kaarta STENCIL 2 - Loop Closure Tool Overview

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rosrun receive_gps binex2GPS_local.sh <.ubx file from the rover> <base unit.obs file> <base
unit.nav file> <PLY output file>
Note that the <PLY output file> should include the .ply extension.
You can now use the corrected gps.ply output in the
.
USING THE LOOP CLOSURE TOOL OVERVIEW
The Loop Closure Tool performs global smoothing by introducing loop closure techniques as well
as using GNSS inputs for absolute references in correcting drift errors common in mobile mapping
systems. It is useful if there are overlapping paths in the trajectory even if you do not start and
stop at the same location, but even more so if you do.
Stencil was originally designed to work without GPS during scanning. However, the Loop Closure
Tool is designed to work with or without GNSS data. This section describes how to use the LC Tool
even if you do not have the GPS_2020-XXX.ply file collected by Stencil while scanning with a GNSS
system attached.
Good planning allows opportunities for the LC Tool to correct linear drift. For example, by starting
and stopping at the same place, you can run the loop closure tool to remove linear drift along
your path. The Loop Closure Tool tries to match the starting and ending scans to each other if you
have indicated the same starting and ending location and includes the orientation as well if you
were facing the same direction at the start and end. The amount of error corrected when a match
is found is back propagated linearly across the trajectory path.
Drift corrections can be done even if you did not start and stop at the same place. The Loop
Closure Tool looks for times when the trajectory path comes close to itself (as set by the Match
Distance parameter) to see overlapping scan frames. The LC Tool then performs a scan match
between those two frames and corrects the trajectory path between them to spread that error out
linearly between the two times it saw the overlapping data. This works when you directly cross a
path, come close to but not crossing the path, or when you go out and back along the same path.
The loop closure software fixes issues related to cumulative drift over the dataset. It will not
correct twists in the dataset and would make the data worse by correcting the twist over the
entire dataset. Twists are better dealt with by cutting the cloud into two pieces by using the scalar
index number and remerging them after taking out the twist section. This is described in the
CloudCompare tips and tricks section of this Users Guide.