Index: trunk/pswarp/src/pswarpTransformTile.c
===================================================================
--- trunk/pswarp/src/pswarpTransformTile.c	(revision 29000)
+++ trunk/pswarp/src/pswarpTransformTile.c	(revision 29331)
@@ -78,5 +78,5 @@
     int yMax = PS_MIN(maxPt.y, outNumRows);
 
-    double jacobian = map->Xx * map->Yy - map->Yx * map->Xy; // Jacobian of transformation
+    double jacobian = fabs(map->Xx * map->Yy - map->Yx * map->Xy); // Jacobian of transformation
     double jacobian2 = PS_SQR(jacobian);                     // Square Jacobian