How to broadcast a transform between /map and /odom

edit

Since static_transform_publisher can be used only to set up transformation between fixed (w.r.t. each other) frames you have to use something like this in your node:

int32_t publish_rate_ = 100;
tf::TransformBroadcaster tf_br_;
tf::StampedTransform tf_map_to_odom_;

// set up parent and child frames
tf_map_to_odom_.frame_id_ = std::string("map");
tf_map_to_odom_.child_frame_id_ = std::string("odom");

// set up publish rate
ros::Rate loop_rate(publish_rate_);

// main loop
while (ros::ok())
{
  // time stamp
  tf_map_to_odom_.stamp_ = ros::Time::now();

  // specify actual transformation vectors from odometry
  // NOTE: zeros have to be substituted with actual variable data
  tf_map_to_odom_.setOrigin(tf::Vector3(0.0f, 0.0f, 0.0f));
  tf_map_to_odom_.setRotation(tf::Quaternion(0.0f, 0.0f, 0.0f));

  // broadcast transform
  tf_br_.sendTransform(tf_map_to_odom_);

  ros::spinOnce();
  loop_rate.sleep();
}

I guess your question comes from the confusion why localization algorithm like amcl or gmapping do not publish /map -> /base_link (robot's position and orientation in map) directly instead of publishing the /map -> /odom transform. Some answers use static_transformation for publishing the /map -> /odom transform, which I can't say is incorrect because if the odometry reading never drifts then the /map -> /odom transform can be static. It will always be the displacement between map's origin and the starting position of the robot. Then you wouldn't need the amcl or gmapping algorithms because Tmap->odom * Todom->base_link = Tmap->base_link will always give you an accurate estimate of the robot's location.

But the fact that odometry reading drifts over time so we need extra localization mechanism including those lidar sensing, visual fiducial system, GPS etc. For amcl using lidar we are capable of getting /map -> /base_link transformation, BUT this breaks the rules of tf where 1 child frame can only have 1 parent frame. If we have /odom -> /base_link, then we can NOT have another direct transformation /map -> /base_link in the tf tree. So a typical tf tree would be /world -> /map -> /odom -> /base_link, where you get /map -> /odom from amcl, and /odom -> /base_link from your wheel encoders and imu fusion.

So to answer your question "where can I get /map -> /odom", it can be an output from localization algorithm like amcl. Together with /map -> /odom and /odom -> /base_link, you can get a more accurate /map -> /base_link transform.

Hi

Boris:
Since static_transform_publisher can be used only to set up transformation between fixed (w.r.t. each other) frames you have to use something like this in your node:

Well is use static_transform_publisher, because in my case odom -> map is static, only two 90° rotations.

I use this in my launch file:

<node pkg="tf" type="static_transform_publisher" name="odom_to_map"
    args="0.1 0  0  -1.570796327 0 -1.570796327  /map /odom 100" />

Search fpr static_transform_publisher in the ROS docu.