diff --git a/cartographer/io/points_batch.h b/cartographer/io/points_batch.h
index 7f6d0b8..f8c8c70 100644
--- a/cartographer/io/points_batch.h
+++ b/cartographer/io/points_batch.h
@@ -35,7 +35,7 @@ using Color = std::array<uint8_t, 3>;
 struct PointsBatch {
   PointsBatch() {
     origin = Eigen::Vector3f::Zero();
-    trajectory_index = 0;
+    trajectory_id = 0;
   }
 
   // Time at which this batch has been acquired.
@@ -49,8 +49,8 @@ struct PointsBatch {
   // is unknown.
   string frame_id;
 
-  // Trajectory index that produced this point.
-  int trajectory_index;
+  // Trajectory ID that produced this point.
+  int trajectory_id;
 
   // Geometry of the points in a metric frame.
   std::vector<Eigen::Vector3f> points;
diff --git a/cartographer/mapping/trajectory_connectivity.h b/cartographer/mapping/trajectory_connectivity.h
index 4e0b8cf..890c90c 100644
--- a/cartographer/mapping/trajectory_connectivity.h
+++ b/cartographer/mapping/trajectory_connectivity.h
@@ -79,7 +79,7 @@ class TrajectoryConnectivity {
 proto::TrajectoryConnectivity ToProto(
     std::vector<std::vector<int>> connected_components);
 
-// Returns the connected component containing 'trajectory_index'.
+// Returns the connected component containing 'trajectory_id'.
 proto::TrajectoryConnectivity::ConnectedComponent FindConnectedComponent(
     const cartographer::mapping::proto::TrajectoryConnectivity&
         trajectory_connectivity,