Name

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  UNITVECANG

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

Major Topics

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  Geometry

Calling Sequence

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  LONLAT = UNITVECANG(U, [/DEC])

Description

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  The function UNITVECANG converts a unit vector into two polar
  angles.
  The input should be one or more unit 3-vector describing points on
  the unit sphere. The input is either a 3-vector, or a 3xN array
  representing N unit vectors (i.e. DBLARR(3,N)).
  The returned array, LONLAT, describes that point in spherical
  polar coordinates.
 
  LONLAT(0,*) is the longitude angle, measured in degrees from +X,
  with positive angles rotating through +Y. The range of
  LONLAT(0,*) is 0 (+X) through 90 (+Y) through 360.
  If DEC=0, LONLAT(1,*) represents a colatitude angle, measured in
  degrees from +Z. The range of LONLAT(1,*) is 0 (+Z) through
  180 (-Z).
  If DEC=1, LONLAT(1,*) represents a latitude angle ("declination"
  in astronomy), measured in degrees from the XY equator (positive
  toward +Z). The range of LONLAT(1,*) is -90 (-Z) through +90 (+Z).
  The input U may either be a single 3-vector (i.e. DBLARR(3)) or it
  may be an array of N 3-vectors, (i.e. DBLARR(3,N)). It is the
  responsibility of the user for U to be a unit vector.
  ANGUNITVEC and UNITVECANG are functional inverses.
  ANGUNITVEC(UNITVECANG(U)) should produce the same unit vector(s).

Inputs

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  U - input unit vector, either a 3-vector or a 3xN array for N unit
      vectors.

Returns

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  The resulting LONLAT array as described above.

Keyword Parameters

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

Example

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  ; Sample unit vector
  U = [-9.1103345E-01,3.7439942E-01,1.7275169E-01]
  print, unitvecang(U)
    ==> [157.65924,80.052155]

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: unitvecang.pro,v 1.4 2012/10/02 12:28:13 cmarkwar Exp $