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+      DOUBLE PRECISION FUNCTION slDPAV ( 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 slDBER 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., 51 Franklin Street, Fifth Floor,
+*    Boston, MA  02110-1301  USA
+*
+*  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+*-
+
+      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
+      slDPAV = ATAN2(SQ,CQ)
+
+      END