107
545.4,M Altitude, Meters, above mean sea level
46.9,M Height of geoid (mean sea level) above WGS84 ellipsoid
(empty field) Time in seconds since last DGPS update
(empty field) DGPS station ID number
*47 The checksum data, always begins with *
If the height of geoid is missing, then the altitude should be suspect. Some non-standard
implementations report altitude with respect to the ellipsoid rather than geoid altitude. Some
units do not report negative altitudes at all.
How often do I need to collect GNSS data?
The loop closure tool works with any number of GNSS points if the timestamp associated with
the GNSS point matches the right point in the Stencil 2 trajectory. If you set the data rate to 5 Hz,
Stencil 2 saves a GPS point for every trajectory point. But even 5 or 10 GNSS points spread out
over the entire data collection will be enough to significantly reduce global drift. The loop closure
tool filters the GNSS points and selects points a specified distance apart and below a certain
position error. GNSS data does not have to generated at a high rate to be effective.
The gps_202X.XX….ply file contains large dropouts of data.
This can be caused by GNSS systems that do not transmit data or transmit dummy values when
the fix quality is too low. Data dropouts are usually okay if there is some GNSS data along the
path. See the GNSS example in this manual for more information.
Does Stencil 2 record raw GPS data when saving the raw data for post-processing (.bag
file)?
Yes, Stencil 2 records the raw NMEA strings in the .bag file.
Does Stencil 2 record a new gps_202X-xxx…ply when replaying the .bag raw data file during
reprocessing?
Replaying .bag files using the Adaptive Data Replay will generate a new GPS_xxx.ply file and
should be identical to the one generated when recording the raw data file originally if the
playback parameters have enabled: true under the receive_gps parameter.
Can I use data in a separate log file to correct global drift in a Stencil 2 processed scan?
See section on Using separately logged GNSS data in the Loop Closure Tool.
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