State Plane Coordinate System
If you've ever worked with GIS data regionally within the United States you've probably come
across the State Plane Coordinate System.
What is the State Plane Coordinate System?
The State Plane Coordinate System (SPCS) is not a projection; rather it is a system for
specifying positions of geodetic stations using plane rectangular coordinates. This coordinate
system that divides all fifty states of the United States, Puerto Rico and the U.S. Virgin Islands
into over 120 numbered sections, referred to as zones. Each zone has an assigned code number
that defines the projection parameters for the region. Figure 1 shows the zone boundaries for
the continental U.S. Note that in some cases, the long axis for the zones is in an east/west
direction. In other cases, the long axis is in a north/south direction. For a portion of Alaska,
the long axis is actually from northwest to southeast (Figure 2).
Figure 1. State Plane Coordinate System Zones
for the Continental U.S.
Figure 2. Oblique zone orientation for
Alaska panhandle region
Origins
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. Many users find State Plane maps more adaptable
than UTMs because the zone boundaries fall along political lines (e.g., State and County) as
opposed to geometric lines. In addition, there is less "scale reduction" with State Planes when
compared to UTMs. The SPCS was created in the 1930's by the U.S. Coast and Geodetic
Survey in order to provide a common reference system to surveyors and cartographers. The
SPCS is a grid system developed by USGS in 1938 for the irregular shapes of the individual
US states. SPCS is widely used in public works and land surveys.
Projections
There are three possible projections for SPCS. The geometric direction of each state
determines the projection utilized. For states that are longer in the east-west direction, the
Lambert Conformal Conic is used. States which are longer in the north-south direction use the
Transverse Mercator Projection. The panhandle of Alaska, which has the sole distinction of
lying at an angle, garners the use of the Oblique Mercator Projection.
Zones or FIPSZones
The main aim in creating the SPCS was to design a conformal mapping system for the entire
country while maintaining a maximum scale distortion of 1 part in 10,000. In 1933 this was
considered the limit of surveying accuracy. In order to attain this accuracy, the larger states
needed to be divided into smaller zones or FIPS (Federal Information Processing Standard).
Each zone or FIPS has its own central meridian or standard parallels to maintain accuracy.
Changes from NAD27 to NAD83
The original zones were based upon a network of geodetic control points known as the North
American Datum of 1927 (NAD27). With improvements within the last 50 years and the need
for compatibility with satellite systems, the origin of the datum was moved and NAD83 was
created. Hence ZONE refers to the older NAD27 system and FIPSZone to the newer NAD83.
Similarities to UTM
The state plane coordinate system (SPCS) is similar to the UTM system in many respects, but
it does have some important differences. In the SPCS, each state has its own zone(s). The
number of zones in a state is determined by the area the state covers and ranges from one for a
small state such as Rhode Island to as many as five. The projection used for each state is also
variable as described earlier. The reasoning behind this is fairly simple; by changing the
projection to maximize the number of zones used for gridding, the distortion within each zone
in the state is minimized. Idaho uses a transverse Mercator projection and is divided into three
zones (west, east, and central).
The NAD-27 coordinates are in English units (feet) while NAD-83 units are metric. The grid
system used is very similar to that used with the UTM system, with the exception of where the
origin for the grids are located. The easting origin for each zone is always placed an arbitrary
number of feet west of the western boundary of the zone, eliminating the need for negative
easting values. The northing origin, however, is not at the equator as in UTM, but rather it is
placed an arbitrary number of feet south of the state border. Because of the limited distortion
in the SPCS (and the associated increase in accuracy of measurements), and the lack of
universal applicability (i.e. it doesn’t work across state lines) it is of greatest use for local
surveying and engineering applications.
Suitability of SPCS
Because many state plane zones use Transverse Mercator projections they are subject to the
same difficulties and limitations of the Universal Transverse Mercator (UTM) system,
which also employs Transverse Mercator projections. It is very important for new users to
realize that each State Plane Zone is a separate projection. Due to distortion effects induced by
Transverse Mercator projections the State Plane system is highly unsuited for creating maps
that combine more than one State Plane zone.