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# Map projection

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• 1. Map ProjectionMap projections are representations of a sphere(the earth) in two-dimensions. A mathematicaltransformation is required in order to convertLatitude & Longitude. A coordinate system isusually defined by a map projection, a spheroid, adatum, one or more standard parallels, a centralmeridian. It is a system to measure horizontal &vertical distance so that a geographic feature’strue position can be established.
• 2. Construction of projection The creation of a map projection involves two steps:• Selection of a model for the shape of the Earth or planetary body (usually choosing between a sphere or ellipsoid).• Transformation of geographic coordinates (longitude and latitude) to Cartesian (x,y) or polar plane coordinates.
• 3. Shape of the earth and the models• A geodetic datum is a reference from which measurements are made. In surveying and geodesy, a datum is a set of reference points on the Earths surface against which position measurements are made and an associated model of the shape of the Earth (reference ellipsoid) to define a geographic coordinate system. Horizontal datums are used for describing a point on the Earths surface, in latitude and longitude or another coordinate system. Vertical datums measure elevations or depths.• As the earth is a geoid accurate representation of the mean sea-level surface becomes more complex.• Hence, the datum WGS84 is used in GPS to represent the entire earth for this purpose.
• 4. Scale Factor The scale of a map is the ratio of distance on the map and the corresponding ground surface.• The scale depends on location, but not on direction.• Scale is constant along any parallel.• Combination of the above: the scale depends on latitude only, not on longitude or direction. This applies for the Mercator projection in normal aspect.• Scale is constant along all straight lines radiating from a particular geographic location. This is the defining characteristic of an equidistant projection such as the Azimuthal equidistant projection.
• 5. Types of Map Projection The choice of a suitable projection on any occasion depends on the extent of area concerned and the purpose of mapping.• Equal Area, Aitoff’s Lambert’s Cylindrical, Sanson- Flamsteed’s Projection: areas in Tropical Zone and Temperate Zone (maintain discred shape & distance at large)• Polar Equidistant, Polar Guonomic, Polar Stereographic Projection: Polar regions• Two Standard Conic Projection: areas of limited extent (eg. France)• Bonne’s Projection : relatively smaller areas
• 6. Transverse Mercator Projection The Transverse Mercator map projection is anadaptation of the standard Mercator projection. When paired with a suitable geodetic datum, the transverse Mercator delivers high accuracy in zones less than a few degrees in east-west extent.
• 7. Transverse Mercator Projection The Transverse Mercator map projection is anadaptation of the standard Mercator projection. When paired with a suitable geodetic datum, the transverse Mercator delivers high accuracy in zones less than a few degrees in east-west extent.