For regular users of GPS. If one is using GPS (based upon leveled NGS BMs)to bring Navd 88 elevations to a project to 2 miles away, what kind of error budget would be reasonable to expect? The project is a third order topo survey? What kind of error budget might one expect by applying the geod to obtain ortho heights , less than 400 feet of elevation difference over all...?
Dane
I can't give you any numerical values for your error budget but I would think it would be dependent on the accuracy classification of the monumented control you were using and the reliability of the geoid model that you choose to use. That being said, I typically find a tenth or less while using first order BM's and geoid 12a in my area of the Southern Oregon coast.
DANEMINCE@YAHOO.COM, post: 395451, member: 296 wrote: For regular users of GPS. If one is using GPS (based upon leveled NGS BMs)to bring Navd 88 elevations to a project to 2 miles away, what kind of error budget would be reasonable to expect? The project is a third order topo survey? What kind of error budget might one expect by applying the geod to obtain ortho heights , less than 400 feet of elevation difference over all...?
In Central Texas, using static GPS vectors to do height transfers over that short distance, I'd expect to get an ellipsoid height difference with an uncertainty of less than 0.03 ft. (95% confidence) from the adjusted results of two vectors. That doesn't include an estimate of uncertainties in NAVD88 elevation differences modeled from the GPS results.
DANEMINCE@YAHOO.COM, post: 395451, member: 296 wrote: If one is using GPS (based upon leveled NGS BMs)
It's going to depend on the length of the observations - OPUS minimum of 2 hours, or can you let them sit for a day apiece? Averaging from multiple BMs or just one?
In my area the NAVD88 system is very stable, applying a geoid model to the elevations generally will produce elevations with less than a cm of "error".
This would mean setting on a bench mark or an offset point to a bench and checking into another bench mark and preferably to multiple ones.
I don't use any ellipsoid heights generated by OPUS or CORS to get those results, they are too unstable.
But then I've heard that where you are there is way more movement in the NAVD88 control.
BMs are first order class II and the ties would be with a minimum of 2 hours of good data I would use OPUS project for the adjustment..
DANEMINCE@YAHOO.COM, post: 395461, member: 296 wrote: ... the ties would be with a minimum of 2 hours of good data I would use OPUS project for the adjustment..
You should be able to get an ellipsoid height difference with an uncertainty of less than 1cm (95% confidence) from 2hrs of observations of good quality. My example above was based upon L1 solutions of a 2 mile GPS vector from 30 - 45 minute occupations.
If the height transfer is critical, I'd think that you'd want to repeat it on at least two days. BTW, I'm assuming that you are contemplating running two receivers at either end of the 2 mile baseline, not trying to work out the height difference from separate OPUS solutions.
DANEMINCE@YAHOO.COM, post: 395461, member: 296 wrote: BMs are first order class II and the ties would be with a minimum of 2 hours of good data I would use OPUS project for the adjustment..
If I used OPUS here I would expect to see it be lower than the bench marks about .15'.
I would also expect it to be consistently "off" through bench mark locations the same amount.
But it's been a few years since I bothered to check OPUS against Bench marks.
Kent McMillan, post: 395462, member: 3 wrote: You should be able to get an ellipsoid height difference with an uncertainty of less than 1cm (95% confidence) from 2hrs of observations of good quality. My example above was based upon L1 solutions of a 2 mile GPS vector from 30 - 45 minute occupations.
If the height transfer is critical, I'd think that you'd want to repeat it on at least two days. BTW, I'm assuming that you are contemplating running two receivers at either end of the 2 mile baseline, not trying to work out the height difference from separate OPUS solutions.
3 4000 series recievers- 4 navd 88 bms to choose from OPUS projects to post process and adjust....good point about mulit day obs... .
DANEMINCE@YAHOO.COM, post: 395513, member: 296 wrote: 3 4000 series recievers- 4 navd 88 bms to choose from OPUS projects to post process and adjust....good point about mulit day obs... .
I'd be interested to know what Jim Frame suggests. He's done quite a bit of height modernization work. The 4000ssi receivers I have are excellent when used with geodetic-grade ground plane antennas. 120-minute sessions for 2-mile vectors may be overkill. I'd rather have two 60-minute sessions on two different days.
If you want to demonstrate the adequacy of the geoid model used, I'd think you'd want to tie benchmarks on both sides of your project. That will flag mark instability and, if the marks are stable, should demonstrate the validity of the geoid model.
Kent McMillan, post: 395514, member: 3 wrote: 120-minute sessions for 2-mile vectors may be overkill. I'd rather have two 60-minute sessions on two different days.
I use 60-minute sessions for vectors in the 10 km range when trying to get vertical at the 2 cm level. Most of the time it works, but you need that second-day session to find the ones that don't.
Jim Frame, post: 395524, member: 10 wrote: I use 60-minute sessions for vectors in the 10 km range when trying to get vertical at the 2 cm level.
The separation in Dane's example was only about 3km, though. My experience would lead me to think that if he does some mission planning (i.e. avoids any peaks in poor VDOP) he ought to expect uncertainties in height difference on the order of about 1.5cm (95% confidence) for vectors from 60 minute sessions, with the uncertainty of the weighted mean of vectors on two days being reduced to about 1.0cm (in just the PC-to-PC ellipsoid height components of the vector).
I plan to use OPUS projects and they require 2 hour sessions, 2 hours of good data that is. I have benchmarks along one side of the project only.
I can only write about our experience. Your area may not be the same.
Two NGS BM's 2 miles apart first order NGS leveling two years ago.
Leveled difference=82.08 US ft.
OPUS Projects Orthometric difference computed using Geoid12B=82.01 US ft
Topcon Tools post processing using Geoid12B=82.08 US ft
These are fairly typical results. We match leveling better a non OPUS post processing software. Sorry OPUS Projects fans.
linebender, post: 395587, member: 449 wrote: I can only write about our experience. Your area may not be the same.
Two NGS BM's 2 miles apart first order NGS leveling two years ago.
Leveled difference=82.08 US ft.
OPUS Projects Orthometric difference computed using Geoid12B=82.01 US ft
Topcon Tools post processing using Geoid12B=82.08 US ftThese are fairly typical results. We match leveling better a non OPUS post processing software. Sorry OPUS Projects fans.
It could be that non opus project processing provides more optimistic results.
The issue in part is trying to figure out a relistic budget. I am not clear which is cost effective 4 miles of level loops or 16 hours of GPS observations?
For NAVD88 "elevation," I would go with the level!
No matter how you screw around with the GPS, or how "tight" you get the Ellipsoid Height, you are still at the mercy of the GEOID MODEL.
Loyal
DANEMINCE@YAHOO.COM, post: 395593, member: 296 wrote: The issue in part is trying to figure out a relistic budget. I am not clear which is cost effective 4 miles of level loops or 16 hours of GPS observations?
4 miles of Third Order double-run levels is about 8 hours in flat ground. (More in the hills of San Francisco, though you could run trig levels.) That's with a 2-man crew, so 16-man-hours, about the same as the OPUS Projects approach. I think I'd lean toward the levels.
