Geographic Coordinate System Definitions

Albers Equal-Area Conic Projection(Albers)

With standard parallels of 29.5 and 45.5 degrees, this projection is one commonly used to depict the United States. It was used by the USGS in their 1970 National Atlas. The atlas design team decided that this projection was preferable because it is an equivalent projection, it is simple to construct, easy to segment and reassemble, contains small errors in scale, and is a popular choice for mapping the largely east-west extending United States. Analysis of the projection shows little distortion in an area the size of the US. The figure below shows the Albers Equal-Area Conic projection of the world with shorelines on a 15 degree graticule. The standard parallels are 20 and 60 degrees N. The central meridian is 90 degrees W. For more about these parameters and other projections, see Map Projection Basics.

Universal Transverse Mercator Grid System (UTM)

This system is a specialized application of the transverse Mercator projection which is both cylindrical and conformal. It divides the world into 60 numbered zones, both north and south, separated by the equator. Each zone spans six degrees of longitude and has its own central meridian. This system was adopted by the US Army Map Service in 1947 for their use in worldwide mapping and continues to be used worldwide. Florida falls into UTM zones 16 and 17.

When using the UTM system, linear parameters are established. The origin of each UTM zone is the intersection of its central meridian and the equator, and the parameters are applied to this origin to make it convenient to work with making all x and y values positive, or reducing their range. The first parameter is the false easting - a linear value applied to the origin of the x-coordinates - or the central meridian. The second parameter is the false northing - a linear value applied to the origin of the y-coordinates - or the equator. The final parameter used in a UTM grid system is a fixed parameter called the scale factor. The scale factor is a unitless value (usually less than one) that is applied to the center point or line of a map projection to reduce the distortion of the projection in the area of interest. The value of the scale factor for the UTM system is 0.9996. For more about this and other projections, see Map Projection Basics.

State Plane Coordinate System (SPCS)

This is not a projection but a coordinate system that came into use in the United States in the 1930s when the United States Coast and Geodetic Survey used it to provide a common reference system to surveyors and mappers. In the 1950s, the US Geological Survey also began to base new topographic quandrangles on it. SPCS divides the 50 states, Puerto Rico, and the US Virgin Islands into more than 120 numbered sections. These sections, referred to as zones, have assigned code numbers that define the projection parameters for that region.

The SPCS system is based largely (Alaska is based on yet another projection) on two different projections depending on the shape of the region being mapped. In the case of states that have an extensive east-west expanse, the Lambert conformal conic projection is used. In the case of a state with a north-south expanse, the transverse Mercator projection is used. Many county and municipal governmental organizations use SPCS in local mapping projects.

There are two phases of the SPCS system, SPCS27 and SPCS83, and they are quite different. While SPCS 27 is based on the local datum NAD27 and is measured in feet, SPCS83 is based on the updated geocentric datum NAD83 and is measured in meters. For more about this and other projections, see Map Projection Basics.

North American Datum (NAD)

There are two phases of the North American Datum - NAD27 and NAD83 - one local the other geocentric. Local datum align a spheroid to closely fit the earth's surface in a particular area. A point on the surface of the spheroid is matched to a point on the earth's surface - this point is the origin of the datum and has fixed coordinates. All other points are calculated from the origin. The North American Datum of 1927 is a local datum based on the Clarke 1866 spheroid and centered on an area named "Meades Ranch" in Kansas.

Geocentric datum relate coordinates to the earth's center of mass. Such datum have been improved by modern satellite data. The North American Datum of 1983 is based on the modern GRS 1980 ellipsoid - almost identical to the most widely used WGS 1984 ellipsoid which is also geocentric. Both earth and satellite data went into the determination of this datum. Because raw GPS data is based on the WGS 1984 ellipsoid, the data is also compatible with NAD83. For more about this and map projections, see Map Projection Basics.

High Accuracy Reference Network (HARN) or High Precision Geodetic Network (HPGN)

HARN and HPGN are both names for the same project that is focused on readjusting the NAD83 datum to a higher level of accuracy state by state. This is an even more accurate form of the North American Datum. For more about this and map projections, see Map Projection Basics.


Linear parameters

For an explanation of linear parameters, please read the above section in the Universal Transverse Mercator Grid System (UTM)system where they are explained. For more about parameters, see Map Projection Basics.

Angular parameters

These parameters are used to set and define the origin of the coordinate system in use. There are several different parameters including azimuths, central meridians, central parallels, longitudes and latitudes of origin and center, and standard parallels 1 and 2. For more about parameters, see Map Projection Basics.