Index: trunk/psastro/src/psLibUtils.c
===================================================================
--- trunk/psastro/src/psLibUtils.c	(revision 5562)
+++ trunk/psastro/src/psLibUtils.c	(revision 5565)
@@ -171,2 +171,34 @@
     return(out);
 }
+
+// returns the rotation term, forcing positive parity
+double psPlaneTransformGetRotation (psPlaneTransform *map) {
+
+    if (map->x->nX < 1) return 0;
+    if (map->x->nY < 1) return 0;
+
+    if (map->y->nX < 1) return 0;
+    if (map->y->nY < 1) return 0;
+    
+    double pc1_1 = map->x->coeff[1][0];
+    double pc1_2 = map->x->coeff[0][1];
+    double pc2_1 = map->y->coeff[1][0];
+    double pc2_2 = map->y->coeff[0][1];
+
+    double px = SIGN (pc1_1);
+    double py = SIGN (pc2_2);
+
+    // both x and y terms imply an angle. take the average
+    double t1 = -atan2 (px*pc1_2, px*pc1_1);
+    double t2 = +atan2 (py*pc2_1, py*pc2_2);
+    
+    // careful near -pi,+pi boundary...
+    if (t1 - t2 > M_PI/2) t2 += 2*M_PI;
+    if (t2 - t1 > M_PI/2) t1 += 2*M_PI;
+
+    double theta = 0.5*(t1 + t2);
+    while (theta < M_PI) theta += 2*M_PI;
+    while (theta > M_PI) theta -= 2*M_PI;
+    
+    return (theta);
+}