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
