State Plane Coordinate System

State plane coordinates are a function of the plane-rectangular coordinate systems developed for each U.S. state in the mid-20th century by the U.S. Coast and Geodetic Survey (now the National Geodetic Survey). A basic premise of the system is to allow geodetic positions (latitudes and longitudes) to be defined as plane (x, y) coordinates, while maintaining a maximum scale error of 1 part in 10,000. This allows land surveyors to use conventional surveying measurements and plane geometry to determine geodetic coordinates (latitude and longitude) to an acceptable degree of accuracy.

A GIS must utilize a coordinate system as its mapping basis. The state plane coordinate system may be the choice, but even when it is not, data are often collected and documented as state plane coordinates and must be converted to the GIS coordinate system when added to the database.

State plane coordinates are published in meters and/or feet, depending on the laws of the respective states of the United States. Some states utilize the U.S. Survey Foot (with an exact conversion of 1 meter = 39.37 inches), while others specify international feet (with an exact conversion of 1 inch = 2.54 cm). Some states do not have a legislated definition for the foot, and the published coordinates in those states depend on a variety of factors, including common usage.

The x-(easting) and y-(northing) coordinate values of a state plane coordinate position depend on the state, projection, and zone (and whether in meters or feet) but will generally include five to seven digits left of the decimal place and three to the right, as in the following examples:

• 1,325,647.187 feet northing, 274,559.012 feet easting
• 267,395.163 meters northing, 22,412.893 meters easting

The state plane coordinate system for each state was specifically designed to adapt the conceptual purpose of the system and its parameters to the geographical shape of the respective state. Each state's system consists of one or more “zones,” each with a plane grid imposed onto a conformal map projection. For states whose greater extent is east-west, the Lambert Conformal map projection is used, and for states whose greater extent is north-south, the Transverse Mercator map projection was chosen. To avoid exceeding the system parameter of 1:10,000, the width of any zone was necessarily limited to 158 miles, this constraint being in an east-west direction for the Transverse Mercator projection and northsouth for the Lambert Conformal projection.

During conversion to or from state plane coordinates, it is important to understand that scale and elevation factors must be applied to ground distances in order to ensure accurate results. The scale factor is a function of the coordinate position's horizontal location in the particular zone (either east-west or north-south, respectively, depending on whether the projection is Transverse Mercator or Lambert Conformal), and the elevation factor is a function of the elevation of the coordinate position. Both factors [Page 456]must be addressed in order to ensure the integrity of the final state plane coordinate.

With the contemporary ease and common usage of the global positioning system (GPS) by surveyors and others to determine highly accurate geodetic coordinates, the need to rely on the state plane coordinate system as originally designed has been diminished to the extent that some states and agencies have modified, simplified, and/or downgraded the importance of their systems in favor of using single zones or geodetic coordinates.

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