I believe your urdf config is incorrect.
After you fixed the urdf config, you should accumulate as many range data sets as you have sensors (or even multiple thereof if the packets to do contain a full scanner revolution.)

Well, the sensor positions are fairly OK as set in the urdf (I did not yet calibrate the sensor positions. It seems that point clouds of multiple measurements are accumulated together and then joined with the map assuming that all data are originating from the same origin which is not the case.

No, cartographer_ros queries tf and transforms the data to the tracking frame.

For more help, please give more details on your config following the issue template https://raw.githubusercontent.com/googlecartographer/cartographer_ros/master/.github/ISSUE_TEMPLATE

I cloned the master branch on 2018-02-28 and executed with the default values of your backpack 2D demo. Running the cartographer_rosbag_validate returned only timing issues not important so far.

With the 4 LIDARs (in this case, onboard a car just standing in the garage), I would expect a map like this (couldn't tune to get this, so I photoshopped):

Cartographer makes the occupancy grid insertions by rays from the return point to the single tracking frame origin.
(In 3D, the insertion ray starts at the return point and is very short.)
So, what you are describing I would call a feature request to insert rays to their respective sensor positions.
Sounds interesting for vehicles with multiple sensors at either side that go through smaller passages.

Yes, that's what I mean.

If it is true that Cartographer makes the occupancy grid insertions by rays from the return point to the single tracking frame origin rather than to their respective ray origins, should we not classify this as a bug?

Just some context, the LocalTrajectoryBuilder::AddAccumulatedRangeData and the RangeDataInserters accept only one origin. (So this is not a regression, but rather a well-justified feature request.)

Is this issue as well as #1242 already fixed with the referenced merge #1357 ? We are experiencing the same problem of the wrong origin of laser rays with version 1.0.0 and think about upgrading Cartographer to a newer version

I think the answer is no. We are also experiencing the same problem as #1242 and just tested the latest cartographer/cartographer_ros yesterday to find the problem being still alive.

I also tested the current master version of Cartographer. The problem still persists. It seems not solved yet.

I am convinced, that this is a critical bug. It can be easily tested if you use range sensors mounted somewhat far from the tracking_frame, which have no overlapping field of view to this frame.

The following image is a good example. The laser is mounted in front of the robot's coordinate frame (base). The highlighted triangle is a dead spot. Cartographer says the area is free, although we don't know if that is true.

This bug is not only related to accumulating multiple sensors, but can be also reconstructed with one single laser scanner.

This does seem to be a very severe bug - any robot which has either of

• multiple lasers
• a laser that is not located at the origin of the child frame used for odometry data

will have freespace calculated incorrectly by cartographer. I think this might take a fair bit of work to fix, however, because you can no longer take the approach of "convert all data to the tracking frame and then deal with it". Instead, each scan will need to keep some information about its origin.