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PROJECTION

[GREG2\]PROJECTION [A0 D0 [Angle]] [/TYPE Ptype]
Define a projection of the (celestial) sphere from point of (A0,D0),
(which are Longitude and Latitude respectively) of the specified type.
Angle is the angle between the Y axis and the North pole. The previous
values are kept if no argument is specified. All angles are in degrees,
except if the SYSTEM is EQUATORIAL in which case A0 is the right ascen-
sion and must be specified in hours. Formats like -dd:mm:ss.s or
hh:mm.mmm in sexagesimal notation up to the point field are allowed. Af-
ter the point, decimal values are assumed.
When a projection is active, the User coordinates are assumed to be pro-
jected coordinates of the sphere, and hence in the case of small field
of view where distortion are negligible, correspond to angular offsets
MEASURED IN RADIANS. The field of view of the projection is defined by
command LIMITS.
The TYPE can be
NONE Disables the projection system. User coordinates then
loose their interpretation in terms of projected coordi-
nates. The ANGLE_UNIT is then totally ignored.
CARTESIAN Cartesian projection with linear coordinates in both di-
rections, with a possible projection angle.
GNOMONIC Radial projection on the tangent plane. Being R and P
the (angular) polar coordinates from the projection
point (tangent point), the projected coordinates are
given by X = Tan(R).Sin(P) and Y = Tan(R).Cos(P) .
ORTHOGRAPHIC View from infinity. X = Sin(R).Sin(P) and Y =
Sin(R).Cos(P)
AZIMUTHAL Spherical offsets from the projection center. X =
R.Sin(P) and Y = R.Cos(P).
STEREOGRAPHIC Uses Tan(R/2) instead of Tan(R), and is thus less dis-
torted than the Gnomonic projection. This is an inver-
sion from the opposite pole.
LAMBERT Equal area projection. Projected distance is
2*Sin(R)/Sqrt(2*(1+Cos(R)).
AITOFF Equal area projection. Angle and D0 are ignored.
RADIO The standard radio astronomy single dish mapping "pro-
jection", in which X = (A-A0).COS(D) and Y = D-D0. The
Angle is obviously ignored.
MOLLWEIDE Equal area projection. Angle and D0 are ignored. The
projection trades accuracy of angle and shape for accu-
racy of proportions in area, and as such is used where
that property is needed, such as maps depicting global
distributions.
NCP North Celestial Pole. Projection to a plane perpendicu-
lar to the pole. Used by the WSRT.

Gildas manager
2020-09-28