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A part of what defines a data set is its coordinate system. A coordinate system includes such information as the unit of measure, the Earth model used,
and sometimes, how the data was flattened. A datum helps to define locations on the Earth’s surface in a three-dimensional coordinate system called a geographic
coordinate system. A geographic coordinate system is defined by a datum, an angular unit of measure (usually either degrees or grads), and a prime meridian.
However, many analysis techniques and data are designed for two-dimensional or planar coordinates. Three-dimensional geographic data is converted to
planar coordinates via a map projection. A map projection is a set of mathematical equations to convert from longitude and latitude (i, j) to planar coordinates
(x, y). Converting from three to two dimensions causes distortions. A map projection is designed to minimize the distortion caused by flattening the Earth’s surface.
A projected coordinate system consists of a linear unit of measure (usually meters or feet), a map projection, the specific parameters used by the map projection,
and a geographic coordinate system.
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