Calculate/Specify the covariance matrix for a omnidirectional robot

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Hello,

I use an omnidirectional robot to get a map with the gmapping algorithm, but this robot gets some odometry drift from the encoder error. To solve this problem I would like to calculate or specify the covariance matrix for this robot, but how?

I read some interesting information about the covariance matrix,like that, but how do I put this into practice?

I hope you can help me.

Regards, Markus