6 Map Projection
The position of a geographic object on the earths surface is described by latitude and longitude. They are measures of the angle (in degrees) from the centre of the earth to a point on its surface. Latitude is the angle measured north or south of the equator, while longitude is measured east or west of the Prime Meridian (an imaginary line running from the North Pole to the South Pole, through Greenwich, England). Bremerhaven is at 53° 35' North latitude and 8.5° 33' East longitude. Whether you treat the Earth as a sphere or as a spheroid, you must transform its three-dimensional surface to create a flat map sheet. This transformation, usually using a mathematical conversion, is commonly referred to as map projection.
There are many different map projections. They are distinguished by their suitability for representing different locations and amount of earth covered, as well as by their ability to preserve distance, area, shape or direction. Matching a map projection to your needs is an important decision. For spatial calculations e.g. of marine fluxes equal area projections are necessary. The power of GIS comes not only from the ability to store geographic data, but also from the ability to analyse it more efficiently and more conveniently than is possible with paper maps.
People often consider latitude-longitude measures to be x,y coordinates; they are not. Latitude-longitude is a unit of measure for a spherical (3-dimensional) coordinate system. A map projection transforms by mathematical formula locations expressed in latitude and longitude from the earths curved surface to Cartesian coordinates (2-dimensional) to a maps flat surface.