diff --git a/gtsam/geometry/PinholeCamera.h b/gtsam/geometry/PinholeCamera.h
index 3b4858a4a..2af7d622b 100644
--- a/gtsam/geometry/PinholeCamera.h
+++ b/gtsam/geometry/PinholeCamera.h
@@ -32,6 +32,14 @@ namespace gtsam {
 template<typename Calibration>
 class PinholeCamera {
 
+public:
+
+  /**
+   *  Some classes template on either PinholeCamera or StereoCamera,
+   *  and this typedef informs those classes what "project" returns.
+   */
+  typedef Point2 Measurement;
+
 private:
   Pose3 pose_;
   Calibration K_;
diff --git a/gtsam/geometry/StereoCamera.h b/gtsam/geometry/StereoCamera.h
index 913b1eab3..8d16de4dd 100644
--- a/gtsam/geometry/StereoCamera.h
+++ b/gtsam/geometry/StereoCamera.h
@@ -38,6 +38,14 @@ public:
  */
 class GTSAM_EXPORT StereoCamera {
 
+public:
+
+  /**
+   *  Some classes template on either PinholeCamera or StereoCamera,
+   *  and this typedef informs those classes what "project" returns.
+   */
+  typedef StereoPoint2 Measurement;
+
 private:
   Pose3 leftCamPose_;
   Cal3_S2Stereo::shared_ptr K_;