# Example 13
In this example, we will be utilizing the [move_base package](http://wiki.ros.org/move_base), a component of the [ROS navigation stack](http://wiki.ros.org/navigation?distro=melodic), to send base goals to the Stretch robot.

## Build a map
First, begin by building a map of the space the robot will be navigating in. If you need a refresher on how to do this, then check out the [Navigation Stack tutorial](navigation_stack.md).

## Getting Started
Next, with your created map, we can navigate the robot around the mapped space. Run:

```{.bash .shell-prompt}
roslaunch stretch_navigation navigation.launch map_yaml:=${HELLO_FLEET_PATH}/maps/<map_name>.yaml
```

Where `${HELLO_FLEET_PATH}` is the path of the `<map_name>.yaml` file.

!!! note
    It's likely that the robot's location on the map does not match the robot's location in real space. In the top bar of Rviz, use `2D Pose Estimate` to lay an arrow down roughly where the robot is located in real space. Below is a gif for reference.

<p align="center">
  <img src="https://raw.githubusercontent.com/hello-robot/stretch_tutorials/noetic/images/2D_pose_estimate.gif"/>
</p>

Now we are going to use a node to send a move_base goal half a meter in front of the map's origin. run the following command in a new terminal to execute the [navigation.py](https://github.com/hello-robot/stretch_tutorials/blob/noetic/src/navigation.py) node.

```{.bash .shell-prompt}
cd catkin_ws/src/stretch_tutorials/src/
python3 navigation.py
```

### The Code

```python
#!/usr/bin/env python3

import rospy
import actionlib
import sys
from move_base_msgs.msg import MoveBaseAction, MoveBaseGoal
from geometry_msgs.msg import Quaternion
from tf import transformations

class StretchNavigation:
    """
    A simple encapsulation of the navigation stack for a Stretch robot.
    """
    def __init__(self):
        """
        Create an instance of the simple navigation interface.
        :param self: The self reference.
        """
        self.client = actionlib.SimpleActionClient('move_base', MoveBaseAction)
        self.client.wait_for_server()
        rospy.loginfo('{0}: Made contact with move_base server'.format(self.__class__.__name__))

        self.goal = MoveBaseGoal()
        self.goal.target_pose.header.frame_id = 'map'
        self.goal.target_pose.header.stamp = rospy.Time()

        self.goal.target_pose.pose.position.x = 0.0
        self.goal.target_pose.pose.position.y = 0.0
        self.goal.target_pose.pose.position.z = 0.0
        self.goal.target_pose.pose.orientation.x = 0.0
        self.goal.target_pose.pose.orientation.y = 0.0
        self.goal.target_pose.pose.orientation.z = 0.0
        self.goal.target_pose.pose.orientation.w = 1.0

    def get_quaternion(self,theta):
        """
        A function to build Quaternians from Euler angles. Since the Stretch only
        rotates around z, we can zero out the other angles.
        :param theta: The angle (radians) the robot makes with the x-axis.
        """
        return Quaternion(*transformations.quaternion_from_euler(0.0, 0.0, theta))

    def go_to(self, x, y, theta):
        """
        Drive the robot to a particular pose on the map. The Stretch only needs
        (x, y) coordinates and a heading.
        :param x: x coordinate in the map frame.
        :param y: y coordinate in the map frame.
        :param theta: heading (angle with the x-axis in the map frame)
        """
        rospy.loginfo('{0}: Heading for ({1}, {2}) at {3} radians'.format(self.__class__.__name__,
        x, y, theta))

        self.goal.target_pose.pose.position.x = x
        self.goal.target_pose.pose.position.y = y
        self.goal.target_pose.pose.orientation = self.get_quaternion(theta)

        self.client.send_goal(self.goal, done_cb=self.done_callback)
        self.client.wait_for_result()

    def done_callback(self, status, result):
        """
        The done_callback function will be called when the joint action is complete.
        :param self: The self reference.
        :param status: status attribute from MoveBaseActionResult message.
        :param result: result attribute from MoveBaseActionResult message.
        """
        if status == actionlib.GoalStatus.SUCCEEDED:
            rospy.loginfo('{0}: SUCCEEDED in reaching the goal.'.format(self.__class__.__name__))
        else:
            rospy.loginfo('{0}: FAILED in reaching the goal.'.format(self.__class__.__name__))

if __name__ == '__main__':
    rospy.init_node('navigation', argv=sys.argv)
    nav = StretchNavigation()
    nav.go_to(0.5, 0.0, 0.0)
```

### The Code Explained
Now let's break the code down.

```python
#!/usr/bin/env python3
```

Every Python ROS [Node](http://wiki.ros.org/Nodes) will have this declaration at the top. The first line makes sure your script is executed as a Python3 script.

```python
import rospy
import actionlib
import sys
from move_base_msgs.msg import MoveBaseAction, MoveBaseGoal
from geometry_msgs.msg import Quaternion
from tf import transformations
```

You need to import `rospy` if you are writing a ROS [Node](http://wiki.ros.org/Nodes).

```python
self.client = actionlib.SimpleActionClient('move_base', MoveBaseAction)
self.client.wait_for_server()
rospy.loginfo('{0}: Made contact with move_base server'.format(self.__class__.__name__))
```

Set up a client for the navigation action. On the Stretch, this is called `move_base`, and has type `MoveBaseAction`.  Once we make the client, we wait for the server to be ready.

```python
self.goal = MoveBaseGoal()
self.goal.target_pose.header.frame_id = 'map'
self.goal.target_pose.header.stamp = rospy.Time()
```

Make a goal for the action. Specify the coordinate frame that we want, in this instance the `map` frame. Then we set the time to be now.

```python  
self.goal.target_pose.pose.position.x = 0.0
self.goal.target_pose.pose.position.y = 0.0
self.goal.target_pose.pose.position.z = 0.0
self.goal.target_pose.pose.orientation.x = 0.0
self.goal.target_pose.pose.orientation.y = 0.0
self.goal.target_pose.pose.orientation.z = 0.0
self.goal.target_pose.pose.orientation.w = 1.0
```

Initialize a position in the coordinate frame.

```python
def get_quaternion(self,theta):
    """
    A function to build Quaternians from Euler angles. Since the Stretch only
    rotates around z, we can zero out the other angles.
    :param theta: The angle (radians) the robot makes with the x-axis.
    """
    return Quaternion(*transformations.quaternion_from_euler(0.0, 0.0, theta))
```

A function that transforms Euler angles to quaternions and returns those values.

```python
def go_to(self, x, y, theta, wait=False):
    """
    Drive the robot to a particular pose on the map. The Stretch only needs
    (x, y) coordinates and a heading.
    :param x: x coordinate in the map frame.
    :param y: y coordinate in the map frame.
    :param theta: heading (angle with the x-axis in the map frame)
    """
    rospy.loginfo('{0}: Heading for ({1}, {2}) at {3} radians'.format(self.__class__.__name__,
    x, y, theta))
```

The `go_to()` function takes in the 3 arguments, the x and y coordinates in the `map` frame, and the heading.

```python
self.goal.target_pose.pose.position.x = x
self.goal.target_pose.pose.position.y = y
self.goal.target_pose.pose.orientation = self.get_quaternion(theta)
```

The `MoveBaseGoal()` data structure has three goal positions (in meters), along each of the axes. For Stretch, it will only pay attention to the x and y coordinates, since it can't move in the z-direction.

```python
self.client.send_goal(self.goal, done_cb=self.done_callback)
self.client.wait_for_result()
```

Send the goal and include the `done_callback()` function in one of the arguments in `send_goal()`.

```python
def done_callback(self, status, result):
    """
    The done_callback function will be called when the joint action is complete.
    :param self: The self reference.
    :param status: status attribute from MoveBaseActionResult message.
    :param result: result attribute from MoveBaseActionResult message.
    """
    if status == actionlib.GoalStatus.SUCCEEDED:
        rospy.loginfo('{0}: SUCCEEDED in reaching the goal.'.format(self.__class__.__name__))
    else:
        rospy.loginfo('{0}: FAILED in reaching the goal.'.format(self.__class__.__name__))
```

Conditional statement to print whether the goal status in the `MoveBaseActionResult` succeeded or failed.

```python
rospy.init_node('navigation', argv=sys.argv)
nav = StretchNavigation()
```

The next line, `rospy.init_node(NAME, ...)`, is very important as it tells rospy the name of your node -- until rospy has this information, it cannot start communicating with the ROS Master.

!!! note
    The name must be a base name, i.e. it cannot contain any slashes "/".

Declare the `StretchNavigation` object.

```python
nav.go_to(0.5, 0.0, 0.0)
```

Send a move base goal half a meter in front of the map's origin.