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USING N ON- GNSS D ATA FOR L OOP C LOSURE
There are other methods of collecting data for use in the loop closure tool. For example, Tracking
Total Stations uses a prism affixed to the top of Stencil 2 to provide position information of the
device that can be used to correct any drift effects due to mobile scanning. Typically, these are
Line-of-Sight applications, but scenarios exist where you may lose and re-acquire Stencil 2
position when going into and out of rooms along a corridor, for example. Aligning the X, Y, and Z
positions of the prism to the X, Y, and Z positions of Stencil 2 is done using the loop closure tool
similarly to what is described in the section above on using separately recorded GNSS data.
A typical file containing position information from a Total Station might look like:
100,4999.967,6925.890,101.001,2/1/2018 10:41:05 AM
101,4999.973,6925.884,100.993,2/1/2018 10:41:06 AM
102,4999.971,6925.886,100.991,2/1/2018 10:41:07 AM
103,4999.971,6925.898,100.987,2/1/2018 10:41:08 AM
104,4999.964,6925.904,100.977,2/1/2018 10:41:09 AM
. . . .
233,5083.435,7013.203,101.179,2/1/2018 10:43:20 AM
234,5082.761,7013.233,101.126,2/1/2018 10:43:21 AM
235,5081.990,7013.079,101.140,2/1/2018 10:43:22 AM
The information here is in the format of index, X, Y, Z, date, and time. This data would need to be
converted to the gps_2020-XX…ply format and the time offset determined to line up the X, Y, Z
data from Stencil 2 and the X, Y, Z data from the Total Station.
If you do not know the coordinate convention of the total station, collect a dataset with the
Stencil 2 moving in 3D. Load both trajectories into CloudCompare to find the correspondence
between Stencil’s XYZ and the total station XYZ. By only moving in a straight line, the loop closure
problem becomes under constrained and may give you a solution that is flipped 180 degrees. It
needs a 3D trajectory to be well constrained. Flip the sign on the Total Station X or Y data if the
pointcloud_loop_closed point cloud is inverted in Z. Flipping the sign on Z only moves the
inverted cloud up and down vertically.
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