# AMCL Covariance : define a "good" covariance

Hello,

hello,I am using AMCL to localize my robot. When i suscribe to `/amcl_pose`

, I get some `geometry_msgs::StampedPoseWithCovariance`

.

I read a lot about AMCL and covariance and I understood what lines of the matrix represent (Position X,Y,Z and Rotation X,Y,Z) => Only position(X,Y) and Rotation(Z) are non null/usefull as my robot is not flying. This thread which contain usefull informations.

I now hace 2 questions and found only partial informations:

1) Are the covariance sent by `amcl_pose`

relative to the last update (eg. at T1 covariance is relative to T0)

2) What define a "good covariance"? A threshold on values? How to determine it?

Thank you by advance !!