The description says it is 'The time at which the map was loaded'- should this be the time the map was initialized? The time it was last updated (making it redundant with the header timestamp)?

PS: I am asking the same question at https://answers.ros.org/question/114726/what-is-map_load_time-in-nav_msgsmapmetadata/ that nobody has answered so far.

  • $\begingroup$ I think the other question will get imported here in a month and one of these will get marked duplicate, not sure if they'll backdate mine $\endgroup$ Nov 18, 2022 at 12:43

1 Answer 1


I have a lot of ros packages checked out into a big catkin workspace, looks like it is redundant with the header stamp:

$ git grep --no-index -n -C 2 --heading map_load_time 
155-      ROS_INFO("Loading map from image \"%s\"", mapfname.c_str());
156-      map_server::loadMapFromFile(&map_resp_,mapfname.c_str(),res,negate,occ_th,free_th, origin);
157:      map_resp_.map.info.map_load_time = ros::Time::now();
158-      map_resp_.map.header.frame_id = frame_id;
159-      map_resp_.map.header.stamp = ros::Time::now();
275-  occupancyGrid.header.frame_id = gridMap.getFrameId();
276-  occupancyGrid.header.stamp.fromNSec(gridMap.getTimestamp());
277:  occupancyGrid.info.map_load_time = occupancyGrid.header.stamp;  // Same as header stamp as we do not load the map.
278-  occupancyGrid.info.resolution = gridMap.getResolution();
279-  occupancyGrid.info.width = gridMap.getSize()(0);
154-      // To make sure get a consistent time in simulation
155-      ros::Time::waitForValid();
156:      map_resp_.map.info.map_load_time = ros::Time::now();
157-      map_resp_.map.header.frame_id = frame_id_;
158-      map_resp_.map.header.stamp = ros::Time::now();
17-    grid.header.frame_id = "map"
18-    grid.header.stamp = rospy.Time.now()
19:    grid.info.map_load_time = rospy.Time.now()
20-    grid.info.resolution = 1.0
21-    grid.info.width = 3

GridMapRosConverter.cpp deserves credit over the others for for re-using the header stamp and putting in a comment instead of having sequential ros::Time::now() calls for time stamps in the same message which will result in very slightly different timestamps which will confuse the issue of whether there is a meaningful difference between the header stamp and map_load_time.

But still I don't fully understand 'Same as header stamp as we do not load the map', probably looking elsewhere in grid_map_ros would reveal what those authors believed map_load_time to mean.


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