So I just received my RTK Express kit. The first thing I did was make a few laps of my property, using SW Maps to record my track with just my phone’s GPS, the RTK Express without corrections, and the RTK Express with corrections from RTK2Go (the base station is roughly 10 miles from my house):
The Blue tracks are Cell GPS (reported accuracy in SW Maps ~1-2m), Red is the RTK Express without corrections (reported accuracy in SW Maps ~.3 m), and Yellow is the RTK Express with corrections (reported accuracy in SW Maps ~0.012-0.018 m).
Since the Yellow line was the most accurate, I expected the other tracks to deviate from that according to their accuracy. I.e. the Red track should be no more than ~0.3 m from the Yellow Track, and the Blue Track no more than ~2m from the Yellow track. But that isn’t the case:
Here, the distance between the red track and the yellow track is 3 m, not the expected 0.3 m.
So, which do I trust more? I understand that the RTK corrections are relative to the base station, so the position accuracy is really only as good as the base station’s, and maybe with a free source like RTK2Go, you get what you pay for.
How accurate is the position in Google maps? If I place my antenna at a landmark I can see in Google Maps, can I judge how accurate the GNSS position is by the distance between where it shows I am in Google maps and where it shows the landmark?
Thanks for any suggestions, ideas, or information you may have!
Lots to unpack, so I’ll give some quick answers and be happy to explain anything in more detail.
“reported accuracy” from any gnss device is an estimate, it’s a guess based on many factors. Don’t put much confidence in the accuracies you listed.
GNSS has a hard time under tree canopy. You will notice your 3 tracks have better agreement in Open Sky.
Free Online Aerial Imagery is generally not expected to be geolocated with an accuracy comparable to RTK.
Take your RTK Express to the middle of that area (in the wide open), connect to the Base, and log a RTK Fixed point. Walk away and come back to the same point and log it again. Then use SW Maps to measure the distance between the 2 points. That’s going to give you an idea on the precision. You should be able to reproduce the position easily.
Thanks for the response. As you suggested, I recorded some waypoints at the center of the pasture, again with the RTK Express w/o corrections, with corrections, and with just my phone’s GPS:
This was the most I could zoom in, but you can see the five waypoints recorded with the corrections (yellow) are very precise, nearly all on top of each other, while the waypoints recorded without corrections (red) have more variation, and the waypoints recorded with just my phone (blue) are still more variable.
But I still see a bias or offset between the data with corrections and that without. I was expecting something more like this:
There is a huge range of answers to that question. I’ll start with the “General” version for simplicity.
RTK positions are considered more “accurate”, because the rover gets the benefit of the base station observations to reduce several atmospheric and timing errors that are inherent to GNSS. The RTK Express can easily reproduce positions to <1cm (precision) under the same conditions with a proper workflow.
But to define the accuracy, one needs to know the true coordinates of the point (the center of the target in your sketches) - which is not a trivial task. You will also need to know the Reference Frame and Epoch of the Base Station Position. You will need the Antenna Calibration, always point the antenna to the North, accurate Rod Height to the ARP, etc. However, you are still hoping whoever owns the Base did their job correctly.
Many times we lean towards post processing when accuracy is required. That is generally long static sessions that are later “corrected” with OPUS, NSRS-PPP, etc, a few weeks later. After many sessions and a lot of work, you can start to gain confidence in the actual accuracy of a single position.
If you verify a Base or Correction Service suits your needs, then RTK is the easy button. Just know that humans will introduce Random and Systematic errors to the process. We are generally the limiting factor, not the GNSS equipment.
Another thing to note is that 10 miles is a bit beyond the usual recommended limit for base station distance (10 km ), which will reduce the accuracy of RTK/corrected one a bit
You can perform a 24-hr survey if you’d like to try testing the maximum accuracy (and then afterward you could use that as a base for corrections!)
