Been using VRS for a couple of years now, but I don't use it for horizontal or vertical control as it seems the solutions can vary from day to day more than using my base set up nearby. The main issue I have is with post processing VRS vectors when I'm not sure about the strength of the solution or it's one of those critical points I'll be building off of and I don't want to be chasing it around every time I go back.?ÿ
It depends.
If the nearest CORS station is full GNSS and 1 mile from your site (Location A), and there is no open location to set your base then the VRS (which is essentially a single baseline to the nearest CORS) will have a better sky view and is probably much better.
If the nearest CORS stations are 15 miles from your site (Location B) and you can set your base in the middle of your job, an open field with no trees or shrubs, then your base should be better than the VRS.
If I am within 5km of a CORS and the VRS can't give me good elevations where a traditional base and rover at the same distance can it's not much use really. I suspect there is a systematic problem in the way VRS calculates its virtual base. I have only ever used Trimble VRS, how do other manufacturers compare?
I'm using Leica SmartNet and I usually get good Horz & Vert here in the DFW area. But every now and then you will have problems with both horz & vert. I think the problem lies with your cell service more than the GPS.
I only do rough staking with VRS and then it's robotic from there on out.
Plus a few years back when we were in an active sunspot cycle you definitely had to check your space weather or you will waste a day or two trying to use any type of GPS equipment.
?ÿ
I think the key is calibrating to the site. I use a Leica GNSS system with a state network. 95% of my work is residential boundary; I see a little bit of float, on control points that I come back to. But for the most part, I'm within acceptable tolerances for the suburbs.
I did an 8 lot plat, several years ago and never used anything but my rover and a public network. The base was very close and I calibrated to the site control, horizontal and vertical. The site was wide open with no obstructions. I set a bench mark in the curb across the street and checked it when I started, finished and sometimes in between and was always right on vertical and usually 0.02' or 0.03' horizontal. The plat had a 200' cul-de-sac and about 100' of storm drain and a retention pond. The site was flat so everything was at a 0.5% grade. I told the contractor what I was doing and to make sure he double checked my work. He said everything checked out perfect.
So like I said; I think you need to calibrate to local, ground control points, to get your best results.
Dougie
?ÿ
?ÿ
110% Agreed! Even running base rover its a little gamble. Fortunately GPS just isn't there vertically (At least RTK). You figure most Sanitary sewer is run at 0.40% slope that's 0.4' per 100'! if your high 0.02' on one end and .04' low on the other you jus graded your sanitary below minimum specs. These numbers are normal case situations also for GPS . Granite the contractor is gonna dial in his laser at the correct slope and may ignore the slop, but he also may call you out on what's going on. That's embarrassing and you know how we surveyors do not like to be called out on our work!!!!
It was shocking to witness what was staked with GPS, especially some time ago when the elevations were really wonky. I had a couple of contractors that wouldn't let me stake with GPS cause they had bad experiences with it. I had to convince them that I would only use it for rough staking applications.
If you are lucky enough to live in Washington and have WSRN available with more CORS than you can shake a stike at:?ÿ if you dont like VRS, do single base.?ÿ
For construction staking, we prefer base and rover. There are a few reasons, but having the same sensor type in essentially the same location simply works best on every level. (Except you have to set up a base, make sure it doesn't get stolen, make sure no one moves it, and double your investment in equipment.)?ÿ
?ÿ
?ÿ
Yes, GPS is not the right tool for minimum slope Sanitary Sewer. And if you know an engineer that designs most of their sewers at minimum slope, please educate them on the reality of the people installing this stuff.
If vertical is critical, you still need to run a level through it. Period. Your robot wont do it, and neither will your $50k GPS. Yes, that means that all those single man crews out there are not following best practices, for the most part.
My only caveat is that there are things that can be done with the robot (checks, etc) that can make it work.
We would set the sanitary with GPS, then run levels. With Carlson SurvCE, you can run the levels in the DC, and have it associate the elevations with the point you are staking. It will run a report to let you check the two against each other, then assign the level elevations to the store point. Then filling out the stakes is easy.
The VRS system in NC is great but the system itself has error in it, in addition you have mobile data timing delays that cause issues and more. I'd agree that horizonal is good, vertical you can still expect (hopefully get better than) 0.1.?ÿ?ÿ
What does that mean "has error"? Don't you calibrate/localize? Is it not consistent?
In Florida,?ÿ FDOT recommends not using network elevations especially if more than 8 miles from stations
Localizing has zero to do with RTN precision. Localizing is transforming coordinates.... a cogo routine.
@linebender
What I imagine he is referencing is that each day on a construction site the RTN is calibrated to local site control, thus shifting the daily error out of the RTN. I would rather not do that myself, but I've seen similar work flow for construction staking crews.
You can't calibrate precision error out of a RTN. period. Unless you calibrate every epoch.
@linebender
Well, it's not something I would do, but we don't have RTNs so I've never tested them. I would imagine they would be nice to use to get a site control point in, then set-up my base over it, instead of an autonomous position.
