diff --git a/simgear/math/SGVec2.hxx b/simgear/math/SGVec2.hxx
index 93655cb1..0979c0a0 100644
--- a/simgear/math/SGVec2.hxx
+++ b/simgear/math/SGVec2.hxx
@@ -375,6 +375,29 @@ interpolate(T tau, const SGVec2<T>& v1, const SGVec2<T>& v2)
   return r;
 }
 
+// Is the first point inside the triangle formed by the other three points?
+// Helper function
+template <typename T>
+inline
+T
+pt_determine(const SGVec2<T>& pt1, const SGVec2<T>& pt2, const SGVec2<T>& pt3)
+{
+  return (pt1.x()-pt3.x()) * (pt2.y()-pt3.y()) - (pt2.x() - pt3.x()) * (pt1.y() - pt3.y());
+}
+  
+template <typename T>
+inline
+bool
+point_in_triangle(const SGVec2<T>& testpt, const SGVec2<T>& pt1, const SGVec2<T>& pt2, const SGVec2<T>& pt3)
+{
+  T d1 = pt_determine(testpt,pt1,pt2);
+  T d2 = pt_determine(testpt,pt2,pt3);
+  T d3 = pt_determine(testpt,pt3,pt1);
+  bool has_neg = (d1 < 0) || (d2 < 0) || (d3 < 0);
+  bool has_pos = (d1 > 0) || (d2 > 0) || (d3 > 0);
+  return !(has_neg && has_pos);
+}
+
 #ifndef NDEBUG
 template<typename T>
 inline