Coordinate System Help

In SPICE terminology, a "coordinate system" is a mathematical means of naming vectors or points in a specified reference frame. Examples are "Cartesian coordinates" or "spherical coordinates." The coordinate systems supported by webGeocalc are:

Rectangular (also called “Cartesian”):

      Coordinates are X, Y, Z.
 
      X, Y, Z    are, respectively, the components of the point (or
                 vector head, with the vector tail at the origin) along
                 the X, Y, and Z axes of a specified reference frame.
                 Units are km.

Right ascension/declination (also called “RA/Dec”):

      webGeocalc supports computation of RA/Dec relative to any specified
      reference frame (not just earth-centered inertial frames).
 
      Coordinates are range, right ascension (“RA”), and declination
      (“Dec”).
 
      range      is the distance of the point (vector head) from the
                 origin (vector tail).
 
                 The units associated with `range' are km.
 
      ra         is the right ascension of the point. This is the angular
                 distance measured toward the east from the prime meridian
                 to the meridian containing the point. The direction of
                 increasing right ascension is from the +X axis of a
                 specified reference frame towards the +Y axis.
 
                 The range of `ra' is [0, 360] degrees.
 
      dec        is the declination of the point. This is the angle from
                 the XY plane of the ray from the origin through the
                 point.
 
                 The range of `dec' is [-90, 90] degrees.

Planetocentric (also called “latitudinal”):

      Coordinates are radius, longitude, and latitude.
 
      radius     Distance of the point (vector head) from the origin
                 (vector tail). The units associated with `radius' are km.
 
      longitude  Longitude of the point. This is angle between the
                 prime meridian and the meridian containing the input
                 point. The direction of increasing longitude is from the
                 +X axis towards the +Y axis.
 
                 The range of `longitude' is [-180, 180] degrees.
 
      latitude   Latitude of the point. This is the angle from the
                 XY plane of the ray from the origin through the point.
 
                 The range of `latitude' is [-90, 90] degrees.

Cylindrical:

      Coordinates are radius, longitude, and Z.
 
      radius     Distance of the point of interest (vector head, with
                 the vector tail at the origin) from the Z axis.
 
      longitude  This is angle between the prime meridian and the meridian
                 containing the point.
 
                 The range of longitude is [0, 360] degrees.
 
      Z          Height of the point above the XY plane. Units are km.

Spherical:

      Coordinates are radius, colatitude, and longitude.
 
      radius      Distance of the point (vector head) from the origin
                  (vector tail). Units are km.
 
      colatitude  Angle between the point and the positive Z-axis.
 
      longitude   Longitude of the point in radians. This is angle between
                  the prime meridian and the meridian containing the input
                  point. Longitude increases in the counterclockwise sense
                  about the positive Z-axis.
 
                  The range of longitude is [-180, 180] degrees.

Planetodetic (also called “Geodetic”):

      Planetodetic coordinates are applicable only to bodies that have a
      reference spheroid.
 
      Coordinates are longitude, latitude, and altitude.
 
      longitude   Planetodetic longitude of the point (or vector head,
                  with the vector tail at the origin). This is the angle
                  between the prime meridian and the meridian containing
                  the point. The direction of increasing longitude is from
                  the +X axis towards the +Y axis.
 
                  The range of `longitude' is [-180, 180] degrees.
 
      latitude    Planetoetic latitude of the point. For a point P on the
                  reference spheroid, this is the angle between the XY
                  plane and the outward normal vector at P. For a point P
                  not on the reference spheroid, the geodetic latitude is
                  that of the closest point to P on the spheroid.
 
                  The range of `latitude' is [-90, 90] degrees.
 
      altitude    Altitude of point above the reference spheroid. Units
                  are km.

Planetographic:

      Planetographic coordinates are applicable only to bodies that have a
      reference spheroid and a spin direction.
 
      Coordinates are longitude, latitude, and altitude.
 
      longitude  Planetographic longitude of the point (or vector head,
                 with the vector tail at the origin). This is the angle
                 between the prime meridian and the meridian containing
                 the point. For bodies having prograde (aka direct)
                 rotation, the direction of increasing longitude is
                 positive west: from the +X axis of the specified
                 reference frame toward the -Y axis. For bodies having
                 retrograde rotation, the direction of increasing
                 longitude is positive east: from the +X axis toward the
                 +Y axis.
 
                 The earth, moon, and sun are exceptions: planetographic
                 longitude is measured positive east for these bodies.
 
                 The default sense of planetographic longitude for a given
                 body may be overridden via text kernel assignments. See
                 the header of the SPICE routine RECPGR for details.
 
                 The range of `longitude' is [0, 360] degrees.
 
      latitude   Planetographic latitude of the point. For a point P on
                 the reference spheroid, this is the angle between the XY
                 plane and the outward normal vector at P. For a point P
                 not on the reference spheroid, the planetographic
                 latitude is that of the closest point to P on the
                 spheroid.
 
                 The range of `latitude' is [-90, 90] degrees.
 
      altitude   Altitude of point above the reference spheroid.