This is one of many variations of a class of projections that is often referred to as Oblique Mercator. The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde. Snyder has also published a variation of Hotine's formulas. All of these formulations produce similar, but not precisely identical, results. Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.
This is the formulation/variation of the Oblique Mercator which most widely used (Alaska excepted). This variation requires three parameters: The Central Point Latitude and Central Point Longitude parameters define the approximate center of the region to be mapped, and the Geodesic Azimuth at Projection Center parameter defines the azimuth of the central geodesic at the central point in degrees east of north. (This is the point given by the first two parameters.)
The true origin of this projection is at the point where "the central geodesic intersects the equator of the aposphere," and is, thus, a rather academic location" (Snyder, p70). Thus, neither the Origin Latitude or Origin Longitude parameter are used. This also makes this projection rather difficult for use when defining a new custom coordinate system. See Rectified Skew Orthomorphic, Origin at Center.
Note that another variation of this projection, Rectified Skew Orthomorphic, Azimuth at Projection Center, supports an azimuth parameter referred to as Geodesic Azimuth at Projection Center. It might seem to the casual cartographer that these would be the same; but on an ellipsoid, these numbers are different by a small (but significant) amount.
|
Nom de paramètre |
Description |
|---|---|
|
PARM1 |
Latitude, en degrés, du point central de la projection. |
|
PARM2 |
Latitude, en degrés, du point central de la projection. |
|
PARM3 |
L'azimut du grand cercle central, en degrés à l'est du nord. |
|
SCL_RED |
Réduction d'échelle à appliquer. |
|
X_OFF |
Le faux Est doit être appliqué à toutes les coordonnées X, pour que toutes les coordonnées X dans le système de coordonnées aient des valeurs positives de taille raisonnable. |
|
Y_OFF |
Le faux Nord à appliquer à toutes les coordonnées Y. |