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Another reason to run the data twice is to make use of the confidence values (see previous
section) to generate a more accurate trajectory by removing a twist in path before aligning it with
the GNSS data.
L O O P C L O S U R E W I T H G N S S D A T A
Stencil records a GPS_20yy-mm-dd-hh-mm-ss.ply file whenever a GNSS device is attached (and
integrated). This file is used as the basis for using GPS in the loop closure process. GPS data
typically has various levels of accuracy, dropouts, and bad readings, especially in elevation. The LC
Tool provides some different tools to help with the process. It includes everything from manually
editing the GPS file to remove bad points, setting the elevation data to the mean value in the
dataset, filtering by Fix Quality or Dilution of Precision, setting a distance between points used,
and recalculating the time offset from Stencil and the GPS data. You could run the LC Tool once to
generate a number of GPS points which would be easier to edit for the final go round. A lot of
GNSS data is collected at 5 or 10 Hz, but you really only need data from 5 – 10 meters apart to
accurately align the Stencil trajectory to GPS.
L O O P C L O S U R E T O O L S E T T I N G S
The Loop Closure Dialog box is divided into four tabbed sections. In addition to the OK button to
start the loop closure process and the Cancel button to terminate the program, there is a
“Compute GNSS Offset Time Only” button which uses the GNSS method selected in the third tab
(GNSS) to compute just the time offsets between Stencil and the GNSS unit. By selecting different
methods and clicking this button, you can determine the best time correlation between GNSS and
Stencil trajectories before deciding on the best one to use when running the loop closure process.
The correlation for the current run is printed out once the calculations have completed. To
continue using the tool, the user will need to close this window. These methods are discussed in
detail in the section describing the options in the third tab.
The first tab allows you to select the point cloud, trajectory, and GPS (optional) files to use for the
loop closure process.
