I need some input please. I have been surveying for more than 50 years :)); but now I have run into a problem....
I have a project for a large company and they want to build a long sanitary sewer pipeline. it will have a nearly flat slope.
There are no bench marks near the treatment plant so i elected to use gps to transfer elevations to the plant.
I submitted my horizontal and vertical info to the engineer in charge.
Turns out that another company is checking my work. SO;
I uses a published NGS point and transfered the info to 2 plant monuments. the other surveyor disagreed with the vertical component of the monuments but agreed with the horizontal portion.
so, my client said for me to use another NGS monument and set new horizontal and vertical on the same two plant monuments.
I use the classical method by occupying the Ngs monument for 2 hours per point.
the other surveyor used OPUS to obtain horizontal and vertical info.
My question is: how accurate is the OPUS solution compared to the classical method.
we are looking at +/- 0.30' difference
HELP PLEASE!!
There may be a systematic shift between OPUS and published values on local bench mark control especially in areas of subsidence.
I know absolutely nothing about California.... but when were the ngs elevations published? I'm sure they must update them. Was his elevation higher than yours? I just read somewhere that the plates in California move at a rate of around 1 inch per year! And I'm sure some areas experience uplift as well. Craziness
I'm not in California and have only textbook knowledge on this, but ...
What is the allowable error, and what is the discrepancy between measurements?
I'm not sure what you mean by "classical method ... 2 hours per point"
Fluids don't know about GPS. They know about gravity. GPS by itself doesn't know about gravity, and OPUS uses a good but imperfect gravity model to estimate NAVD88. As Dave said, there may be a local discrepancy in the geoid model.
-So did you somewhere in your process also employ a gravity (geoid) model to relate the points?
-Was it the same geoid model used by OPUS for the other guy?
-Is that geoid model known to be good in your region?
-Have the NGS bench marks suffered subsistence, uplift, or seismic shifts? Although NGS re-ran the data for NAVD88 those marks may not have been leveled for decades before that.
For something like this that is critical in the vertical, it's hard to beat high-order optical leveling along the route for accuracy, with proper orthometric correction. It really doesn't matter how high the pipe is, it's the slope that gets you. So agree on an elevation somewhere in the route and go from there. GPS can be a great blunder check, but may not achieve the same accuracy.
For super-critical fluid work (in the few mm per km range) it is better to work in dynamic height instead of orthometric height, which requires a similar but different correction formula.
Why don't you run levels? Everyone knows vertical is the weak link in GPS. Especially with a flat slope I would make sure all my vertical was dialed in straight away!
Bill93, post: 363129, member: 87 wrote: I'm not in California and have only textbook knowledge on this, but ...
What is the allowable error, and what is the discrepancy between measurements?
I'm not sure what you mean by "classical method ... 2 hours per point"
Fluids don't know about GPS. They know about gravity. GPS by itself doesn't know about gravity, and OPUS uses a good but imperfect gravity model to estimate NAVD88. As Dave said, there may be a local discrepancy in the geoid model.
-So did you somewhere in your process also employ a gravity (geoid) model to relate the points?
-Was it the same geoid model used by OPUS for the other guy?
-Is that geoid model known to be good in your region?
-Have the NGS bench marks suffered subsistence, uplift, or seismic shifts? Although NGS re-ran the data for NAVD88 those marks may not have been leveled for decades before that.For something like this that is critical in the vertical, it's hard to beat high-order optical leveling along the route for accuracy, with proper orthometric correction. It really doesn't matter how high the pipe is, it's the slope that gets you. So agree on an elevation somewhere in the route and go from there. GPS can be a great blunder check, but may not achieve the same accuracy.
For super-critical fluid work (in the few mm per km range) it is better to work in dynamic height instead of orthometric height, which requires a similar but different correction formula.
You are right, the project datum is far more important than an NGS datum.
Elevations on fairly flat flows are critical. I would run a level loop and make sure the project elevations differences are accurate. If all you can recover is one undisturbed Bench mark, use it as your basis of elevation. 0.3' don't really mean much, unless its between your survey bench marks. As far as the correct elevation as per NGS datum, I don't think you really want to go there.