diff --git a/gtsam/slam/ProjectionFactor.h b/gtsam/slam/ProjectionFactor.h
index 2d25d70a9..ee5649f78 100644
--- a/gtsam/slam/ProjectionFactor.h
+++ b/gtsam/slam/ProjectionFactor.h
@@ -87,6 +87,11 @@ namespace gtsam {
 			return reprojectionError.vector();
 		}
 
+    /** return the measurement */
+    inline const Point2 measured() const {
+      return z_;
+    }
+
 	private:
 
 		/// Serialization function