@fobos8?ÿ
The numbers on a GNSS controller screen are only RMS values which should be repeatible 68% of the time. If you double them then close to 95% will be within that range and double again 99% will, roughly. So if controller says 10mm in horizontal CQ which is not an unusual number in good conditions then I double it in my head for most work (20mm) and triple it (30mm) for tight work. Also would never rely on a single RTK shot for any control/critical point.
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Then you move on to environmental factors. Trees/buildings blocking satellite signals and multipath from reflective surfaces near the antenna can bias a solution so you get the same wrong result twice on a point. As a graduate GNSS was the best tool in the truck but the longer I practice the more the total station comes out for urban work, rural work can often absorb the additional errors GNSS throws out.
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The only systematic win GNSS has over total station is usually a fixed pole height for rover which helps avoid pole height errors.
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GNSS use is a bit like boundary surveying for me, there is no substitute for good experience whereas with modern total stations and some good training to start you can be getting repeatible results pretty much straight away.
When checking repeatability of GNSS measurements, it is important to let some time elapse between so that the satellite constellation is significantly different and any multipath effects are randomized.?ÿ If you are using long baselines then you need hours or a day between to allow the iono/tropo to change.
I'm not condemning GNSS, but I certainly don't know much about it as you guys do. How do you get your redundancy using GNSS alone, just by occupying a point more than once? Surely that's the same as measuring a point twice with a Total Station.
It's not just about redundancy, but independent measurements. For total station work, that means breaking setup or observing a point from a different setup/backsight. In GNSS surveying that means observing under a minimum of two different satellite configurations. Wait a minimum of 30 minutes - an hour is better - and then re-observe. If you're getting repeatable results under different constellations, you're mostly good to go. Then it becomes a matter of how tight do you need those final positions - tighter is going to require more observations, or static work.
Running double bases, as some of our offices do, is a different form of independence, but still relies upon essentially the same satellite configuration, which can sometimes burn you.
Also, when your GNSS controller says you have accuracy of say 10mm, do you believe it? I don't, I've run GNSS on very tight traverses with braces and found differences of up to 60mm.
?ÿI set my job properties to 95% or 99% confidence levels. Our crews' job templates default to 95%.
I believe those numbers, in all seriousness, about 95% of the time. As others have pointed out, GNSS is a tad more fickle than a total station, but modern receivers + good practices is perfectly reliable - good enough that I'll put my stamp on it.
Now that more receivers are able to work in what used to be impossible conditions for satellite positioning, I'm definitely seeing folks push the boundaries farther than they should, while at the same time tossing out that redundancy and independent measurements. As someone mentioned upthread, GNSS has more art to it than total station work - which is why there is so much confusion about it.
@lukenz?ÿ
Of course that's why I said close to foolproof. The system that is.
The operator is a different story.
I will say that GPS systems have been the simplest instrument I've ever used. Surveying geodetically with T2 and distance meters was wwaaayyyy more difficult and involved than GPS surveying. The very first day I got my unit was an eye-opening experience. I was wary that I wouldn't understand how it all worked but it was more like, OH this is so simple.?ÿ
Even tasks such as running line, staking offsets, topo took much more thought and effort before GPS came along. Using GPS really is simple, uncomplicated,,,,,,,,, if you have the background.?ÿ
The errors I usually see and (that is rare where I survey) are usually related to calibration or someone unable to understand how the system should be used. But I've retraced townships and townships of sectional GPS surveys and found one questionable monument. That's it!! All were done with GPS and I found nothing more than that. I doubt that one was a GPS issue. And those were late 90's early 2000s GPS surveys. Not the new equipment that is so much more advanced.?ÿ
Same with local smaller boundaries, if they were GPS they are good almost universally. Doesn't mean the boundary theory is good, but the geometry is.?ÿ
It's a tool, it's simple, it's easy, it's freed me from complicated calculations, time consuming set-ups and movements, chopping lines, it's changed surveying for the better. I don't wish to do another solar, been there, done that many, many times. I don't wish to calculate areas by hand, my DC does it in the field as I survey, I don't want to spend time at lunch adjusting traverses, not needed anymore. I don't need to set up random points to run line into ravines and through timber, that's taken care of with my radio.?ÿ
And far as bad points, don't see em now, they used to be a daily occurrence, but even though we check everything it's now become a CYA (an important) task.
If you are seeing many bad fixes or large errors in your points, you must be working in way more challenging environments than I am or my equipment is more advanced.?ÿ
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I'd rather have 60 mm precision on an accurate old boundary mark than 10 mm precision on a shiny new inaccurate boundary mark.?ÿ
I'd rather have 60 mm precision on an accurate old boundary mark than 10 mm precision on a shiny new inaccurate boundary mark.?ÿ
So would I. But it isn't too much to ask to have high precision on the correct boundary mark.?ÿ
@lukenz?ÿ
Of course that's why I said close to foolproof. The system that is.
The operator is a different story.
I will say that GPS systems have been the simplest instrument I've ever used. Surveying geodetically with T2 and distance meters was wwaaayyyy more difficult and involved than GPS surveying. The very first day I got my unit was an eye-opening experience. I was wary that I wouldn't understand how it all worked but it was more like, OH this is so simple.?ÿ
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This is my perspective. The "black box" of the GNSS reduces all sorts of mistakes. But, it is also much harder to know when something went wrong, especially for us who are coming from many years of total station use, where we have a kind of sixth sense about its use.
But, with today's robots, they are just as much of a "black box" as the GNSS, so...
I'd rather have 60 mm precision on an accurate old boundary mark than 10 mm precision on a shiny new inaccurate boundary mark.?ÿ
So would I. But it isn't too much to ask to have high precision on the correct boundary mark.?ÿ
100% this.?ÿ Don't know why some people talk like this is an either/or proposition.?ÿ For heaven's sake, let's walk and chew gum at the same time.
Not the point I wanted to make. The other day I found two boundary corners constructed of #5?ÿ rebar. They were exposed above grade about as much as they were buried leaning way out of plumb. If we are going to walk the walk and chew gum should we not only be interested in surveying precisely and accurately but also constructing monuments that are stable for more than a season? Seems like the monument should have a higher importance than the precision of the measuement used to depict it on a map.?ÿ
Ok, what precision is the minimum? Where I work, it is codified. If you cannot attain that for some reason, you are asked to note the precision.
Generally, from what I can tell, throughout the years the answer to, "What is the minimum?" has correlated to that level of precision which can generally be obtained by a competent technician without undue burden (expense). If we take precision as a curve with diminishing returns, then the answer lies somewhere at the start of the steep slope.?ÿ
Personally, I would rather find an original corner 1 foot from record distance than find a pincushion of unrecorded marks within 0.5'. That pincushion are people that felt that their need for precision outweighed the realities of measurement.?ÿ
And that is where GNSS comes in. It is much less precise than a total station, call it an order of magnitude. But, it is also the same order of magnitude more accurate than traversing, if the distances exceed anything more than a small project. Sure, you can run it through star*net, but look at those error ellipses. Take a 30 leg traverse and tell me the 95% ellipse. (And do not pick the best, pick the worst one you accepted.) I would bet you they are about what you would get from GNSS. And, if you have not create a network with that traverse, then the error ellipses are a bit of guess work, and the error will not be randomly distributed.
This is kind of the point of why we like traverses. Everything fits locally. The closer we are to each closing point, the more accurate things are, but since they are precise to each other in the middle, it is all ok.
But let us not be deceived.
Classic application:
PLSS subdivision corners control a plat layout. We run a traverse and we use that balanced traverse to set street centerline monuments. They all fit each other to 0.01'.
Another guy locates the PLSS corners with GNSS and uses RTK to set all the centerline monuments. They all fit each other to within 0.1' (say +/- 0.05' on each location).
BUT...traverse guy has an error elipse of 0.15 in the center of that traverse, while RTK guy is still within 0.05' of actual location on every shot. So which one is "right"??ÿ I prefer the total station, because in practice local precision creates a better outcome for landowners (IMHO). But, that doesn't make me "right", it just means it is my practice.
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I'd rather have 60 mm precision on an accurate old boundary mark than 10 mm precision on a shiny new inaccurate boundary mark.?ÿ
So would I. But it isn't too much to ask to have high precision on the correct boundary mark.?ÿ
100% this.?ÿ Don't know why some people talk like this is an either/or proposition.?ÿ For heaven's sake, let's walk and chew gum at the same time.
It's because some surveyors equate precision work - and technical knowledge - to "button pushing".
If we are going to walk the walk and chew gum should we not only be interested in surveying precisely and accurately but also constructing monuments that are stable for more than a season?
Sure. This is the "two-bit rebar" argument, and I agree with you.?ÿ A bulldozer will destroy a concrete bound just as readily as it will a #5 rebar. I like the idea of setting offsets in a place that is safe and stable rather than dogmatically setting the actual corner in harms way. The survey using public has trouble with that. If in a non-recording state the surveyors do also.?ÿ?ÿ
If we are going to walk the walk and chew gum should we not only be interested in surveying precisely and accurately but also constructing monuments that are stable for more than a season?
Sure. This is the "two-bit rebar" argument, and I agree with you.?ÿ A bulldozer will destroy a concrete bound just as readily as it will a #5 rebar. I like the idea of setting offsets in a place that is safe and stable rather than dogmatically setting the actual corner in harms way. The survey using public has trouble with that. If in a non-recording state the surveyors do also.?ÿ?ÿ
Just had a 2-bit rebar arguement.
Plat has all the corners monumented with 3/4" iron pipes with caps. They show up on multiple surveys, including mine for about 20 years. My co-worker points out that the county will probably say we have to monument the plat corners. I remind him that they have monuments...he says, "But they aren't MONUMENTS." I shake my head and say we will fight that battle when it comes. Ancient bounds are near and dear to my heart, they hold a sacred place in the cadaster.
I believe those numbers, in all seriousness, about 95% of the time. As others have pointed out, GNSS is a tad more fickle than a total station, but modern receivers + good practices is perfectly reliable - good enough that I'll put my stamp on it.
At this point, which is more likely? A sloppy backsight setup combined with a total station on maladjusted legs, or a bad fix with GNSS.
I know my opinion on that matter. (and the answer might be different for different shops)
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Yes, neither is acceptable, but both happen, and both have happened in the experience of us all. Perhaps WE didn't do it, but we know someone who did. So, the "why can't we have both argument" is completely irrelevant. We play the odds, really. We choose that path most likely to produce a good result, but only ignorant children and politicians think that there is a such thing as a perfect measurement. We do not live in that world. We have blunders and error and imprecision and inaccuracy all around us. Mitigation, redundancy, repeatability, and check check check...this is what we live by. And all of that is why you can use either GNSS or a total station, or a right angle prism and a plumb bob with your steel tape.
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@fobos8 I am working on a Survey of a Section of land (square mile) in the Sierra Nevada Mountains, steep, rocky in places, forested. It is open enough to use GNSS. I would expect 4 miles of total station traversing to produce an accuracy of 30cm or so.
We use a lot of redundancy, running up to 6 receivers at once for multiple hours then processing and reviewing the data every evening. TBC flags questionable points, I use the point derivation report to disable problem vectors.
A few monuments can??t be observed directly so we set control pairs then turn angles and distances to the needed monument.
We use RTK but not alone, long static is needed in difficult areas. Hoping to use RTK to set line points for boundary signs, a fire burned it last year so there??s hope.
iIn the redwood forest on the Northern California coast, forget it, traversing is the only way.
One of the keys to RTK is to choose a base location which is as clean and WFO as possible. (Wide F?? Open)
