shrug-l: WGS84 vs NAD83 and data transformation

Eric_Songer at Eric_Songer at
Wed Jun 7 12:00:48 EDT 2006

5) Finally, State Plane Coordinate systems are a planar system (hint, look
the name).  Other coordinate systems are usually spherical or, better yet,
ellipsoidal, so the conversion from SPCs to Geographical Coordinates, for
example, are done by a "Best Fit" data method.  The errors from this
conversion can be in the 10 - 20 cm range at worst (in a state like
which has many fit points, it is much better).

Whoa.  Wait a minute.

SPC's are coordinate systems, but they are derived using map projections.
The following is from ESRI's documentation.

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. Smaller states such as Connecticut
require only one zone, whereas Alaska is composed of ten zones and uses all
three projections.

Eric Songer
URS Corporation
1625 Summit Lake Drive
Tallahassee, FL 32317
Direct: 850.402.6327
Main: 800.842.9671 ext. 327
eric_songer at

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