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After learning ROS for a while, I am trying to reorganize/correct some understandings. My current understanding in dealing with ROS packages/stacks is:

1. Best to install from Ubuntu if available:

   sudo apt-get install ros-electric-stack_name

2. Otherwise, check out and rosinstall:

   i. cd ~/ros_workspace (I put all checked out packages in here)

   ii. roslocate info package_name > package_name.rosinstall

   iii. rosinstall . /etc/ros/electric package_name.rosinstall

   iv. rosmake package_name (rosdep --install package_name to install system dependencies)

   v. deal with ROS dependencies by iterating 1 or 2

   I have the following line in .bashrc:

   source /opt/ros/electric/setup.bash

   and this line in /opt/ros/electric/setup.bash

   . $SCRIPT_PATH/setup.sh

   and this line in the /opt/ros/electric/setup.sh

   export ROS_PACKAGE_PATH=~/ros_workspace:/opt/ros/electric/stacks

   Due to the above setup, I never bother to source the setup.bash from every rosinstall, neither I edit my .bashrc.

For packages installed in ~/ros_workspace, if I do not need them, I simply delete the folder. I didn't do any other cleanup, assuming I didn't specifically change setup.sh for every rosinstall.

Question:

1. Are there any problem in the above workflow? What is the best practice?

2. In rosinstall ~/ros_workspace /opt/ros/electric package_name.rosinstall, why do we need both ~/ros_workspace /opt/ros/electric and how will /opt/ros/electric be used? The packages are always stored in ~/ros_workspace. Am I right that the "/opt/ros/electric" field is only used to add this path to the newly created setup.sh?

3. I did not add "?rosdistro=electric&variant=robot&overlay=yes (or no)" to the end of rosinstall. What difference will this make? I don't understand the difference between "overlay=yes" and "overlay=on". What does overlay do? Does it check that if things are already available in /opt/ros/electric and only update the difference? If so, see 4.

4. What does "Developing a stack against a full tree" means, e.g. "rosinstall ~/workspace package_name.rosinstall"? Will we still use all packages available from /opt/ros/electric, e.g. dependencies?

5. What does "Updating a rosinstall tree" do? Does it update ROS_PACKAGE_PATH in setup.sh with all the paths to packages? Will having "~/workspace:/opt/ros/electric/stacks" in the ROS_PACKAGE_PATH be sufficient if all stacks/packages are in these folders?

6. I seem to be rosinstalling packages (due to dependencies) most of the time. Will it be necessary to rosinstall the stack instead?

7. rospack list-duplicates shows a number of duplicates. Should I deal with them? What is the right way to do so (may not know which is latest or dependency for which package)?

8. I can svn up in stack/package in /opt/ros/electric since they were not svn checked out. I tried update them with sudo apt-get install ros-electric-stack_name. Is this correct? What is the best practice?

9. When I svn up in those packages in ~/ros_workspace, I received "At revision *". Is that it?

My apology for the long list of fundamental questions. I know this kind of questions don't ... (more)

After went through tf tutorial, I thought the transformation between two frames of a shall be fixed.

In the odometry tutorial in navigation/Tutorials/RobotSetup/Odom, the transformation is following the changes in odometry readings (x,y,th). I am lost here. I can't visualise "odometry frame". I was expecting that the odometry readings be used without any transform, i.e. if it says x=2m, y=1m, z=0m, th=1rad, then that indicates the position of the base_link, provided the odometry was initialised at the beginning. I am surely terribly wrong in this. I hope some one can help point me to the right direction.

Thanks.

EDIT 1:

@DimitriProsser Many thanks for the explanation.

If the odom transform was published once at startup, then I might understand it fixes the relationship between the odom measurements and world. However, in the navigation/Tutorials/RobotSetup/Odom, the odom transform is between odom and base_link, and it is in the while loop as below:

  ros::Rate r(1.0);
  while(n.ok()){
    current_time = ros::Time::now();

    //compute odometry in a typical way given the velocities of the robot
    double dt = (current_time - last_time).toSec();
    double delta_x = (vx * cos(th) - vy * sin(th)) * dt;
    double delta_y = (vx * sin(th) + vy * cos(th)) * dt;
    double delta_th = vth * dt;

    x += delta_x;
    y += delta_y;
    th += delta_th;

    //since all odometry is 6DOF we'll need a quaternion created from yaw
    geometry_msgs::Quaternion odom_quat = tf::createQuaternionMsgFromYaw(th);

    //first, we'll publish the transform over tf
    geometry_msgs::TransformStamped odom_trans;
    odom_trans.header.stamp = current_time;
    odom_trans.header.frame_id = "odom";
    odom_trans.child_frame_id = "base_link";

    odom_trans.transform.translation.x = x;
    odom_trans.transform.translation.y = y;
    odom_trans.transform.translation.z = 0.0;
    odom_trans.transform.rotation = odom_quat;

    //send the transform
    odom_broadcaster.sendTransform(odom_trans);

    //next, we'll publish the odometry message over ROS
    nav_msgs::Odometry odom;
    odom.header.stamp = current_time;
    odom.header.frame_id = "odom";

    //set the position
    odom.pose.pose.position.x = x;
    odom.pose.pose.position.y = y;
    odom.pose.pose.position.z = 0.0;
    odom.pose.pose.orientation = odom_quat;

    //set the velocity
    odom.child_frame_id = "base_link";
    odom.twist.twist.linear.x = vx;
    odom.twist.twist.linear.y = vy;
    odom.twist.twist.angular.z = vth;

    //publish the message
    odom_pub.publish(odom);

    last_time = current_time;
    r.sleep();
  }

May be I am unable to clearly explain my doubt (my wrong understanding). I am relating this to the explanation in navigation/Tutorials/RobotSetup/TF. I assume odometry is a sensor (encoder) like a laser range finder, hence a transform from odom frame to the base_link frame should be fixed just like the transform from base_laser to base_link. Where do I go wrong?

Thanks again.

EDIT 2:

@ahendrix Many thanks for the explanation. My understanding of "transform" has been wrong. I am thinking along this way now:

In the tf tutorial, the scan (laser) is a measure of distance (of points) from the robot (base_link). Since scan is with reference to lrf (base_laser), it has to be transformed to with reference to robot position (base_link). Since base_laser (lrf) is always fixed ... (more)