Name

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  ANGUNITVEC

Author

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  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
  [email protected]
  UPDATED VERSIONs can be found on my WEB PAGE:
      http://cow.physics.wisc.edu/~craigm/idl/idl.html

Purpose

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  Convert longitude and (co)latitude (RA/Dec) to unit vector

Major Topics

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  Geometry

Calling Sequence

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  U = ANGUNITVEC(LON, LAT, [/DEC])

Description

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  The function ANGUNITVEC converts spherical polar angles into a
  unit vector.
  The inputs, LON and LAT, describes a point in spherical polar
  coordinates on the unit sphere. The output is that point as a
  unit 3-vector.
  LON is the longitude angle, measured in degrees from +X, with
  positive angles rotating through +Y. The range of LON is
  0 (+X) through 90 (+Y) through 360.
  If DEC=0, LAT represents a colatitude angle, measured in
  degrees from +Z. The range of LAT is 0 (+Z) through
  180 (-Z).
  If DEC=1, LAT represents a latitude angle ("declination"
  in astronomy), measured in degrees from the XY equator (positive
  toward +Z). The range of LAT is -90 (-Z) through +90 (+Z).
  ANGUNITVEC and UNITVECANG are functional inverses.
  UNITVECANG(ANGUNITVEC(LON,LAT)) should produce the same LONLAT
  pairs.

Inputs

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  LON - input longitude values, scalar or vector. See above.
  LAT - input (co)latitude, scalar or vector. See above.

Returns

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  The resulting unit vector, either a 3-vector or a 3xN array for N
  unit vectors.

Keyword Parameters

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  DEC - if set, then the input LONLAT(1,*) component is latitude;
        if not set then LONLAT(1,*) is a colatitude.

Example

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  print, angunitvec([157.65924,80.052155])
    ==> [-9.1103345E-01,3.7439942E-01,1.7275169E-01]
    (compare to example in UNITVECANG)

See Also

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  UNITVECANG, ANGUNITVEC, CROSSP, QTNORMALIZE

Modification History

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  Written, 1999, CM
  Documented, 2012-10-02, CM
  $Id: angunitvec.pro,v 1.2 2012/10/02 12:28:13 cmarkwar Exp $