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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  coordinates.  The
ANGLE_UNIT is then totally ignored.
- 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
distorted  than  the  Gnomonic projection. This is an inversion 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
"projection", in which X = (A-A0).COS(D) and Y = D-D0. The Angle  is
obviously ignored.
```

Gildas manager 2014-07-01