VRS for construction staking?

I agree with him. I have found vertical on VRS to be unusable even within 20km of a station.

I have run a VRS for a couple of years and absolutely agree with him. If I have to do anything with tight vertical tolerances I use the robot or base & rover.?ÿ

I often run control for our large mobile LiDAR projects and I can tell you VRS is fine for horizontal but is terrible for elevations and will never match OPUS/RTX. We always roll with Base/Rover then run levels and us a TS for all our check shots.

What they've said.

Agreed. RTK or static to get good elevations. 0.05 m is the cutoff for me. If I need 0.03 m (0.1 feet) or better, I use local base RTK or static.?ÿ

Our experience with NC VRS is elevation spread is near 0.1' whereas having a local base generally yields much tighter results.?ÿ We use VRS all day long for rough grade operations on the dozers, but all fine grading is robotic control on the motor graders or using topcon millimeter augmentation on GPS.

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You should test this yourself, no doubt many of the responders have done it. I don't have a VRS but we continually test our GPS, robots and levels to determine the correct tool for the job. Much of that testing can be incorporated into existing workflows.

We static in from 2 or 3 controls to establish where we are, and as a check compare OPUS results. Then use base/rover on wide open stuff, and total station/levels for more difficult environments.

VRS works fine for vertical except when it doesn't.?ÿ

I agree with the others, the vertical just isn't that good.?ÿ About twice the error as a base rover.?ÿ I figure base rover might be with in 0.03 to 0.05' vertical and VRS double that error.

It seems the positional accuracy of the receivers has gotten better with the newer models. For RTK, the vertical accuracy for the Topon Hiper II was listed as 15mm+1ppm, so a baseline of say 20km, for VRS, the vertical accuracy would be 35mm and that is probably +/- which would mean worst case is the elevation between two points could be 70mm.?ÿ With the newer Hiper VR, the vertical accuracy is listed as 10mm+0.8ppm, so over the same baseline the potential error would be +/- 26mm. Now it is not clear if the positional accuracy is taking in account as base/rover setup or VRS.

Even RTK would once in a while give a false positive result and those usually have short baselines.

I have found Elevation Certificates based on a VRS benchmark to often be around 0.4 of a foot out. But its cheaper, so what's the problem?

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.

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

Posted by: @mark-silver

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.

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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

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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!!!!