Hi Garth,
Thank you for some very useful data. Mine own experience in archaeology
fully supports your findings. Like most things in archaeology, money is scare,
time is plentiful :-) So when you talk about getting sub-metre accuracy using
something called "iGS3", my interest peaked :-) However, despite a reasonably
thorough Google, nothing relevant turned up. What is a "iGS3" please?
Thanks in advance.
Ray Carpenter
Chapel Archaeology.
-----Original Message-----
From: Qgis-user [mailto:[email protected]] On Behalf Of Garth
Fletcher
Sent: 27 May 2020 04:29
To: [email protected]
Subject: Re: [Qgis-user] wishing for accurate lattitude/longitude from, a cell
phone
I gathered some field data using my Garmin eTrex 20.
It was placed in a fixed location in a small field and 17 readings were
taken (using Mark Waypoint) at various intervals between 11 AM and 10 PM
(EST). The Garmin generally estimated the error band at 3 to 3.6 m. The
Garmin was receiving both US GPS and Russian GLONASS satellites, plus
WAIS (as indicated by a "D" in the Garmin's satellite signal bars).
The same location was accurately measured with a dual-freq iGS3 by
recording during the first ~ 6 hours and post-processing via CSRS-PPP
which resolved to a 95% probability error ellipse of 2.4 cm x 1.2 cm.
The CSRS report indicates that on average 10 to 11 satellites were being
tracked (range of 8 to 13). The Garmin's display indicated a similar
number of satellites being tracked.
The Garmin waypoints were plotted on a 1 meter grid, along with the
precisely determined location. This plot can be downloaded from
<http://www.Mason-NH.org/Specials/GARMIN_TESTS.pdf> (21 KB).
Worst case Garmin errors are > 6 m, close to twice the Garmin's
estimates - see above linked plot.
I am not convinced that averaging any couple such readings can do much
to increase the confidence level...
My conclusion is that if precision better than ± dozens of feet is
needed for a number of points (such as in an archeological dig), simple
hand-held instruments such as the Garmin (or cell phones) are not
suitable.
Using base/rover pairs, such as discussed earlier, is a much more
effective solution, albeit considerably more costly (~ 1500 USD). They
have the advantage of rapidly providing precise differential
measurements - which are what are all that is needed to map a site.
High position accuracy can be achieved with a single dual-frequency GNSS
receiver, such as the iGS3 I use, but at the cost of long duration
observations at each point. In my experience an observation time of at
least 30 minutes is needed (in my wooded environment) to achieve ± 1
meter, and of 1 or more hours to achieve sub-meter accuracy in absolute
positional accuracy (as opposed to relative positional accuracy).
--
Garth Fletcher
_______________________________________________
Qgis-user mailing list
[email protected]
List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
_______________________________________________
Qgis-user mailing list
[email protected]
List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
