A couple of weeks ago, I participated in a roundtable discussion at the Land Surveyors Association of Washington (LSAW) annual conference on the subject of RTK Networks (RTN). Gavin Schrock, administrator of the Washington State Reference Network (WSRN), did a good job of selecting a number of industry folks who’ve got personal experience with RTN to be on the panel.
I always enjoy listening to heavy RTK users about their thoughts, their procedures and how they arrived at them. We danced around a number of subjects with one being the “RTN’s biggest flaw.” My first thought was the communications link. That always seems to me to be the biggest problem with RTK in general. When it’s not working, the first thing I check is the communications link.
“Wrong,” said the panel members.
According to them, the biggest weakness of RTK/RTN is the vertical accuracy. They want vertical accuracy to be equal to horizontal. Duh, why didn’t I think of that? My only excuse is that I’m so used to expecting vertical to be 2x-2.5x worst than horizontal that I already have my expectation set and don’t see it improving until we have a lot more satellites in orbit that will bring very low VDOP values. But I guess if I really think about it, vertical accuracy is the Achilles heel (well, maybe behind the line-of-sight limitation).
It was great to hear thoughts from real-life RTK users. Two panel members in particular espoused the value of RTK/RTN in their operations.
Douglas Casement, PLS, a solo land surveyor using a Leica receiver on the Leica Spider Network, talked about the efficiency of RTK/RTN and doing projects in a half-day that would have taken a couple of days using conventional surveying equipment with a two-man crew.
Mike McEvilly, PLS, works for a surveying/engineering firm in Washington State. He uses the WSRN for RTK corrections. He talked about using RTK on most of their projects in one way or another with the limitation being the vertical accuracy on some projects. I asked him if he had any problems with “brownouts” (lack of satellites), he said he didn’t, but then I found out he is using GPS+GLONASS receivers.
Larry Signani, PLS, is responsible for the geodetic framework behind three RTNs in Washington State. He talked about how he constrains the networks and ties them into the National Spatial Reference System (NSRS). This is the behind-the-scenes grunt work that really makes an RTN perform. It really makes me wonder how other RTNs handle this.
Gavin spoke a bit about procedures and the testing they’ve done, with RTN rovers, on NGS Calibrated Baselines (CBL) during the life of the WSRN. They’ve got a myriad of data that they’ve collected and used to develop their RTK operating procedures. It’s fascinating to look at the data they’ve collected…that’s another article altogether, but I will share with you a slide that summarizes their RTK field practice.
There’s always been a lot of discussion about RTK procedures and occupation times. Last year, I wrote an article called “What’s Your Occupation Time?” that garnered quite a few e-mail responses. I want to address that subject again in the next couple of months.
In the meantime, for those who haven’t read it, an extensive report was published by the UK Survey Association regarding RTK performance and procedures. I highly suggest downloading and reading the report. You can download it by clicking here. I would also suggest downloading and reading the National Geodetic Survey’s User Guidelines for Single Base Real Time GNSS Positioning. Although it doesn’t agree with the UK Survey Association on the time splits (the NGS suggests four-hour time splits) for setting project control, it is the most complete “RTK User’s Guide” I’ve run across. I think it’s a must-read for any RTK beginner as well as a refresher for veteran users.
I could write a lot more about this, and will over the coming months. I’d love to hear about your RTK field procedures and how you arrived at them. E-mail me at email@example.com and let me know your procedures for setting control and topo surveying.
Thanks, and see you next time.
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Edit: Link updated to User Guidelines for Single Base Real Time GNSS Positioning. Previous link was to a draft version of the document.