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UNITVECANG

UNITVECANG

## Author

Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
craigm@lheamail.gsfc.nasa.gov
UPDATED VERSIONs can be found on my WEB PAGE:
http://cow.physics.wisc.edu/~craigm/idl/idl.html

## Purpose

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

Geometry

## Calling Sequence

LONLAT = UNITVECANG(U, [/DEC])

## Description

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

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

## Returns

The resulting LONLAT array as described above.

## Keyword Parameters

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

## Example

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

UNITVECANG, ANGUNITVEC, CROSSP, QTNORMALIZE

## Modification History

Written, 1999, CM
Documented, 2012-10-02, CM
\$Id: unitvecang.pro,v 1.4 2012/10/02 12:28:13 cmarkwar Exp \$

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