://geoengine.nga.mil/geospatial/SW_TOOLS/NIMAMUSE/webinter/geotrans2/help/projectionDescriptions.htm
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