PROJECTION

##
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.

          SFL           Sanson-Flamsteed  projection,  similar to the RADIO pro-
                        jection with  one  exception:  X = (A-A0).COS(D-D0)  and
                        Y =D-D0.  Beware that an Euler rotation of the celestial
                        sphere is necessary as an additional step, i.e.  (A0,D0)
                        and  (A,D)  here  are not the absolute celestial coordi-
                        nates (see Calabretta & Greisen 2002).

          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.