What Skeeter1996 said. Why wouldn't an OPUS solution work? I understand that when the 2020 datum is adopted physical benchmarks will be obsolete. At that time an OPUS solution would seem to be the ONLY acceptable vertical datum...
That's 2022.?ÿ Obsolete is too strong a word.?ÿ Obsolescent, maybe.?ÿ
You need to use whatever datum makes sense for the particular work, not necessarily the latest.?ÿ It will take a while for flood maps to convert to a new datum, and extensions to older projects will continue to use the original datum.?ÿ There are cities and Corps of Engineers facilities still on NGVD29.?ÿ
My $.02....I would steer away from using OPUS to establish/verify elevations but that's just based on my personal experience.
Why! It doesn't get any better than OPUS. NGS even knows it has bad benchmarks and wants to verify them with OPUS.
Actually, NGS is trying to get it's system to better match bench marks, they are getting closer.
In our area the question is which NGS or USGS passive BM's best represent datum? Static surveys usually reveal they aren't very well related anymore due to stability, 80 year old leveling, longevity, etc.?ÿ NGS I'm betting would say use passive marks to access datum until 2022. In our area I'm not too sure the RTN base stations are not the best estimate to datum and most well related. But that is not the accepted access point to datum (yet).?ÿ Perhaps it should be but doesn't the cert require a passive tie? The last time I saw one it did.?ÿ
I would say the one who checked into the Benchmark is going to be right.
WSDOT (Washington State DOT) Maintains an excellent network of monuments; up and down all of the state highway corridors.?ÿ
Survey Monument Database
I subscribe to the WSRN (Washington State Reference Network) and my SOP is:
- I use my Leica Viva GNSS GS12, connected to the network and shoot a minimum of 5 shots, each, on at least 3 WSDOT mons
- I set a minimum of 3 temporary bench marks, on-site, and shoot each one at least 5 times
- I repeat on the 3 WSDOT mons
- I repeat on the 3 TBM's
- Set up the robot and shoot the 3 TBM's, collect all the appropriate data and maybe a little more
- This field procedure usually takes about an hour and a half, plus travel time
When I analyze the data, I tend to try to find a happy medium. Sometimes I throw out an outlier, most of the time the average is within a few hundredths of all of them. I don't do a lot of these, the most error I've seen is a couple tenths.
It's all about your comfort level; how comfortable are you, in saying: my elevation is the right one...
mine varies somewhat, but the idea is similar.?ÿ and prefaced with the statement that?ÿmoreso than any other exercise?ÿthe setting of control is a purposefully deliberate and slow exercise for me.
likewise, running a GS12 and per some suggestions from a regional leica guy, i'll do 5 minutes per point.?ÿ then leave for at least an hour (let the constellation move around sufficiently), come back and do another 5 (and orient the antenna differently).?ÿ repeat.?ÿ 3 shots per, minimum.?ÿ and if in the meanwhile i can shoot some corners in or run some topo or whatever- great.?ÿ if not, oh well- it's always planned, budgeted, and proposed as though it's a day's exercise (which includes the same above-described process on 1-3 known nearby points as available)- unless it's more than 5-6 points, then adjust accordingly.?ÿ day two is getting out the robot and the level and making sure all is as it should be.?ÿ 9.8675 times out of 10 it is.
> (and orient the antenna differently)
I thought you were always supposed to orient the antenna a particular way (some have a north arrow), at least for horizontal accuracy.?ÿ The antenna model may include some offset of the phase center (APC) from the reference point (ARP). If you point 180 degrees differently, you are introducing an error of double that offset.
Maybe it doesn't matter for vertical??ÿ Does the model allow an orientation-dependent vertical offset?
> (and orient the antenna differently)
I thought you were always supposed to orient the antenna a particular way (some have a north arrow), at least for horizontal accuracy.?ÿ The antenna model may include some offset of the phase center (APC) from the reference point (ARP). If you point 180 degrees differently, you are introducing an error of double that offset.
Maybe it doesn't matter for vertical??ÿ Does the model allow an orientation-dependent vertical offset?
Bill,
Things have come a long way since the early "relative" antenna calibration techniques, and some of the NEW robotic systems (calibration models) should make all "reasonable" allowances for both horizontal and vertical azimuth variations. I haven't followed this technology all that closely in recent years, but I do know that "The Times They Are a-Changin."
Edit...assuming of course that the antenna is PROPERLY aligned in the first place (as you pointed out).?ÿ
?ÿLoyal
Thread hijack:?ÿ I've heard that the subtleties of a water hose level mean it isn't as accurate as a good optical level run. ?ÿ I note that the Coast and Geodetic Survey chose not to save a lot of money on equipment by using the water level, so I doubt it works as well. I can see that differences along the hose in the amount of dissolved air, surface tension/wetting giving difference in meniscus at the ends, and sun loading on part of the hose would bias the result.?ÿ There might be other factors?
Anyone have info on this?
It was discovered centuries back that a syphon could only work up to around 30+ ft. It was this problem that led the scientific minds of the time to discover the barometer.?ÿ
Thread hijack:?ÿ I've heard that the subtleties of a water hose level mean it isn't as accurate as a good optical level run. ?ÿ I note that the Coast and Geodetic Survey chose not to save a lot of money on equipment by using the water level, so I doubt it works as well. I can see that differences along the hose in the amount of dissolved air, surface tension/wetting giving difference in meniscus at the ends, and sun loading on part of the hose would bias the result.?ÿ There might be other factors?
Anyone have info on this?
It was discovered centuries back that a syphon could only work up to around 30+ ft. It was this problem that led the scientific minds of the time to discover the barometer.?ÿ
doh! ?ÿwater levels were the question. Disregard the syphon reference.
I use OPUS but I like to have a four hour static session.?ÿ ?ÿI can tell you that in North Carolina, home to the most accurate flood maps in the United States, an OPUS report can be more valuable than a level loop off a benchmark as the metadata can forever be transformed to the datum of the day.
I've spoken to NCGS and NGS (I think it was Scott Lokken) about physical benchmarks vs OPUS and both agencies seem to prefer the latter as long as there is metadata.?ÿ The NGS guy kind of hurt my feelings.?ÿ He went on and on about how baseless the common surveyor's concept of "control" is.?ÿ Apparently NGS?ÿ has a few places throughout the country where they experiment with different control monuments in various soil types.?ÿ I got the impression that it is difficult to predict when, where, and how much a BM has moved.?ÿ From a liability standpoint, I would be more nervous about using a BM that I had not run a static session on, than I would be about using an OPUS report (assuming that it was a good report).
I doubt this is your problem, but some of the CORSs have poorly placed and/or mounted receivers.?ÿ Take a quick look at the picture of the CORS on NGS's website to make sure it is solid.?ÿ In the OPUS Projects class they show a bunch of examples of questionable CORS and advised us not to use them in solutions (especially if you follow their advice and include one very long vector when processing).?ÿ
For flood certificates, the important thing is to match whatever the map is based on, regardless of whether that is accurate in the larger world.
Sounds like NC has a good system. Some of us aren't as fortunate.?ÿ It can be high risk, low reward is my only point.?ÿ
I don't do elevation certificates but if I took one on in an area that could flood, and was only going to use GPS for elevation control, I would set a mark on my site, and run a 2 + hour static session on it.?ÿ Then I would break the setup and do a second 2+ hour session on the same mark.?ÿ Then I would submit the data to OPUS Projects and jump through all the hoops of Blue Booking it.?ÿ If the mark passes all the tests, it will become part of the NSRS and would be hard to argue with.?ÿ?ÿ You would have about 5 hours in the field and 2 in the office once you got your workflow down.?ÿYou could even add some benchmarks to your network and constrain to them. but that would increase field and office time.
Now for the Monkey Wrench.?ÿ In this area, OPUS observations on Benchmarks produce an elevation that is 0.4' to 0.5' below the published elevation.?ÿ That makes me suspect subsidence.?ÿ If the flood study was based on Passive benchmarks, you GPS observations would show all of the elevations lower than the flood study datum.?ÿ That is the safe direction, but your data could cause your client to pay more for flood insurance than they would if you brought in an elevation from nearby benchmarks with a level.?ÿ The problem is that we don't have many benchmarks left.?ÿ
Many years ago, I did the field work for a flood certificate.?ÿ We ran a 3 mile level loop and closed +0.03'.?ÿ The finished floor on the house was 0.02' above the BFE.?ÿ After adjusting the elevation it was 0.01' below the BFE.?ÿ
The way FEMA maps were created was to base them on NAVD88 bench marks. At least the maps I was involved with were. They have little to do with CORS/OPUS. Understanding the process will lead you the to the correct procedure.
If the mapping is based on a VRS NAD83(2011), Geoid12, then that's what should be used for BFEs. If it's based on bench marks shown on FIRMS then use them, tie them in with GPS snd figure out the differences.
The last two posts are excellent. They both outline well thought out ways to approach the outcome. The outcome would probably be two different elevations. Which one will you be willing to certify and possibly risk your business on? I repeat high risk - low reward.?ÿ
So, there is lots of good advice and procedures here.?ÿ One thing I would say is that of course, you should always use a benchmark as shown on the FIRM.?ÿ That said, there were no benchmarks shown on the firm or adjoining FIRMs.?ÿ The original FIRM from 1979 shows 2 RMs listed in 29 datum of course.?ÿ The updated FIRM shows no BMs and is in 88 datum.?ÿ A search of the NGS data sheets shows the nearest true benchmark in the next town, about 8 miles away.?ÿ The nearest mark with any vertical is about 2 miles away, but is listed by NGS as "GPS and Approx Height".?ÿ ?ÿThe height shown is to the 1oth of a meter.?ÿ To me this is not an acceptable benchmark to use for elevation.?ÿ The other company using network RTK accepted this benchmark as good.?ÿ Am I crazy, or would DPOS not give me a better elevation than a benchmark with +/- 0.1 meters?
Your explanation is sound.
Are you crazy? ?ÿMaybe?
There are no Benchmarks shown on Firms in Montana. The Benchmarks shown on the old are different datum. Plus the elevations are usually pretty bad.
OPUS will give an Elevation .to hundredth an evaluate the qualaity. I'm not familiar with DPOS,?ÿ
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There are no Benchmarks shown on Firms in Montana. The Benchmarks shown on the old are different datum. Plus the elevations are usually pretty bad.
OPUS will give an Elevation .to hundredth an evaluate the qualaity. I'm not familiar with DPOS,?ÿ
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Well, you made me look, I downloaded a MT FIRM map onto my phone, there are at least 13 BMs shown, the BFEs were developed from base mapping done in 2005, its very clear that the BMs are the elevation basis.
