Georeferencing

Georeferencing trajectories and metric maps is implemented in the mola_sm_loop_closure package.

The concept of using simple-maps as intermediary map format together with the layered metric map format (see [BC24]) enables embedding georeferenced coordinates to any kind of map typically used in robotics: grid maps, voxel maps, point clouds, etc.

1. Frames for geo-referenced maps

When working with ROS tf (transformations), MOLA packages use the following frame convention, which extends the standard REP-105 with additional enu and utm frames:

These are the existing frames:

• base_link: The robot reference frame. For ground vehicles, normally placed at the center of the rear axle.

• odom: The arbitrary origin for odometry measurements.

• map: The origin of the reference metric map used for localization.

• enu: For geo-referenced maps, the North (y axis), East (x axis), Up (z axis) frame for which we have reference geodetic coordinates (latitude and longitude). Different maps built in the same zone will surely have different enu frames, since it is defined by collected GNSS measurements.

• utm: The origin of the UTM zone in which enu falls. Unlike enu, it is independent of the trajectory followed while building the map.

And this is who is responsible of publishing each transformation:

• odom → base_link: Odometry module. High-frequency, accurate in the short term, but drifts in the long term.

• map → odom: Localization module, which corrects the odometry drift.

• enu → {map, utm}: Published by mrpt_map_server (github), if fed with a geo-referenced metric map (.mm) file.

2. How to build a georeferenced map

First, build a simple-map from a dataset or a live robot as described in Tutorial: build a map. Make sure of having a GPS (GNSS) sensor source emitting observations, and that they were captured by MOLA-LO (see the corresponding variable in the LO pipeline).

Then, build the corresponding metric map by applying a metric map generation pipeline (see [BC24] or this step in the tutorial:

# Build metric map (mm) from simplemap (sm):
sm2mm -i datasetWithGPS.simplemap -o myMap.mm -p sm2mm_pipeline.yaml

Now, to find out the optimized map-to-ENU transformation and write it into the map file, use:

# georeference it:
mola-sm-georeferencing -i datasetWithGPS.simplemap --write-into myMap.mm

Alternatively, the georeferenciation metadata can be also stored, independently of a metric map, in an independent file with:

# georeference it:
mola-sm-georeferencing -i datasetWithGPS.simplemap --output myMap.georef

Full CLI reference

USAGE:

   mola-sm-georeferencing  [-v <INFO>] [-l <foobar.so>]
                           [--horizontality-sigma <1.0>] [-o <map.georef>]
                           [--write-into <map.mm>] -i <map.simplemap> [--]
                           [--version] [-h]


Where:

   -v <INFO>,  --verbosity <INFO>
   Verbosity level: ERROR|WARN|INFO|DEBUG (Default: INFO)

   -l <foobar.so>,  --load-plugins <foobar.so>
   One or more (comma separated) *.so files to load as plugins, e.g.
   defining new CMetricMap classes

   --horizontality-sigma <1.0>
   For short trajectories (not >10x the GPS uncertainty), this helps to
   avoid degeneracy.

   -o <map.georef>,  --output <map.georef>
   Write the obtained georeferencing metadata to a .georef file

   --write-into <map.mm>
   An existing .mm file in which to write the georeferencing metadata

   -i <map.simplemap>,  --input <map.simplemap>
   (required)  Input .simplemap file

   --,  --ignore_rest
   Ignores the rest of the labeled arguments following this flag.

   --version
   Displays version information and exits.

   -h,  --help
   Displays usage information and exits.

3. Georeferenced trajectories

Once you already have a trajectory file in the local map frame of reference, for example, as generated by MOLA-LO in TUM format, and after georeferencing the generated map as shown above, you can use the CLI tool mola-trajectory-georef to convert it into geodetic coordinates, for example in KML format suitable for visualization in Google Earth.

Full CLI reference

USAGE:

   mola-trajectory-georef  -o <path.kml> -t <traj.tum> [-g <map.georef>]
                           [-m <map.mm>] [--] [--version] [-h]


Where:

   -o <path.kml>,  --output <path.kml>
   (required)  The name of the google earth kml file to write to

   -t <traj.tum>,  --trajectory <traj.tum>
   (required)  Input .tum trajectory, in map local coordinates

   -g <map.georef>,  --geo-ref <map.georef>
   Input .georef file with georef info

   -m <map.mm>,  --map <map.mm>
   Input .mm map with georef info

   --,  --ignore_rest
   Ignores the rest of the labeled arguments following this flag.

   --version
   Displays version information and exits.

   -h,  --help
   Displays usage information and exits.

4. Georeferenced maps in mm-viewer

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