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FrancisH, post: 419396, member: 10211 wrote: huh? aren’t the data for RTK and static sessions coming from the same sets of satellites?
only difference between RTK and static sessions is the RT in RTK —> real timeSurveyors use GPS for relative positioning. RTK ties to local benchmarks will give you heights relative to the datum of the established benchmarks. OPUS results will give you heights relative to the datum(s) of the CORS. In both scenarios you need to understand the appropriate way to use the current Geoid in establishing orthometric heights.
Your accuracy will vary based on occupation times and vector length to the stations you are “tying”.
If you’re interested in using GPS for heights, check out NGS’s Height Modernization papers, this excellent paper https://www.ngs.noaa.gov/PUBS_LIB/NGSRealTimeUserGuidelines.v2.1.pdf, and above all, find out how good the Geoid model is in your area. I see you’re in Indonesia right now. I have no idea how good the Geoid is there. Even with a very poor Geoid, you could still use GPS (geodetic) heighting techniques over smaller areas.
FrancisH, post: 419396, member: 10211 wrote: huh? aren’t the data for RTK and static sessions coming from the same sets of satellites?
only difference between RTK and static sessions is the RT in RTK —> real timeIf my base is setting on and holding a stable first order bench mark and I RTK a point a mile away, set on it and RTK a second first order bench that resulting adjusted elevation will be better than a hundred hours of OPUS will produce. Not that ive ever done a 100 hour session, but I’ve done just what I described and compared them with OPUS and the RTK elevation has always been superior, the reasons why should be self-evident.
FrancisH, post: 419396, member: 10211 wrote: only difference between RTK and static sessions is the RT in RTK
The difference is in using local BMs instead of CORS to constrain the vertical. In my area OPUS-derived ortho heights aren’t very reliable because land subsidence isn’t adequately reflected in the CORS heights. One local CORS is about 8 cm below its published height.
MightyMoe, post: 419408, member: 700 wrote: If my base is setting on and holding a stable first order bench mark and I RTK a point a mile away, set on it and RTK a second first order bench that resulting adjusted elevation will be better than a hundred hours of OPUS will produce. Not that ive ever done a 100 hour session, but I’ve done just what I described and compared them with OPUS and the RTK elevation has always been superior, the reasons why should be self-evident.
The RTK tie (or even better, a static tie) to a local bench mark will be better for getting you on that bench mark’s datum than just using an OPUS result and adding the Geoid. But 100 hours of OPUS would give you a very repeatable answer on whatever OPUS + the Geoid turns out to be. Sometimes that might be what you want.
I think your point here is that the current Geoid model in your area must not match the local bench marks “nuts on”. So yes, in that case, you’ll never get there with OPUS + the Geoid.
FrozenNorth, post: 419410, member: 10219 wrote: The RTK tie (or even better, a static tie) to a local bench mark will be better for getting you on that bench mark’s datum than just using an OPUS result and adding the Geoid. But 100 hours of OPUS would give you a very repeatable answer on whatever OPUS + the Geoid turns out to be. Sometimes that might be what you want.
I think your point here is that the current Geoid model in your area must not match the local bench marks “nuts on”. So yes, in that case, you’ll never get there with OPUS + the Geoid.
The newer Geoid models do quite well, what doesn’t work is OPUS. Disconnecting the model from OPUS is of course the key to better elevation results using GPS. OPUS is getting better, but from the time it became available its been obviously inferior to the bench mark system.
MightyMoe, post: 419431, member: 700 wrote: The newer Geoid models do quite well, what doesn’t work is OPUS. Disconnecting the model from OPUS is of course the key to better elevation results using GPS. OPUS is getting better, but from the time it became available its been obviously inferior to the bench mark system.
OPUS is probably much more accurate than running a level loop 60+ miles, so just consider where your benchmarks are.
I will state that OPUS-RS is much better for elevation than OPUS. I attribute that to several advantages: OPUS-RS can better estimate atmospheric conditions, it’s use of C1/P1 and L2/C2 ranging observations and the fact that it can incorporate up to 9 CORS into one solution.
PDOP of satellite configuration is always important to a good GPS solution, I propose that OPUS-RS has better CDOP, CORS Dilution of Precision.
Paul in PA
FrancisH, post: 419012, member: 10211 wrote: It seems that surveyors are becoming lazy to run a level loop from a benchmark to work site.
That happened when the magic boxes came along.
FrancisH, post: 419012, member: 10211 wrote: It seems that surveyors are becoming lazy to run a level loop from a benchmark to work site.
What?? Really? Lazy huh. Well, maybe you call it lazy. I call it working smarter not harder. Why run a level loop from 5 miles or more away, when GPS can give a differential elevation difference in a fraction of the time. Also, the further distance from the originating BM, the less likely that number needs to be relative. What needs to be relative are the working points on site and for that, I agree that the level loop is still needed
Mark Mayer, post: 419227, member: 424 wrote: No. It’s to facilitate the use of GIS data in the design process. Close enough is good enough.
BTW, I am confident that I can get a lot closer than 4-5 cm in the vertical using RTK. More like +/- 1 cm.
Roger that!
FrancisH, post: 419396, member: 10211 wrote: huh? aren’t the data for RTK and static sessions coming from the same sets of satellites?
only difference between RTK and static sessions is the RT in RTK —> real timeIn my mind a big difference is the length of the baseline. A typical RTK vector might be under a mile and under no circumstances would it be more than a dozen miles (In an RTN it may be more than that to the nearest base receiver but the “virtual base” will be much closer – that’s how RTNs work). Static vectors to CORS are rarely less than that and usually more like 20-60 miles in my area.
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Paul in PA, post: 419441, member: 236 wrote: OPUS is probably much more accurate than running a level loop 60+ miles, so just consider where your benchmarks are.
I will state that OPUS-RS is much better for elevation than OPUS. I attribute that to several advantages: OPUS-RS can better estimate atmospheric conditions, it’s use of C1/P1 and L2/C2 ranging observations and the fact that it can incorporate up to 9 CORS into one solution.
PDOP of satellite configuration is always important to a good GPS solution, I propose that OPUS-RS has better CDOP, CORS Dilution of Precision.
Paul in PA
When bench marks are far away I will use OPUS, actually I prefer RTX-PP to OPUS since it returns better z values, however, this discussion was about using bench marks and RTK, this technique and it is superior to an OPUS solution since it fixes the standard value. As I say this is self-evident.
When NAVD88 is no longer the standard then it will change. But as it stands now, that is the standard, and from what I’ve seen it’s a very stable and good standard, I realize this isn’t so in other areas of the country, but here NAVD88 (bench marks) has proven itself to be a more stable and accurate system of control than OPUS/CORS for elevations.
So many tasks are to be based on NAVD88 from city sewers, DOT construction, FEMA elevations, ect., why not use it?
MightyMoe, post: 419454, member: 700 wrote: I realize this isn’t so in other areas of the country
Last week I did a 4-hour session on a First Order Class I mark for the GPSonBM program. I got a note back from OPUS saying that the OPUS solution missed the published ortho height by 24.4 cm. That’s about par for the course around here.
Jim Frame, post: 419459, member: 10 wrote: Last week I did a 4-hour session on a First Order Class I mark for the GPSonBM program. I got a note back from OPUS saying that the OPUS solution missed the published ortho height by 24.4 cm. That’s about par for the course around here.
OPUS elevations have been all over the map over the years in my area. From 3cm to the 24cm that you see. The more recent EPOCH and Geoid Model have greatly improved results. I’m actually seeing elevations returned at appx. 3 cm.
Joe the Surveyor, post: 419235, member: 118 wrote: I would also agree that the bench marks on site must work within themselves, that’s gotta happen. But unless, you’re in a flood zone, the number on the elevation is mostly irrelevant.
I agree with this statement. I will often use RTK to set a couple of bench marks at a site and run a level between them and hold one for elevation and adjust the others to fit. Often you could assume an elevation and that would be good enough.
My experience is also that RTK give better results than OPUS.
RTK has a vector limit of ~5km?10km? depending on how good your radios/cell signals are between work & benchmark sites.
If distance is too far away for RTK to work then you revert to static which is 1 hour?4 hours? repeated sessions? That in itself is
1 whole day going to base/rover sites. What are your errors?1cm? You must be kidding me. Repeated static sessions have
accuracies of 2-3 cm. vertical. That is the instrument inherent accuracy. It means with the best satellite orientation available
and no obstructions then you are lucky to get within this range.
In 4 hours you could run a 5 km loop and your results would be within 1st/2nd order if you will be using a digital level.Repeated static sessions can readily have errors of less than 1 cm. That is why I do static surveying, to minimize the errors.
That is not inherent instrument accuracy, it is the accuracy of the survey algorithms.
Paul in PA
That is not inherent instrument accuracy, it is the accuracy of the survey algorithms.
Paul,
I have checked static/RTK sessions with level loops (<10km) and difference NEVER EVER came to <1 cm.
Most are in ~5-10cm mark.
This is reason why GPS height transfers are ok for general topographic work but never for flood/mean sea level requirements or
for projects that are stretched out over several phases with different surveyors.Not to mention using RTK for verticall values for road engineering surveys that run for +100km.
FrancisH, post: 419576, member: 10211 wrote: In 4 hours you could run a 5 km loop and your results would be within 1st/2nd order if you will be using a digital level.
And in 2 hours you could run a 5 km loop with a stadia board and a hand level and get a First Order closure. But in neither case are you going to be able to demonstrate First or even Second Order accuracies. Closure is only meaningful when it’s backed up by equipment and procedures.
The last Second Order Class II run I did was in January. 4 km out and 4 km back in 10 hours. There were a couple time-consuming quirks to the run, and we don’t do these often, so we’re no doubt slower than a dedicated geodetic level crew running miles and miles day after day. But geodetic leveling is so so slow that it’s rarely done anymore due to the cost.
And in 2 hours you could run a 5 km loop with a stadia board and a hand level and get a First Order closure. But in neither case are you going to be able to demonstrate First or even Second Order accuracies. Closure is only meaningful when it’s backed up by equipment and procedures.
enlighten me but how can you have 1st order closures and not have 1st order accuracies?if your forward and backward runs differ by 1st order limits then you are accurate within 1st order requirements.you are never going to get within 1st order limits using a stadia rod/hand level, if you did no one would have a need to get an NAK1 or a digital level.
I have never used a stadia rod or even a hand level, why bother when you have an automatic or digital level lying around.Repeated static sessions can readily have errors of less than 1 cm
really? the error brought about by just levelling your base/rover setups and getting your HI using a tape would introduce an error or what? 5mm??
how many times have you measured an HI and repeated it and then made your own reading at where the tape curls up when you placed it over the rod?the curled up portion already has several mm of inherent error.
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