RTK Precision
We set a pair of points today using RTK. Each point was shot twice consecutively for three minutes. To begin with, the residuals were terrible. Something like 0.04′ H and 0.09′ V. The satellite geometry was good. I tweaked the connection using different combinations of single baseline and MRS. I also switched from primary network to secondary and back again. No significant improvement in the residuals. I was told to average the coordinates of each point. I noticed the second shot on each point was off mostly to the north. Why?
- 25 Replies
One satellite difference can be all it takes to get a large N-S or E-W difference. If you want precision you do at least 3 shots with different satellite geometry. No matter what some people say a pair of points is useless for precision. It is much, much better to set 3 points that at intervisible and can be confirmed with a set of total station observations. Your residuals were a valid prediction of what you were going to end up with. Time for a do over.
Paul in PA
Why?
Because RTK.
How long were the vectors to the network stations?
That’s not a good result, .04 x .09 translstes to .12 x .3. I would say you don’t have control points yet and you need to resolve it, no one can tell you why with the info provided, call the network provider or do a base rover to get it tight.
We set a pair of points today using RTK. Each point was shot twice consecutively for three minutes. To begin with, the residuals were terrible. Something like 0.04′ H and 0.09′ V. The satellite geometry was good. I tweaked the connection using different combinations of single baseline and MRS. I also switched from primary network to secondary and back again. No significant improvement in the residuals. I was told to average the coordinates of each point. I noticed the second shot on each point was off mostly to the north. Why?
Why? In a word, multipath. You haven’t told us anything about the site conditions, PDOP, etc. You have measured the points in various ways which seems to prove that there is nothing wrong with the network or your equipment. The thing that is left is the observing conditions.
So, you averaged multiple shots. That tactic is much more effective when the shots are spaced hours apart in time, so that the satellite geometry is different. You are still going to get huge residuals if the multipath conditions are bad, but the average (might be?) should be closer to true. No guarantees, though.
No simple answer as to why. As stated above, likely a combination of satellite geometry and multipath. Network solutions always seem to be a bit more loose when compared to a nearby base/rover observation. To get points really tight between each other the best way in my experience is to observe both simultaneously for a good 5-15 minutes, with good PDOP window, which is where a planning utility comes in handy. Then post process against a base receiver in a favorable location not too far away. If you were to do your observations during a big spike in the PDOP and/or bad multipath, doesn’t really matter what you do the results are going to be poor. By doing your observations simultaneously a good many of the random errors will be cancelled out. With RTK you’re likely to find a good 1-2 cm. floating around. Just is what it is.
Willy
@ Paul in PA
Thanks for replying! We have another day left on the job. It’s a boundary survey of a single family home in a suburban subdivision. The pair of points are situated so that half the found property corners can be observed from one point and half from the other point. If we do a third RTK session in the afternoon after observing the property corners in the morning, can the office re-calculate the property corners? I assume they can.
MH@ Jim Frame
Thanks for replying! I have no idea how long the vectors were. How do I find out?
MH@ MightyMoe
Thanks for replying! You think it might be a network problem? I normally see residuals tighten up over a 3 minute session. The residuals stayed the same.
MH
If I’m assuming correctly and your errors are referenced in feet then I don’t see a problem with your results. RTK is only reliably good to a tenth of a foot horizontal. What was the time split between observations? The purpose of multiple observations is to use different constellations. A couple of minutes does not cut it. Two hours would be the minimum with four hours preferable.
Multi- path issues or network issues seem to be the two choices. If you have clear skies, then it would be network issues, no doubt there are many traps from user errors, setting errors, network anomalies, the list is probably long.
We don’t see this with a base/rover setup; expect <.02′ horizontally using R10’s and if tight vertical is required the digital level is used. That’s RTK control points, base/rover, a few minutes, open sky, never had a bad shot on a control point with RTK and the R10’s.
Sure have seen lots of complaints about bad shots in a network.
Hey mate,
I tend to agree with John.P. above, the figures you’ve stated convert to 12mm H 27mm V, while it’s not great I’ve seen alot worse for network RTK, which seems to be the nature of the beast. Maybe you need to use a total station to achieve the accuracys you need.
How far were yo from the closest base?
You had 2 3 minute observations, what and where was your base? Can you use your base for occupying one of them? Since you say these two points are for total station observations are they intervisible? 2 15 minute observations could get you independent OPUS-RS solutions. If you do not have RTK units capable of logging field data for post processing there is little you can do later to recover from poor observations.
As to your question; “I have no idea how long the vectors were. How do I find out?” I suggest thorough education.
Your residuals suggest you may have had too few satellites visible, the only solution to too few satellites is a lot more time. You have to have enough understanding of your results in the field to know when it is necessary to invest more time on the job. GPS works in the dark and I have often stayed well past Dark:thirty so that I was not having to make a return trip.
Paul in PA
Give full details. Even post the lat lon, and someone will help you with some of them.
Dittos on the onsight base…
A base within a mile or 2 improves it.
If you want/need high accuracy, set your rtk ctrl pnts at least 500′ apart… 1000′ is better. Set 3-4 ctrl around your site. Run tight traverse, doubled angles etc. Closed loop. Least squares, (or send to Kent). Then, develop a “best fit” between total station, and rtk.
Rtk is sloppier than total station… Ie, it’s always got 0.02′ to 0.07′ error per shot, but, it’s non- cumulative.
Total sta is tighter, but it cumulates.
When used TOGETHER, it can produce wonderful results.
Understanding your tools is essential.
Also, if you always point your gps pole the same direction, it tends to eliminate pole bubble error.
If your pole bubble is off 0.02′ @ 2 meters height, and you observe ctrl station number 1, facing north, then ctrl station no. 2 facing south, you just threw 0.04′ horizontal error between these 2 pnts.
Jus a sec… Kent is calling on Line 2….
🙂
N
As to your question; “I have no idea how long the vectors were. How do I find out?” I suggest thorough education.
Reading between the lines I believe Field Dog is using a VRN. If so the base is a virtual one and the length of vectors is irrelevant. You are right that he should know that but lets give him credit for asking questions here and thereby trying to get that education.
If so the base is a virtual one and the length of vectors is irrelevant.
The OP stated that he shot the points both VRS and single-base.
I don’t use a VRS, so my knowledge of them is limited, but I’m under the impression that virtual base accuracy depends on the accuracy of the modeling, which is geometry-dependent. I would expect that if the points are outside the modeling limits of the VRS (e.g., on one side of the network or too far from the network stations), the virtual base accuracy will degrade.
If so the base is a virtual one and the length of vectors is irrelevant.
The OP stated that he shot the points both VRS and single-base.
I don’t use a VRS, so my knowledge of them is limited, but I’m under the impression that virtual base accuracy depends on the accuracy of the modeling, which is geometry-dependent. I would expect that if the points are outside the modeling limits of the VRS (e.g., on one side of the network or too far from the network stations), the virtual base accuracy will degrade.
In my limited experience VRS is much more susceptible to multipath issues because of the length of the vectors to the ‘virtual’ base solution, so to say the lengths are irrelevant isn’t exactly accurate. Our VRS system is fairly new and from personal experience I can say that the farther from the polygon being modeled I get, or if a station drops out of the network solution, the precision goes down while the effects of multipath on the rover go up. One reason I still rely on a base/rover setup for most work. I’m sure Gavin could shed some light on this.
Willy
@Field Dog did I understand correctly that you observed both pair points twice? If so, what was the actual difference between the first observation and the second observation for each point (the actual residual)? (Or is that the 0.04′ and 0.09′ value you are citing?)
Also, if you always point your gps pole the same direction, it tends to eliminate pole bubble error.
If your pole bubble is off 0.02′ @ 2 meters height, and you observe ctrl station number 1, facing north, then ctrl station no. 2 facing south, you just threw 0.04′ horizontal error between these 2 pnts.
I really like what Nate was saying until I reached the above section – then I thought… Maybe Nate just forgot to add the sarcasm smiley icon.
To the OP this method doesn’t eliminate error – it just hides it – the antenna locations may be good relative to each other but the points have ALL of the error that the rod bubble has introduced. You’d be much better off to average two shots on the point – one shot with the bubble on the north side of the rod and then rotate the rod 180 to put the the bubble on the south side. The averaged point has no bubble error.
John P is correct (especially about the vertical). I cringed yesterday when I saw some guys staking out curb with RTK in an area with minimal grade – Hello birdbaths.
Nate’s spot on about keeping a long distance between the RTK control points when you are going to use those points as the basis of total station work.
Simultaneous observations between the points will give you the best relative accuracy – (BETWEEN the points); however, a single set of observations will have unreliable absolute accuracy (location relative to the rest of the world).
Lot’s of good input here but there’s a lot more to this than you are getting here – you’ll be miles ahead if you spend the time to get good formal training.
I am doing a large topo now for a school. I set several control points using the GPS and will use for the robot where the GPS won’t work well near the buildings and trees. I then held one point as bench mark and ran a level around to all the control points. As I recall most of the points were with in 0.04’vertically and one was out 0.07′ . This is for a single GPS shot for about 20 seconds using my own base on the site, the longest baseline would be less than a 1/4 mile. I haven’t set up the robot yet so I don’t know the horizontal errors yet but I expect them to be about the same as the verticals. So looking at your numbers, maybe they aren’t out of line.
We don’t own a VRS system but I have rented one for a few weeks. My short time with a VRS system gave me the impression it wasn’t a lot worse for precision than our base rover system but maybe not quite as good.
Also, if you always point your gps pole the same direction, it tends to eliminate pole bubble error.
If your pole bubble is off 0.02′ @ 2 meters height, and you observe ctrl station number 1, facing north, then ctrl station no. 2 facing south, you just threw 0.04′ horizontal error between these 2 pnts.
Shoot points twice with the same point number, rotating the rod 180?ø in between. That should tend to average out error due to pole plumbing. If the variance between the two is as much as 0.02′ it’s time to adjust the bubble or increase the time on station. I generally get about 0.01′ “splits” with a pair of 15 second occupations taken within a minute of each other.
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