Pitney Bowes MapXtreme - Manual (Page 581)

Appendix H: Elements of a Coordinate System
Datum Conversion
MapXtreme v7.1 588 Developer Guide
The Polyconic projection is neither equal-area nor conformal. Along the central meridian, however, it
is both distortion free and true to scale. Each parallel is true to scale, but the meridians are
lengthened by various amounts to cross each parallel at the correct position along the parallel, so
that no parallel is standard in the sense of having conformality (or correct angles), except at the
central meridian. Near the central meridian, distortion is extremely small.
This projection is not intended for mapping large areas. The conversion algorithms used break down
when mapping wide longitude ranges. For example, the sample table WORLD.TAB may exhibit
anomalies if reprojected using Polyconic.
Datum Conversion
When converting coordinates from one datum to another, MapInfo has used the Molodensky (3-
parameter) and Bursa-Wolf (7-parameter) methods. These are general-purpose methods that can
convert coordinates from any datum to any other datum.
After the NAD 83 datum was introduced, NOAA developed a program called NADCON, which
stands for North American Datum CONversion. This is a very specialized program that converts
coordinates only from NAD 27 to NAD 83 and vice versa. For this specialized task, it’s much more
accurate than the Molodensky general-purpose method; NADCON is accurate to about 0.1 meter,
and Molodensky is accurate to only 10–30 meters. Most U.S. government agencies, including the
Census Bureau, have standardized on NADCON for converting between NAD 27 and NAD 83.
Beginning with MapInfo 4.1.2, the NADCON algorithm is used to convert coordinates between NAD
27 and NAD 83 if those coordinates lie within the areas covered by NADCON (United States, Puerto
Rico, and the Virgin Islands). If the coordinates lie outside those areas, or if they use datums other
than NAD 27 or NAD 83, MapInfo uses the Molodensky or Bursa-Wolfe conversion methods.
Due to the file access required, the NADCON conversion method can be slightly slower than the
Molodensky method. If you want to turn off the NADCON conversion, remove the *.las and *.los files
from the MapXtreme program or the MapXtreme Common directory.
Custom Datums
A datum is a mathematical description of the earth’s shape and orientation. Because the earth’s
shape is not uniform, there are many different local datums used in different parts of the world.
These local datums provide a close approximation to the earth’s surface in a particular area.
Each Earth coordinate system uses a specific datum to approximate the earth’s surface. If two
coordinate systems use different datums, then Pitney Bowes Software’s mapping products must
perform a datum transformation when it converts coordinates from one coordinate system to the
other. Pitney Bowes Software uses the Bursa-Wolfe datum transformation method, which is
generally accurate to within 10 meters. (When the conversion is between two coordinate systems
that use the same datum, no datum transformation is performed, and the results are generally
accurate to within 0.1 meter.)