Pitney Bowes MapInfo Professional - Manual (Page 400)

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.
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
Professional 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, add a "NADCON" entry to the
registry. The registry entry should have this path:
HKEY_LOCAL_MACHINE\Software\MapInfo\MapInfo\Common\NADCON
If this entry is set to zero, then the Molodensky conversion method will be used instead of NADCON.
NADCON=0
Units
The units indicate the measurement that the projection uses to keep track of space. To find a complete
list of units and their corresponding projection entries, see
Units .
Coordinate System Origin
The coordinate system origin is the point on the Earth (specified as longitude and latitude degrees) from
which all coordinate distances are to be measured. X = 0 and Y = 0 at the origin point, unless a false
easting and/or false northing is used (see below). It is chosen to optimize the accuracy of a particular
coordinate system. As we move north from the origin, Y increases; X increases as we move east. These
coordinate values are generally called northings and eastings.
For the Transverse Mercator projection, the origin's longitude defines the central meridian. In constructing
the Transverse Mercator projection a cylinder is positioned tangent to the earth. The central meridian is
the line of tangency. The scale of the projected map is true along the central meridian.
In creating a Hotine Oblique Mercator projection it is necessary to specify a great circle that is not the
equator nor a meridian. MapInfo Professional does this by specifying one point on the ellipsoid and an
azimuth from that point. That point is the origin of the coordinate system.
Standard Parallels (Conic Projections)
In conic projections a cone is passed through the earth intersecting it along two parallels of latitude.
These are the standard parallels. One is to the north and one is to the south of the projection zone. To
use a single standard parallel specify that latitude twice. Both are expressed in degrees of latitude.
Oblique Azimuth (Hotine Oblique Mercator)
When specifying a great circle (such as the Hotine Oblique Mercator) using a point and an azimuth (arc),
the azimuth is called the Oblique Azimuth and is expressed in degrees.
Scale Factor (Transverse Mercator)
A scale factor is applied to cylindrical coordinates to average scale error over the central area of the
map while reducing the error along the east and west boundaries. The scale factor has the effect of
MapInfo Professional 12.5 400
Building Blocks of a Coordinate System