L1 only is perfectly adequate for all the close observations. I have even seen cases were an L1 only observation came out at higher quality than L1/L2.
I have never ever tried to do any GPS work with only a single receiver because I want to get the best data I can for my time investment and that requires more than 1 unit.
Paul in PA
MightyMoe, post: 347778, member: 700 wrote: The rules for static surveying have always included the connection between control points, for this example Kent accomplished that with conventional surveys, a bit of an exhausting exercise, but acceptable.
Actually, there's no way that GPS ties would have accomplished the same thing that the conventional ties between control points did. Keep in mind that the standard error of an EDM distance is barely +/-2mm. GPS ties between control points would have been a very inefficiient way to try to get the results I did.
The only thing Paul and I are saying is that it's a hanging figure, probably just fine, but without a second fixed point one can't really publish as NAD83 (2011), which I doubt is the point of the survey.
I think you may want to rethink what you consider surveying to amount to if you regard 7.7km long GPS vectors surveyed from the best positioned CORS antenna in Central Texas to be "hanging". The only way it is uncertain is if the CORS antenna TXAU is unstable, and that is a mouse click to verify. This is pretty elementary stuff, gents.
Well, let's not lose sight of the fact that the results speak for themselves in that the standard errors WRT NAD83 of all of the coordinates positioned by the survey are under 0.01 ft. If a person is interested in much better than that without conventional measurements, he or she is probably in for a surprise.
Considering that the control points have to be occupied with the total station anyway to tie other features, trying to do it all with GPS is even more inefficient.
Kent McMillan, post: 347812, member: 3 wrote: Actually, there's no way that GPS ties would have accomplished the same thing that the conventional ties between control points did. Keep in mind that the standard error of an EDM distance is barely +/-2mm. GPS ties between control points would have been a very inefficiient way to try to get the results I did.
I think you may want to rethink what you consider surveying to amount to if you regard 7.7km long GPS vectors surveyed from the best positioned CORS antenna in Central Texas to be "hanging". The only way it is uncertain is if the CORS antenna TXAU is unstable, and that is a mouse click to verify. This is pretty elementary stuff, gents.
Yes it's elementary, for painfully obvious reasons a static survey REQUIRES at least TWO fixed points in the system you survey in.
DOT would reject it for that reason, one of the first items taught in static surveying 101 class.
MightyMoe, post: 347818, member: 700 wrote: Yes it's elementary, for painfully obvious reasons a static survey REQUIRES at least TWO fixed points in the system you survey in.
DOT would reject it for that reason, one of the first items taught in static surveying 101 class.
This may be contrary to what the engineers at the DOT are telling you, but static surveying can be done perfectly well from one point. I think you probably have situation in mind where the coordinate system of the control points has some ill-defined relationship to ITRF and you need at least two points to solve that relationship.
In the case of a CORS antenna, there is, of course, no such problem and, in any event, the effect of non-identity of NAD83 and ITRF over vectors under 10km in length is trivial for any conceivable DOT-type purpose. I'm a bit surprised that this is news in any way.
Kent McMillan, post: 347821, member: 3 wrote: This may be contrary to what the engineers at the DOT are telling you, but static surveying can be done perfectly well from one point. I think you probably have situation in mind where the coordinate system of the control points has some ill-defined relationship to ITRF and you need at least two points to solve that relationship.
In the case of a CORS antenna, there is, of course, no such problem and, in any event, the effect of non-identity of NAD83 and ITRF over vectors under 10km in length is trivial for any conceivable DOT-type purpose. I'm a bit surprised that this is news in any way.
For this survey you SAY you are using a CORS point, but there is no way of KNOWING that, since you are processing these points yourself there is no way anyone KNOWS this.......
Therefore, the uncertainty of that point is high with respect to the state plane system. That's ONE important reason to include more than one known point. Simply put it helps eliminate a bunch of possible errors such as Imputting the coordinates with a typo error.
But surely you know this, it's a major reason for issues with RTK surveys. It's part of the rules for static surveys, you don't see OPUS hang a survey off one point.
The question is 0.01' relative to what?
Precision does not equal accuracy. Personally I would happier with each of my GPS points 0.02' from multiple CORS than precisely 0.01' from some single point somewhere.
Paul in PA
MightyMoe, post: 347826, member: 700 wrote: For this survey you SAY you are using a CORS point, but there is no way of KNOWING that,
So, once a person starts to use RTK, does he or she forget all about how to determine whether a CORS site is actually a CORS site? (I assume that you didn't really mean what you posted above, but I can't resist the comic opportunity you've provided.)
Paul in PA, post: 347828, member: 236 wrote: The question is 0.01' relative to what?
Uh, in this case the L1 Phase Center of TXAU?
Kent McMillan, post: 347833, member: 3 wrote: So, once a person starts to use RTK, does he or she forget all about how to determine whether a CORS site is actually a CORS site? (I assume that you didn't really mean what you posted above, but I can't resist the comic opportunity you've provided.)
Lol, you are making the same argument made in static class from 20 years ago..rejected by then instructor then as it is now.
Just go get another point into your survey, it will be good for the stats........
MightyMoe, post: 347838, member: 700 wrote: Lol, you are making the same argument made in static class from 20 years ago..rejected by then instructor then as it is now.
I guess your "instructor" never considered that the coordinates of a CORS antenna are easily checked, eh? I mean, he or she had a point when dealing with monumented passive control points that might be subject to disturbance, but that would have been before there was a reliable active CORS network to use.
MightyMoe, post: 347838, member: 700 wrote: Lol, you are making the same argument made in static class from 20 years ago..rejected by then instructor then as it is now.
I guess your "instructor" never considered that the coordinates of a CORS antenna are easily checked, eh? I mean, he or she had a point when dealing with monumented control points that might be subject to disturbance, but that would have been before there was a reliable CORS network to use.
dmyhill, post: 347770, member: 1137 wrote: ... the reality is that using our RTK with the WSRN produces results without useful differences from static ....
Well you're always free to post examples in support of that claim, but right now, the ante is a standard error of +/-0.01 ft. in N and E as demonstrated by conventional ties between points.
MightyMoe, post: 347458, member: 700 wrote: Urban control? Lots of it. There are rules for control surveys. I follow them, no exceptions..........
Could you elaborate on that? I've seen some "minimum standards" and a few "suggested procedures" but never anything that rises to "rules for", and I'd very much like to.
Kent McMillan, post: 347834, member: 3 wrote: Uh, in this case the L1 Phase Center of TXAU?
Kent, I don't think anyone is attacking your work. I think they are discussing best practices. What (and I think respectfully) people are trying to point out, is that without a second base to produce baselines, your precision and accuracy from TXAU is derived from a geometrically weak sample comparing only itself to itself.
All I see is that many of us advocate including a second baseline from some source that would enable a more robust assessment. The method of obtaining that second base is described in different ways, from using CORS, or simply dusting off an extra receiver.
There is no doubt that your work is adequate, but perhaps some of us feel the need for more validation.
dmyhill, post: 347928, member: 1137 wrote: What (and I think respectfully) people are trying to point out, is that without a second base to produce baselines, your precision and accuracy from TXAU is derived from a geometrically weak sample comparing only itself to itself.
And what I was pointing out was that is an unfounded assertion where (as was the case in this example):
(a) various GPS-derived positions are highly correlated by means of high-quality conventional measurements, and
(b) some subset of GPS-derived positions are connected by vectors from two different days (or widely spaced times) to demonstrate the absence of systematic effects.
The first condition (a) is in fact the entire point of this post, i.e. that conventional measurements connecting GPS-derived positions provide an additional condition in an adjustment by least squares that both tests for blunders and significantly decreases the uncertainty in results. While various posters have approached the problem as a GPS-only problem, it obviously is not, particularly in a cluttered urban setting.
All I see is that many of us advocate including a second baseline from some source that would enable a more robust assessment. The method of obtaining that second base is described in different ways, from using CORS, or simply dusting off an extra receiver.
Well, considering that the results can be checked for blunders by the method I outlined, the only advantage to using a second CORS antenna would be to detect instability in the antenna. However, as mentioned, that is easily done simply by either:
(a) inspecting the short-term time series of the CORS site or
(b) getting an OPUS solution for that CORS on the day in question and including the time when the connection to it was made.
The idea that using two bases where both of the bases are CORS antennas with unobstructed sky view will somehow produce significantly better results when the vectors from both are solutions using the exact same rover observations strikes me as unlikely to be true for short (<10km) vectors.
That's a long subject, but at the end of the day it's about redundancy.
All the steps and checks in the field to assure when you are in the office there aren't any surprises.
For instance, we were required to measure our antennas with the big ground planes in three spots (doesn't really apply anymore with the tiny antennas of today).
Each adjoining control point having receivers sitting on them at the same time, each point having at least two occupations.
It's always best to use different types of antennas on the points if you can, mix it all up. Different tribrachs on each point if you can.
If you are using HARN monuments it's necessary to "break" the set up on each one.
Of course with CORS points instead it gets easier.
If you are setting say 40 control points along a highway and it's 3d then a level run between them at the end.
Usually a control project will need a minimum of 5 recievers, 8 if I can get my hands on them.
There are minimum time rules, we always used at least 10 minute occupations, of course there are recievers collecting longer sessions through the day.
Everything gets written down, dates, HI's ft and mt, receiver#, tribrach, start time, end time, ect........
The tribrachs are checked, and adjusted they get a stamp when they are adjusted.
It depends on what you are doing also, a tight highway survey is different than a lidar control panel survey for a 50 sq mile block of land. No one is going to pay for levels to the panel survey of course.
Then there are the calcs which are usually pretty simple, that's always the easy part.
The first condition (a) is in fact the entire point of this post, i.e. that conventional measurements connecting GPS-derived positions provide an additional condition in an adjustment by least squares that both tests for blunders and significantly decreases the uncertainty in results. While various posters have approached the problem as a GPS-only problem, it obviously is not, particularly in a cluttered urban setting.
What Kent appears to be emphasizing is that he likes to do least squares combining his individual GPS positions and his traverse points. I find that usually unnecessary as by using multiple GPS receivers my GPS software performs an LS adjustment without any specific input from me. If I can overly the GPS with my traverse within my tolerance I am done, GPS has confirmed I have met certain conditions of accuracy. Depending on the complexity of my traverse I may elect to hold my GPS elevations over my traverse elevations as I compare the care used in the field work. In some cases I may hold GPS over traverse points in some areas but not the whole project, especially if I am still adding to my work.
Currently I am prepping for a LOMA that may or may not satisfy FEMA. It is 34 acre residential lot with very significant elevation changes, 1000' to 790' to 930'. My best GPS area is at the North 930' elevation end, my dwelling is at about 800-810' area. The majority of the site is heavily wooded and a significant ridge to the South. If and when I have to traverse 3/4 mile along the driveway I am still 1000' in opposite directions from corner pins. Step two would entail substantially more work. The driveway is not fully within the lot flagpole and with no written easement. Yesterday I flagged up an iron pin and pipe 48.5' apart about halfway down the driveway that match nothing called for in that area. I know where the gas line is but not it's easement yet. I will back out for a half day of more locations and then will GPS. After that it is back in the huddle and decide to run or pass.
Well, considering that the results can be checked for blunders by the method I outlined, the only advantage to using a second CORS antenna would be to detect instability in the antenna. However, as mentioned, that is easily done simply by either:
Again Kent is missing the point, multiple CORS are used to get different possible combinations of common satellites that you do not have with a single CORS. A longer observation is to get a better handle on the OPUS-S atmospheric correction. Up to 9 CORS are used in OPUS-RS to get a better advance calculation of atmospheric correction. None of those three things can be accomplished using the Kent method.
Paul in PA
Kent McMillan, post: 347941, member: 3 wrote: The idea that using two bases where both of the bases are CORS antennas with unobstructed sky view will somehow produce significantly better results when the vectors from both are solutions using the exact same rover observations strikes me as unlikely to be true for short (<10km) vectors.
And even though it strikes you as unlikely, in practice it has been the case. I am confident that you are producing great results. Perhaps it is only my lack of confidence that is assuaged by multiple baselines.
Paul in PA, post: 347953, member: 236 wrote: I find that usually unnecessary as by using multiple GPS receivers my GPS software performs an LS adjustment without any specific input from me.
Yes. I think that most of us, in our processing software use the LSA, whatever it looks like, after deciding what to hold as our control. This likely leads us to preferring multiple baselines, simply because it helps us analyze our results, not to mention increasing our confidence. The extra step of exporting to StarNet is fine, if your post processing software doesn't support conventional measurements.
