Pitney Bowes MapInfo Professional - Manual (Page 479)

Units Number
Millimeters 5
Nautical Miles
2
9
Rods 32
US Survey Feet (used for 1927 State Plane)
3
8
Yards 4
1
One International Foot equals exactly 30.48 cm.
2
One Nautical Mile equals exactly 1852 meters.
3
One US Survey Foot equals exactly 12/39.37 meters, or approximately 30.48006 cm.
Coordinate System Origin
The origin is the point specified in longitude and latitude from which all coordinates are referenced. 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 (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
recessing the cylinder into the earth so that it has two lines of intersection. Scale is true along these lines
of intersection.
You may see the scale factor expressed as a ratio, such as 1:25000. In this case it is generally called
the scale reduction. The relationship between scale factor and scale reduction is:
scale factor = 1-scale reduction
In this case the scale factor would be 1-(1/25000) or 0.99996.
479 MapInfo Professional User Guide
Appendix B: Elements of a Coordinate System