State Plane Technical Characteristics
Standard Projection for KCGIS

• What is the Standard Projection for King County GIS Data?
• What is the FGDC Spatial Reference Information for King Co. GIS Data?
• What is State Plane?
• Why use State Plane?
• State Plane and the North American Datum
• Datums and datum conversion
  • NAD27
  • NAD83
  • State readjustments HPGN / HARN
• Unit of length

What is the Standard Projection for King County GIS Data?

Projection: State Plane*
Zone: 5601  (Washington State Plane North; FIPS Zone 4601)
Datum: HPGN
Units: feet

*Technically, State Plane is a coordinate system, not a projection. See "What is State Plane?" below.

What is the FGDC Spatial Reference Information for
King Co. GIS Data?

What is State Plane?

The State Plane Coordinate System (SPCS) is a coordinate system designed for mapping the United States. It was developed in the 1930s by the U.S. Coast and Geodetic Survey to provide a common reference system to surveyors and mappers. The goal was to design a conformal mapping system for the country with a maximum scale distortion of 1 part in 10,000, then considered the limit of surveying accuracy. Three conformal projections were chosen: the Lambert Conformal Conic for states that are longer in the east-west direction, such as Tennessee and Kentucky, the Transverse Mercator projection for states that are longer in the north-south direction, such as Illinois and Vermont, and the Oblique Mercator projection for the panhandle of Alaska, because it is neither predominantly north nor south, but at an angle. To maintain an accuracy of 1 part in 10,000, it was necessary to divide many states into zones. Each zone has its own central meridian or standard parallels to maintain the desired level of accuracy. The boundaries of these zones follow county boundaries.

There are two zones in Washington State (NAD83 based on a Lambert Conformal Conic Projection).

Why Use State Plane?

Governmental organizations or groups who do work with them primarily use the State Plane Coordinate System. Most often, these are county or municipal databases. The advantage of using SPCS is that your data will be in a common coordinate system which is the same as that of other databases covering the same area.

State Plane and the North American Datum

Technological advancements of the last fifty years have led to improvements in the measurement of distances, angles, and the Earths size and shape. This, combined with moving the origin of the datum from Meades Ranch in Kansas to the Earths center of mass, for compatibility with satellite systems, made it necessary to redefine SPCS 27. Consequently, the coordinates for points are different for SPCS 27 and SPCS 83. There are several reasons for this. For SPCS 83, all State Plane coordinates published by NGS are in metric units, the shape of the spheroid of the Earth is slightly different, some states have changed the definition of their zones, and values of latitude and longitude are slightly changed.

Datums and datum conversion

A datum is a set of parameters defining a coordinate system, and a set of control points whose geometric relationships are known, either through measurement or calculation (Dewhurst 1990). A datum is defined by a spheroid, which approximates the shape of the Earth, and the spheroid's position relative to the center of the Earth. There are many spheroids representing the shape of the Earth, and many more datums based upon them. A horizontal datum provides a frame of reference for measuring locations on the surface of the Earth. It defines the origin and orientation of latitude and longitude lines. A local datum aligns its spheroid to closely fit the Earth's surface in a particular area and its origin point is located on the surface of the Earth. The coordinates of the origin point are fixed and all other points are calculat ed from this control point. The coordinate system origin of a local datum is not at the center of the Earth. NAD27 and the European Datum of 1950 are local datums.

In the last fifteen years, satellite data has provided geodesists with new measurements to define the best Earth-fitting ellipsoid, which relates coordinates to the Earths center of mass. An Earth-centered or geocentric, datum does not have an initial point of origin like a local datum. The Earths center of mass is, in a sense, the origin. The most recently developed and widely used datum is the World Geodetic System of 1984. It serves as the framework for supporting locational measurement worldwide. GPS measurements are based upon the WGS84 datum.

North American datums: There are two horizontal datums used almost exclusively in North America. These are the North American Datum of 1927 NAD27 and the North American Datum of 1983 NAD83. The US. National Geodetic Survey (NGS) uses a transformation program known as NADCOM, the Canadian method for transformation between the two datums is known as the Canadian National Transformation.

NAD27: The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent the shape of the Earth. The origin of this datum is a point on the Earth referred to as Meades Ranch in Kansas. Many NAD27 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years. Therefore, errors varied from station to station.

NAD83: Many technological advances in surveying and geodesy since the establishment of NAD27 - electronic theodolites, GPS satellites, Very Long Baseline Interferometry, and Doppler systems revealed weaknesses in the existing network of control points. Differences became particularly noticeable when linking existing control with newly established surveys. The establishment of a new datum would allow for a single datum to cover consistently North America and surrounding areas. The North American Datum of 1983 is based upon both Earth and satellite observations, using the GRS80 spheroid. The origin for this datum is the Earths center of mass. This affects the surface location of all latitude-longitude values enough to cause locations of previous control points in North America to shift, sometimes as much as 500 feet. A ten-year multinational effort tied together a network of control points for the United States, Canada, Mexico, Greenland, Central America, and the Caribbean.

State readjustments HPGN / HARN: There is an ongoing effort at the state level to readjust the NAD83 datum to a higher level of accuracy using state-of-the-art surveying techniques that were not widely available when the NAD83 datum was being developed. This project, known as the High Accuracy Reference Network (HARN), or High Precision GPS Network (HPGN) is a cooperative project between the National Geodetic Survey and the individual states.

Unit of length

The standard unit of length measurement for SPCS 27 is the U.S. Survey foot. For SPCS 83, the most common unit of measure is the meter. Those states that support both feet and meters have legislated which feet-to-meters conversion they use. The difference between the two is only two parts in one million, but that can become noticeable when data sets are stored in double precision.

Map scale is expressed as 1: Representative fraction (RF) = Map Distance (MD) * Conversion Factor (CON) / Ground Distance (GD), for example, given a conversion factor of 63,360 inches to a mile:

  

The National Map Accuracy Standards defines the requirement for horizontal accuracy as:

• no more than 10% of features shall be more than 1/30th of an inch from their intended location on maps of scale larger than 1:20,000.
• no more than 10% of features shall be more than 1/50th of an inch from their intended location on maps of scale smaller than 1:20,000.

Revised January 21, 2005