Jim in AZ, post: 432229, member: 249 wrote: What makes you think your DOT is working on the same datum as NGS
I'm not assuming that it does, but it seemed like a good place to start and check into since it is the nearest monument I could find and had an elevation stamped on it.
You can run a level across 2 miles a lot faster than all this "OPUS" work - I'm just saying...Remember, Keep it Simple Surveyor (KISS).
aliquot, post: 432252, member: 2486 wrote: But OPUS is NGS.
Yeah, but it's a different vintage NGS
StLSurveyor, post: 432257, member: 7070 wrote: You can run a level across 2 miles a lot faster than all this "OPUS" work
True, though elevation transfer via GNSS is practical for a solo operator, while leveling really isn't. That said, most of the projects on which I actually use an employee anymore are leveling jobs.
First of all, I would never trust an elevation stamped on a monument. The data sheet in Bill93's post does include all the various stampings on the photo so it seem this is the monument, PID:AJ7779, to which the data sheet refers.
Examining the projects associated with the point indicates only one project. This means the different heights on the data sheet are the result of adjustments and NOT re-observations. Information for this monument has never been validated since the original submission. An all too common problem with points in the NGSIDB.
The project during which the point was determined,GPS1649, consisted of 19 points. Many of the point designations appear to be benchmarks?? No leveling observations were submitted to the NGS for this monument.
As far as the current status, the scraping of the disk and the reference to its siting in a head wall of a small structure raises the possibility of it having been disturbed. The photo does not show the vicinity of the monument only the disk itself.
As for your uncertainty about your ARP height, might I suggest that your measure in both metric and English units as a field check. As recommended elsewhere, carefully examine the antenna diagram to determine the correct point for the measurement up from the monument. In the case of a CHCX91+S it is to the bottom of the antenna mount.
Looking at your OPUS-RS solution, I note a 9.7 cm standard deviation (1 sigma) in the ellipsoid height component. That is not too good.
On the matter of changes to the NGS geoid models, there is only 1 cm difference between the GEOID99 and GEOID12B at the point. I also note the GEOID12B model uncertainty is 5 cm at the two sigma level. Geoid modeling in mountainous areas is problematic. A 5 cm uncertainly is good.
Without a good OPUS or OPUS-RS solution, you can neither verify the published value nor use the determined value with confidence. Without multiple independent sessions on the monument it is impossible to detect an antenna height problem. By independent I mean tearing down a setup and resetting the equipment completely. For an even better check observe at different times of the day.
I would incorporate as many of the procedures described in the NGS orthometric height guidelines as possible within your budget and time constraints.
My best guess is that this problem is another example of our decimated, unmonitored network of monumented points.
HTH,
DMM
billvhill, post: 432189, member: 8398 wrote: ...Or should I just use the level.
If I were determining elevations using the level I probably wouldn't use this monument, per datasheet, as the benchmark. The datasheet only quotes the ortho elevation to the nearest foot. That should give you some idea.
Note that your OPUS-RS horizontal position is around 11cm from the datasheet postition for the monument. Vertical errors in GPS are frequently double the horizontal. Yeah, its not a great OPUS result, or maybe the datasheet is not so great. Or both.
The elevation stamped on the cap may well be NGVD29.
If you take the average of a couple of +/-4 hour OPUS-S sessions, and if the site has good sky, and if you are measuring the HI to the ARP correctly, you will usually get a NAVD88 elevation that is good to about +/-0.2', probably somewhat better.
Jim Frame, post: 432265, member: 10 wrote: True, though elevation transfer via GNSS is practical for a solo operator, while leveling really isn't. That said, most of the projects on which I actually use an employee anymore are leveling jobs.
Okay, fair enough, then set the total station up and trig some levels, that wouldn't take too long.
I emailed Mark Silver who set me on the correct path, ended up using the calculated orthometric elevation at the base station of my project using several OPUS solutions. There were other BMs that were along a N-S highway a couple miles west of my project, the problem is that they were originally set approximately 30' from the road centerline in the 1930s, the road bed is much larger now and I couldn't find any survivors within a reasonable distance.
billvhill, post: 432994, member: 8398 wrote: There were other BMs that were ... a couple miles west of my project ....that ... were originally set ... in the 1930s
Even if those existed I think that you will be better off in all ways (cost & data quality) using the average of a number of long duration OPUS-S sessions for your vertical control basis.
Maybe look into doing an OPUS-DB and getting a psuedo-data sheet for your new monument.
This right here is my standard procedure:
GeeOddMike, post: 432266, member: 677 wrote: As for your uncertainty about your ARP height, might I suggest that your measure in both metric and English units as a field check. As recommended elsewhere, carefully examine the antenna diagram to determine the correct point for the measurement up from the monument. In the case of a CHCX91+S it is to the bottom of the antenna mount.
I had one of those 2M fixed height tripods at my last job, stopped using it after a few years. Neal, my partner at that time, hated it more than me, when I finally went back to tripod/tribrach he said WE ARE SURVEYORS, FOR CRYING OUT LOUD, WE MEASURE THINGS! Using both M and USfeet I've never not caught a bad height.
OPUS is a pretty cool set of tools. It does NOT replace having adequate tools in your posession or knowledge in your head. You are doing the right thing seeking and heeding advice.
I just evaluated a data set where the control was run in network style with 7 hour occupations. It was then processed as a series of OPUS solutions and used as a basis for processing a huge lidar and photo set. The vectors were never even generated as a check. Completely ridiculous. The best thing that could happen to our Profession is the loss of OPUS for a year, followed by random outages of varying lengths.
thebionicman, post: 433014, member: 8136 wrote: The best thing that could happen to our Profession is the loss of OPUS for a year, followed by random outages of varying lengths.
[SARCASM]Sounds like the CEO at Trimble....[/SARCASM] (I whole heartedly agree with your entire comment.)
thebionicman, post: 433014, member: 8136 wrote: The best thing that could happen to our Profession is the loss of OPUS for a year, followed by random outages of varying lengths.
Yeah...while we're at it, lets turn off the Internet, close the Interstate Highway System, and scramble the whole NAVSTAR Network too. Just because there are LOTS of folks who can't seem to utilize these things properly, lets punish those who can.
:confused:
Loyal
Dave Karoly, post: 433013, member: 94 wrote: WE ARE SURVEYORS, FOR CRYING OUT LOUD, WE MEASURE THINGS!
I agree that variable-height setups can be used effectively, but mistakes do happen, and when they do the fewer variables the better. Not having to deal with the nagging possibility that it was a bad HI that blew up your network is worth the hassle of using those cumbersome fixed-height tripods for static or RTK base setups.
Thanks for all the helpful posts
There is a lot to be learned on this site, once you look past a few sarcastic posts. This my first time using OPUS for the orthometric elevation since I purchased this ability 6 months ago, why not use it, just have to get past the learning curve.
To those who discourage others not to use the technology just because they don't currently have the knowledge, should climb off their high horse and offer helpful comments. You weren't born with the knowledge, but aquired it and you too were there once.
Dave Karoly, post: 433013, member: 94 wrote: This right here is my standard procedure:
I had one of those 2M fixed height tripods at my last job, stopped using it after a few years. Neal, my partner at that time, hated it more than me, when I finally went back to tripod/tribrach he said WE ARE SURVEYORS, FOR CRYING OUT LOUD, WE MEASURE THINGS! Using both M and USfeet I've never not caught a bad height.
This has always been my feeling about those stupid 2M find height tripods. If you can't trust yourself to figure out a bullet-proof way to measure an HI something is wrong.
I measure slant height up to the rim of the antenna ground plane in meters and feet to check for misreading. Then I consult a spreadsheet table pasted into the field book to find how much to subtract to get the ARP height (bottom of antenna assembly).
That's a lot easier and less blunder-prone than doing the Pythagorean calculation every time. A small table covers more than the range of slant heights I need. I've checked that calculation within a mm of the height from a concrete floor to a board laid across the tribrach adapter.
Edward Reading, post: 433036, member: 132 wrote: This has always been my feeling about those stupid 2M find height tripods. If you can't trust yourself to figure out a bullet-proof way to measure an HI something is wrong.
2M fixed height is a bullet proof method. What do you use for a rod for your rover?
Rankin_File, post: 433041, member: 101 wrote: 2M fixed height is a bullet proof method. What do you use for a rod for your rover?
Adjustable height rod. We have been measuring instrument and rod heights for a long, long time. Why is it with GPS all of a sudden we can't measure an HI or HT? Makes no sense to me.
Not to mention with the cost of the fixed heght tripods, I can but 2 nice Dutch Hill tripods
