I have added some data to my earlier report, both from my Garmin eTrex and from a Bad Elf Surveyor.
As described earlier I established an accurately measured test location (± 2.4 cm) and taken 17 readings with my Garmin eTrex 20, distributed over a 10 hour period. The eTrex was receiving both US GPS and Russian GLONASS satellites, plus WAAS (as indicated by a "D" in the Garmin's satellite signal bars). These points were plotted on a 1 meter grid with a link provided in my earlier report to the PDF graph. That graph <http://www.Mason-NH.org/Specials/GARMIN_TESTS.pdf> (56 KB). has now been updated with more Garmin eTrex 20 data plus some data using a Bad Elf Surveyor. On the updated graph the original 17 points are plotted as blue unlabeled dots. I added another 12 pairs from the Garmin eTrex, taken at the same location at variable intervals throughout a 10 hour period on May 27. The first reading in each pair was taken (Mark Waypoint) and stored, followed by a second reading in which I told the Garmin to "average waypoint" before storing. This averaging takes from 4 to 10 minutes until a bar reaches "100% confidence", whatever that means. Note that this is not Garmin's recommended use - which is to average waypoints with at least 90 minutes between. However my immediate averaging should filter out short term "noise". These points are plotted as green (immediate) and red (averaged) dots, labeled x and xA, e.g., 1 and 1A, for the immediate and averaged points, respectively. I also added thin magenta lines linking the two dots in each pair. That data indicates a ±7 meter "wander", even in the averaged data. I also set a Bad Elf Surveyor at the same location and recorded data over a 2 hour period into a RINEX file. The estimated position returned by CSRS-PPP post processing (Rapid) is plotted as a yellow star. CSRS also reported a distressingly large error ellipse: 10.8 x 8.7 meters! While the Surveyor is recording via its Bad Elf app one can also display a "Deviation Plot" which shows the wandering of the measured location, centered over its recent average (past 15 minutes). This "wander" was observed to drift slowly within a 20' (6m) radius circle; i.e., in any 15 minute period the maximum drift seen was ±20' (up to 40'), but could have been larger over longer intervals. While the Surveyor is described as being compatible with US GPS and Russian GLONASS, it can use only one constellation at a time. Consequently the Surveyor tracks many fewer satellites than the eTrex which tracks both GPS and GLONASS. This further reduces its performance in less than the optimal "360° open sky" environments. The Surveyor, like the eTrex, is also a single-frequency receiver, thus its RINEX recordings cannot be post-processed by USGS OPUS. CSRS-PPP will process single-frequency data (includingg GPS + GLONASS), but is constrained by the limitations of single-frequency data and wandering data. My conclusions: under realistic field conditions (not all satellites visible) neither the Garmin eTrex nor the Bad Elf Surveyor can be relied upon for mapping-grade accuracy. the Surveyor's RINEX capability is of very limited use due to its single constellation and single-frequency limitations. the slow "wander" in single-frequency GPS location suggests that averaging, even if done with several widely spaced observations, is unlikely to improve accuracy. It would be necessary to average many dozens of readings evenly distributed throughout a satellite orbital period (~12 hours) in order to get a reasonable sampling capable of reducing uncertainty. -- 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
