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DOUBLE PRECISION FUNCTION sla_DPAV ( V1, V2 )
*+
* - - - - -
* D P A V
* - - - - -
*
* Position angle of one celestial direction with respect to another.
*
* (double precision)
*
* Given:
* V1 d(3) direction cosines of one point
* V2 d(3) direction cosines of the other point
*
* (The coordinate frames correspond to RA,Dec, Long,Lat etc.)
*
* The result is the bearing (position angle), in radians, of point
* V2 with respect to point V1. It is in the range +/- pi. The
* sense is such that if V2 is a small distance east of V1, the
* bearing is about +pi/2. Zero is returned if the two points
* are coincident.
*
* V1 and V2 need not be unit vectors.
*
* The routine sla_DBEAR performs an equivalent function except
* that the points are specified in the form of spherical
* coordinates.
*
* Last revision: 16 March 2005
*
* Copyright P.T.Wallace. All rights reserved.
*
* License:
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program (see SLA_CONDITIONS); if not, write to the
* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
* Boston, MA 02111-1307 USA
*
*-
IMPLICIT NONE
DOUBLE PRECISION V1(3),V2(3)
DOUBLE PRECISION X1,Y1,Z1,W,X2,Y2,Z2,SQ,CQ
* The unit vector to point 1.
X1 = V1(1)
Y1 = V1(2)
Z1 = V1(3)
W = SQRT(X1*X1+Y1*Y1+Z1*Z1)
IF (W.NE.0D0) THEN
X1 = X1/W
Y1 = Y1/W
Z1 = Z1/W
END IF
* The vector to point 2.
X2 = V2(1)
Y2 = V2(2)
Z2 = V2(3)
* Position angle.
SQ = Y2*X1-X2*Y1
CQ = Z2*(X1*X1+Y1*Y1)-Z1*(X2*X1+Y2*Y1)
IF (SQ.EQ.0D0.AND.CQ.EQ.0D0) CQ=1D0
sla_DPAV = ATAN2(SQ,CQ)
END
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