diff --git a/cpp/Point3.h b/cpp/Point3.h
index e6d3dc7b3..48350edc5 100644
--- a/cpp/Point3.h
+++ b/cpp/Point3.h
@@ -20,6 +20,9 @@ namespace gtsam {
 
   /** A 3D point */
   class Point3: Testable<Point3>, public Lie<Point3> {
+  public:
+	  /// dimension of the variable - used to autodetect sizes
+	  static inline size_t dim() {return 3;}
   private:
     double x_, y_, z_;