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From an issue I submitted to Hector SLAM's Github page:
We are violating the specification of the geometry_msgs/PoseWithCovariance message here. What is output in the poseupdate message is not a covariance, but the Hessian calculated during the scan matching process (see ScanMatcher.h:184). It can be interpreted as the inverse of the covariance matrix up to a constant factor that depends on the variance of the distance measurement of a single laser ray and the map resolution.
The Hessian (or information matrix) has the advantage that its diagonal elements go to zero for maximum uncertainty instead of having to represent this situation with -1 like in other occurrences of variances in ROS.
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