Coordinate Systems in Maps
Coordinate systems are comprised of spheroids, datum, and projections, and are specified in terms of units (i.e., feet, meters, yards, etc.). Coordinate systems locate features by x and y coordinates in the maps.
Spheroids in Maps
A spheroid is a mathematical description of the earth. Over time these mathematical expressions have changed from describing the earth as a perfect circle to a spheroid (i.e., an egg shape). For years the US standard has been Clarke 1866 but with improvements in measurement techniques, the US standard is moving toward GRS80.
Datum in Maps
A datum is a set of control points whose geometric relationships are known, either through measurement or calculation, and is used to define a coordinate system. Datum are based on a particular spheroid. There are two datum used almost exclusively in the US, the North American Datum of 1927 (NAD27 based on Clarke 1866) and the North American Datum of 1983 (NAD83 based on GRS80). Converting digital data based on NAD27 to NAD83 can migrate features slightly. New USGS 7.5 minute topographic Maps show the corners of the map in both datums.
Projections in Maps
Projection is the process of representing a three-dimensional surface in two-dimensions. Projections are mathematical expressions that convert data from a geographic location (latitude and longitude) on a sphere or spheroid to a representative location on a flat surface (a map). This process distorts at least one of these properties: shape, area, distance, or direction.
The most popular projections are:
1. Conic Maps
- A cone is placed over the globe touching along one or two standard parallels, and information is transposed onto the cone.
- Of all the conic projections, the Equidistant Conic, Lambert Conic Conformal, and Albers Equal-Area Conic projections are the most popular.
2. Cylindrical Maps
- A cylinder is placed over the globe touching along one or two standard parallels, and information is transposed onto the cylinder,
- Of all cylindrical projections, the Mercator projection is the most popular.
- Planar projections transpose information onto a flat surface which is touching the earth at one point,
- Planar projections are most often used to map the poles.
The most common projections used in commercial or publicly available mapping data sets are: