odom frame keeps moving in ROS GPS robot_localization

Question

I am using two instances of ROS robot_localization package to get the local and global odometry. In the local one, I am fusing IMU and twist messages. It is working fine. But when I use the second instance where map and odom frame both should be fixed points, one of them is moving. I set map as the fixed frame, then odom is moving. Why is that? Odom should remain fixed at the initial starting point. base_link with respect to odom is working fine but since odom is moving, base_link with respect to map is not working.

In the image, you can see the map frame is the big axes. odom frame was there too. But as I move the robot, the odom frame starts moving. The red lines are local odometry and green ones are global odometry.

1st instance:

ekf_se_odom: # Used only for broadcasting odom to base_link transforms
  frequency: 30
  sensor_timeout: 0.1
  two_d_mode: true
  transform_time_offset: 0.0
  transform_timeout: 0.0
  print_diagnostics: true
  debug: false

  map_frame: map
  odom_frame: odom
  base_link_frame: base_link
  world_frame: odom

2nd instance:

ekf_se_map:
  frequency: 30
  sensor_timeout: 0.1
  two_d_mode: true
  transform_time_offset: 0.0
  transform_timeout: 0.0
  print_diagnostics: true
  debug: false

  map_frame: map
  odom_frame: odom
  base_link_frame: base_link
  world_frame: map

  publish_tf: true

navsat_node:

navsat_transform:
  frequency: 30
  delay: 3.0
  magnetic_declination_radians: 0.0842
  yaw_offset: -1.57080
  zero_altitude: true
  broadcast_utm_transform: true
  publish_filtered_gps: true
  use_odometry_yaw: false
  wait_for_datum: false
  broadcast_utm_transform_as_parent_frame: false

I am using gazebo husky simulation.

Answer 1

Odom should remain fixed at the initial starting point.

This assumption is not correct, as map->odom is allowed to change. You have configured your 2nd instance to publish this update.

An update to the map->odom TF is typically done to correct for drift in odom->base_link since it comes from an odometry source.

The purpose of this to make the overall transform map->base_link a globally accurate source for localisation and mapping purpose which may have discontinuous jumps (e.g. Due to loop closure from SLAM), while maintaining odom->base_link as an approximately accurate but more importantly continuously varying TF for control and state estimation purposes.

The purpose of this structure is formalised and documented in REP-105