Initial commit

1 files changed, 72 insertions, 0 deletions

diff --git a/src/slalib/dpav.f b/src/slalib/dpav.f
new file mode 100644
index 0000000..63ad759
--- /dev/null
+++ b/src/slalib/dpav.f
@@ -0,0 +1,72 @@
+      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.
+*
+*  Patrick Wallace   Starlink   13 July 1997
+*
+*  Copyright (C) 1997 Rutherford Appleton Laboratory
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION V1(3),V2(3)
+
+      DOUBLE PRECISION X1,Y1,Z1,W,R,XU1,YU1,ZU1,DX,DY,DZ,SQ,CQ
+
+
+
+*  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
+
+*  Unit vector "north" from point 1
+      R=SQRT(X1*X1+Y1*Y1)
+      IF (R.EQ.0.0) R=1D-5
+      W=Z1/R
+      XU1=-X1*W
+      YU1=-Y1*W
+      ZU1=R
+
+*  Vector from point 1 to point 2
+      DX=V2(1)-X1
+      DY=V2(2)-Y1
+      DZ=V2(3)-Z1
+
+*  Position angle
+      SQ=DX*YU1*Z1+DY*ZU1*X1+DZ*XU1*Y1-DZ*YU1*X1-DY*XU1*Z1-DX*ZU1*Y1
+      CQ=DX*XU1+DY*YU1+DZ*ZU1
+      IF (SQ.EQ.0D0.AND.CQ.EQ.0D0) CQ=1D0
+      sla_DPAV=ATAN2(SQ,CQ)
+
+      END