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CHAPTER 5
C A S E S T U D I E S
C A S E S T U D Y 1 : L O O P C L O S U R E M A P P I N G W I T H O U T G N S S D A T A
When mapping over long distances, global smoothing using loop closure techniques can fix
double registrations in maps caused by drift. This can be seen sometimes be seen when Stencil 2
sees the same 3D structure at different times in a single scan. This could be caused by enough
drift occurring between the sightings that Stencil 2 did not fix the issue through implicit loop
closure (for errors less than
¼
meter) or by the localizationDecayDis or mappingDecayDis
parameters set to
“forget”
earlier scan data for the purposes of matching back to the map after a
certain distance.
One method to limit drift during scanning is to localize against a loop-closed map to create
another map registered to the loop-closed map but with more detail and no drift.
This method corrects data after scanning. Choose a Mapping Mode (use with camera if needed)
to build a map up to several kilometers. Try to revisit the same point every few hundred meters.
If driving, move slowly (< 10m/s; 20-25 mph) when revisiting same point. Starting and stopping at
the same point will also help when creating longer maps.
If you started and stopped at the same point during scanning and the trajectory shows a gap
between the start and end, you will need to fix this in loop closure. By colorizing the point cloud
by time or scalar index, you can look at the overlapping paths and verify that all 3D structure is
registered correctly to each other for each time at that point. If you do not see any double
registrations and there is no gap between the starting and ending points, then you do not need
to run loop closure.
If you do see issues, measure the XY distance and the Z distance between the two distinct
registrations. Double-click the Loop Closure icon. A detailed description of the loop closure tool
and the various parameters are given in the GPS section of this manual.
If you see a large gap (>4 meters) on a map that you started and stopped at the same place, use
the default parameters and select the “End at start point (and Keep Orientation if applicable)”.
This will remove the large gap and apply most of the drift corrections back along the entire
trajectory. Using this result, you can then rerun Loop Closure tool on the corrected map with
smaller patches to get the inter-trajectory crossing points to line up better.
If you are only correcting smaller misalignments in the map, you can use the XY and Z
measurements earlier to help in setting the parameters for loop closure. Set the match distance
to a value that is about 50% higher than the distance between the registrations. Remember to
