amcl_pose echo results explanation

asked 2016-04-13 11:39:03 -0500

murdock
75 ●12 ●17 ●20

updated 2016-05-21 15:57:27 -0500

Hello,

I have amcl_pose running. After I echoed it, this is what I see. I can see that covariance consists of 36 values, so I guess it is a 6x6 matrix, however position + orientation have 7 different values. What am I misunderstanding here? How do I interpret this?

header: 
 seq: 81
  stamp: 
 secs: 1839
nsecs: 212000000
frame_id: map

pose: 
 pose: 

position: 
  x: -7.76006046064
  y: -1.15741912051
  z: 0.0
orientation: 
  x: 0.0
  y: 0.0
  z: 0.841545398193
  w: 0.540186396331

  covariance: [0.0026612296465486907, -0.0006793180004436294, 0.0, 0.0, 0.0, 0.0, -0.0006793180004454058, 0.003341409476325552, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0012125460173182706]

Thanks!

EDIT: Why are X and Y for orientation equal to 0?

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answered 2016-04-13 12:34:13 -0500

al-dev
883 ●7 ●14 ●20

The documentation of geometry_msgs/PoseWithCovariance gives some explanation :

# Row-major representation of the 6x6 covariance matrix
# The orientation parameters use a fixed-axis representation.
# In order, the parameters are:
# (x, y, z, rotation about X axis, rotation about Y axis, rotation about Z axis)
float64[36] covariance

For the conversion from quaternions to fixed-axes, see this question and the links therein.

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I thought that first (0.0026612296465486907) variable was covariance value of x axis, so if I used sqrt on it, result would be 0.0515, which shows how much robot is certain about its current position (which is ~5cm).

murdock ( 2016-04-14 07:18:45 -0500 )edit

covariance[0][0] should be the variance of the linear position x and its square root is the standard deviation, if that is what you are asking.

al-dev ( 2016-04-14 16:17:32 -0500 )edit

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answered 2016-04-13 12:39:20 -0500

pgigioli
354 ●29 ●33 ●38

Your orientation is given in quaternion coordinates which has 4 values. The covariance matrix is based on the 3 euler angles roll, pitch, yaw. You can convert between the two coordinate systems. See this post for more details.

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Hey, do you know why X and Y for orientation are equal to 0?

murdock ( 2016-05-21 16:06:44 -0500 )edit

3

@murdock Because mobile robots navigating in a flat 2D workspace do not rotate about the X and Y axis (roll and pitch). They only rotate about the Z axis (yaw). If you convert the quaternion to Euler angles like the answer suggests you'll see that more clearly.

spmaniato ( 2016-05-22 14:51:54 -0500 )edit

Thank you!

murdock ( 2016-05-22 15:39:30 -0500 )edit

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