Large covariance values in /poseupdate from Hector mapping

edit

I'm trying to fuse an IMU with Hector SLAM's /poseupdate topic using robot_localization (move_base needs odometry), but /poseupdate has huge covariance values, especially on yaw, which are getting in the way:

---
header: 

  seq: 6229
  stamp: 
    secs: 1476302184
    nsecs: 887035694
  frame_id: map
pose: 
  pose: 
    position: 
      x: -0.968845367432
      y: 0.239940643311
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: 0.717722267805
      w: 0.696329481134
  covariance: [139.23130798339844, -4.342318534851074, 0.0, 0.0, 0.0, -5340.9833984375, -4.342318534851074, 93.05287170410156, 0.0, 0.0, 0.0, 718.5166015625, 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, -5340.9833984375, 718.5166015625, 0.0, 0.0, 0.0, 1196099.625]
---
header: 
  seq: 6230
  stamp: 
    secs: 1476302184
    nsecs: 912063694
  frame_id: map
pose: 
  pose: 
    position: 
      x: -0.966373443604
      y: 0.238426208496
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: 0.717676196271
      w: 0.696376964945
  covariance: [138.5482635498047, -5.373541355133057, 0.0, 0.0, 0.0, -5026.064453125, -5.373541355133057, 105.44342803955078, 0.0, 0.0, 0.0, 1032.7490234375, 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, -5026.064453125, 1032.7490234375, 0.0, 0.0, 0.0, 1170601.375]