# 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.

(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.

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.

(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

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.

```