This discussion is interesting and very helpful. Some remarks from our experiences in a densely forested region with narrow valleys and steep slopes, lots of water floating in creeks and abandoned rice paddies -- water all around.
ANTENNA: The antenna of Garmin 64 is a lot quicker in receiving signals than that of the <100$ Garmin etrex10, but it was not more precise, just a lot quicker, which may be due to the chips inside rather than the antenna. The Garmin Oregon, which has an antenna similar to the etrex10 was a quick as the 64. We did not try smartphones, but a colleague who is doing similar research told us that he uses the Oregon and an iPhone with similar results. Therefore, it seems that Garmin handhelds are really not necessary. SATELLITES: We receive GLONASS and GPS, max is about 18 satellites, but this gives good results where the conditions are good. AVERAGE AND LOCATION: We got very good experiences with averaging. This means, I just measured shortly until I got enough satellites and measured after a period of at least two hours again. I repeated this on different times on other days, but after all, the measurements concentrated in less than one meter, the precision of the average as compared to the LiDAR data being under 40cm. We had equally "precise" measurements at places beneath slopes and with lots of water floating around, but the precision was in the wrong place, about 5 to 10 meters apart from where it should be. These are, as we were told, systematical errors that cannot be overcome, by any GPS. COMBINATION Solutions like the https://emlid.com/reachrs/ mentioned in this thread seem to help with at lest two receivers working. Can anybody report whether it helps to place the base station in a location far from the slopes and the waters of the broken paddy fields or creeks and then walk around with the second receiver to measure places down at the slopes? Maria > Am 24.05.2020 um 04:51 schrieb Garth Fletcher <ga...@jacqcad.com>: > > I got involved in a project to locate the actual boundary monuments > which delimit our town. The USGS 7.5' topographic maps appeared to be > in error at some locations by several hundred feet. About 30 monument > locations were involved. > > We are in rural NH which is mostly wooded, which results in a lot of > satellite signal attenuation, i.e., fewer receivable satellites. > > I did a first effort using a Garmin eTrex 20 (~ $200) which received > both the US GPS satellites and the Russian GLONASS satellites. Receiving > both is important in our high attenuation environment because it > significantly increases the number of receivable satellites. > > Those measurements supported my suspicions about topo errors, but had > error bounds in the tens of meters - not accurate enough for my purpose. > > Next I used a Bad Elf Surveyor (~ $600 + Mac iPad) to record 30 minutes > of data into RINEX files which were then sent to CSRS-PPP for > post-processing. This somewhat reduced the error bounds, but they were > still ~10 meters wide, even for some 60 minute recordings. > > > Finally I used an iGage iGS3 receiver (~ $2400) to record US GPS and > GLONASS satellites for at least 30 minutes each (up to 1 hour under > heavy foliage) into RINEX files sent for post-processing to CSRS-PPP. > This approach finally realized the ±1 meter with 95% probability I needed. > > A graph of the error ellipses for the 30 monuments as predicted by > CSRS-PPP post-processing, shown on a 1 foot grid, can be seen here: > <http://www.Mason-NH.org/Specials/Mon_Ellipses_Final_1_ft_grid.jpg> > Note that almost all are within ± 2 feet. > > > GPS satellites broadcast on two frequencies, L1 and L5. A key difference > is that the hand-held Garmin units and the Bad Elf Surveyor only use the > L1 frequency whereas the iGS3 is a dual frequency receiver (L1 and L5). > > One large source of errors is the variable signal propagation delays in > the ionosphere, which have predictable differences between the L1 and L5 > frequencies. Recording both signals allows a better estimation of, and > correction for, the ionospheric delays. > > A word about post-processing. > > I use the Canadian Geodetic Survey's CSRS-PPP processing because they > accept data from both US GPS and Russian GLONASS satellites whereas the > US Geodetic Survey's OPUS only accepts US GPS satellite data. In our > heavily wooded environment the ability to use both constellations of > satellites provides a crucial boost in performance. > > Post-processing services continuously record L1/L5 signals from hundreds > of fixed sites. This allows them to accurately model the time-changing > errors in GPS signals, primarily ionospheric delays but also errors in > the satellite orbits and their clocks. > > When RINEX data is submitted, the service can look at its > contemporaneous data from fixed receivers to model the errors at the > time and location of the RINEX recording and thereby apply corrections. > > CSRS-PPP offers 3 levels of correction: ultra-rapid (90 minutes), rapid > (24 hour) and final (2 weeks). Which is applied depends on how long you > wait between recording the RINEX file and submitting it for post-processing. > > The ultra-rapid will use the 90 minutes of data preceding the recording; > the rapid will use 24 hours of data, and the final will use 2 weeks of > data in calculating the corrections. More data gives better models and > corrections. In my experience the differences between 24 hour and 2 week > corrections are minor compared to my ±1 m target accuracy. > > > A fixed + rover approach is the gold standard for accuracy. This > technique uses 2 receivers - one fixed for the duration and the other > "roving" around to the various locations to be measured. As long as the > two stations are fairly close they will be affected identically by the > same GPS errors which can then be cancelled out. The cancelling can be > done in real-time if the fixed station broadcasts its data to the rover, > or it can be done by post-processing. Of course using two receivers > doubles (or more) the equipment cost.-- > Garth Fletcher > > _______________________________________________ > Qgis-user mailing list > Qgis-user@lists.osgeo.org > List info: https://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
