This is a new position2d driver "ekfvloc" that implements EKF-based localization on a vectorial map, from book
"Mobile Robot Localization and Map Building: A Multisensor Fusion Approach"
J.A. Castellanos and J.D. Tardós
Kluwer Academic Publishers, Boston, 1999
It's a good alternative when you have a very large map that would require lots of memory for the grid map, since this one uses a vector map.
The driver publishes the localization via a position2d interface and the covariance via an opaque interface. Includes documentation in the ekfvloc_driver.cc file.
The attached patch has been taken from the playerstage/code/player/tags/release-3-0-1/server/drivers/position folder with svn diff.