The Oblique Mercator projection is a cylindrical, conformal map projection. It is similar to the Mercator projection, except that the cylinder is wrapped around the ellipsoid so that it touches the surface along the great circle path chosen for the central line, instead of along the earth's equator. Scale becomes infinite 90 degrees from the central line and is true along a chosen central line, along two straight lines parallel to the central line, or along a great circle at an oblique angle. The Oblique Mercator projection is used for geographic regions that are centered along lines that are neither meridians nor parallels, but that may be taken as great circle routes passing through the region, such as the Alaskan panhandle. In this variation of the Oblique Mercator projection, two points define the central line where the cylinder touches the ellipsoid.
The planar points determined by this projection may be left “unrectified” (often these are referred to as the u, v coordinates in published formula) or they may be “rectified” (often these are referred to as the x, y coordinates in published formula). Rectification consists of rotating the coordinates by a certain angle. This implementation of the Oblique Mercator projection allows for a flag to be set to determine if the points should be rectified, or not. By default, the points will be rectified. If an unrectified version of the projection is desired, the “Unrectified Flag” parameter should be set to 1.
In the case where the points are rectified, the angle of rotation may be one of three values. Most commonly, it may be a user-specified rotation, or a rotation such that the y-axis will be parallel to the meridian through the center of the projection (the angle used in this case is often referred to in published formula as alpha). Less commonly, it may be a rotation such that the y-axis will be parallel to the meridian through the natural origin of the projection (the angle used in this case is often referred to in published formula as gamma). If the “Rotation Angle” parameter is a non-zero number, it will be used in the rectification. If the “Rotation Angle” is zero, the default behavior will be to rectify so that the y-axis will be parallel to the meridian through the center of the projection. If it is desired that the rectification cause the y-axis to be parallel to the meridian through the natural origin of the projection, the “Use Gamma Flag” parameter should be set to 1.
The "Oblique Mercator Two Point" projection has the following parameters:
-
Latitude of the Origin of the Projection
-
Latitude of the First Point
-
Longitude of the First Point
-
Latitude of the Second Point
-
Longitude of the Second Point
-
False Easting
-
False Northing
-
Rotation Angle (Defaults to 0)
-
Unrectified Flag (Defaults to 0)
-
Use Gamma Flag (Defaults to 0)