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Your code looks fine. The problem here is your interpretation of the "orientation" -- which is in fact a quaternion, not euler angles. Therefore, orientation.z is not the rotation about the z axis, but rather a component of the quaternion. For instance, using the tf.transformations module in python, we can see that when your marker is perpendicular (euler rotation about z of 1.57 radians), we have:

quaternion_from_euler(0,0,1.57) = array([ 0. , 0. , 0.70682518, 0.70738827])

So the quaternion's "z" component will be 0.707 as you saw.

If you run the following (while your node is publishing tf messages):

rosrun tf tf_echo odom base_link

You can see the transformation in both euler angle and quaternion form.

Your code looks fine. The problem here is your interpretation of the "orientation" -- which is in fact a quaternion, not euler angles. Therefore, orientation.z is not the rotation about the z axis, but rather a component of the quaternion. For instance, using the tf.transformations module in python, we can see that when your marker is perpendicular (euler rotation about z of 1.57 radians), we have:

quaternion_from_euler(0,0,1.57) = array([ 0. , 0. , 0.70682518, 0.70738827])

So Quaternion components are labeled (x,y,z,w), so the quaternion's "z" component will be 0.707 as you saw.

If you run the following (while your node is publishing tf messages):

rosrun tf tf_echo odom base_link

You can see the transformation in both euler angle and quaternion form.