What means 'ground truth'

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Thanks for the reply ahendrix. What absolute origin of coordinates does groundtruth refers to in gazebo? And what should be in a real SLAM enviroment? The exact position known by the observer? Do you know what i mean?

I'm not sure where gazebo puts the origin for the ground truth frame. You should be able to figure it out by visualizing it in rviz.

On a real robot, ground truth can be provided indoors by a highly-accurate motion capture system such as a Vicon, or outdoors ground truth can be obtained through differential GPS.

Very thanks ahendrix, now its more clear! I did not find anything relevant on the web :(

I'll quibble with Austin's definition a little. Ground truth is the actual information that you're trying to estimate with a sensor. For odometry, it's the actual position of the robot, delivered by a simulation or estimated (with high accuracy) by high-accuracy sensors...

... Technically, no sensor can give you ground truth, since they're making measurements, but good ones (like the ones Austin mentions) can give you measurements that are close enough to ground truth that they can be treated as being such.

@ahendrix did u try with motion capture/stereo for ground truth?