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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