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Why do you use OPUS
Posted by ryancj31 on June 12, 2021 at 6:04 pmEvery once in a while I will read on this forum about people using OPUS for post processing gps observations. In all my 20ish years I??ve used it maybe 2-3 times. And I can??t clearly recall the reason.
My work has typically been summed up as large and small boundary surveys, large and small topos, occasional elevation certificates or LOMA, and construction staking.
If we need a solid elevation reference we find a nearby NGS monument and typically our single 3 minute observation checks out within hundredths of the published elevation.
Are people also using for horizontal corrections?
Should I be using OPUS more? What triggers you to spend the time to post process?
summerprophet replied 3 years, 3 months ago 14 Members · 23 Replies -
23 Replies
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I’ve been using it lately because my Network RTK can be kind of wonky about connecting. I only use it when I have to tie to state plane or need an actual elevation, otherwise I don’t break out the GPS.
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I use OPUS — mostly in the form of OPUS Projects, but sometimes just OPUS itself — whenever I want a survey tied to the NSRS in an easily-documented manner that’s generally acceptable, notable to public agencies. Note also that an OPUS-S elevation is acceptable to FEMA for Elevation Certificates.
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One reason is there is no nearby NGS benchmark (or any kind of benchmark)
Another is that it is and effortless way to tie an RTK survey into real world horizontal coordinates.
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I do mostly utility support work spread out over an area larger than some east coast states. For the last 8-9 years, I’ve used OPUS to reference nearly all of our work to CORS and it’s paid huge dividends when I get a new job in an area and I can relate one survey done in 2013 to another survey done in 2020, a mile or more apart. Verticals aren’t a particularly important element to it but a tie from the ’13 survey to a section corner and a tie from ’20 to a quarter corner a mile or two away from another job and to be able to relate them within a couple centimeters, allows me to compute relatively accurate search coordinates in an area I’ve never worked, before ever setting foot on the ground. The value of our survey data is dramatically enhanced by using OPUS to reference it to the real world. It’s also paid off handsomely in other ways I won’t go into, but let’s just say that the mapping my clients use is based off tax maps that were built on protracted townships. To know where they actually are on the ground in the real world? Priceless.
Willy -
I use it as a check occasionally. I do so much static that OPUS can’t handle so it’s not a very useful tool for me.
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What are you using for a Base unit elevation to compare into an NGS benchmark to within “hundredths”? I’ve certainly never seen that using a “Here” base. Frankly I’ve never seen that using another NGS benchmark or an OPUS based elevation.
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Posted by: @ryancj31
If we need a solid elevation reference we find a nearby NGS monument and typically our single 3 minute observation checks out within hundredths of the published elevation.
I would really, really love to see some of those checks, because I am highly skeptical of that claim.
EDIT: I should have said that it would be unusual, in my experience, to check in to 95% of NGS passive monuments within even a tenth or two, especially utilizing RTK methods. Even the BMs set during the NAVD88 levelling campaigns, and held fixed in the current hybrid geoid models, are rarely found to be within that range of published.
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil Postman -
A super simple workflow for putting your typical project control on the current realization of the NAD83 datum is to set up your base to collect data for OPUS while you conduct your RTK ties. Presumably you will be set up for an hour of two. Send the base data into OPUS, get a solution, and shift your whole set of points onto that coordinate. Presto – you have the whole lot on a correct geodetic basis. That is at least 98% of what I use OPUS for.
As for just tying to a nearby NGS mon instead – apart from the time involved – there are exactly 4 such monuments within 10 miles of my city. One is completely unusable and two others are less than optimal for GPS due to the trees that have grown up around them. The last is a solid 10 miles away. Not a practical everyday solution.
For larger control projects I would make an effort to include those NGS monuments in my program but I would still weight OPUS more heavily in the adjustment. The CORS network OPUS is tied to is actively maintained and adjusted. Those passive marks were last tied 2 decades ago, and while they appear to be OK, it is hard to be certain that they are still in exactly the same position they purport to be.
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@skeeter1996 and this is using WisCORS dial up or MN on a rover. Maybe we work in a sweet spot? SW Wisconsin and SE Minnesota. I??ll look closer at more data sheets on monuments when I get on a full screen tomorrow. This is what I was able to get on my phone from a recent one:
https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=DJ4546
The orthometric height was determined by differential leveling and DJ4546.adjusted by the WI DEPT OF TRANSP DJ4546.in May 2012.
Maybe this leveling and adjustment in 2012 is what is giving us great results?
Our typical work flow on large projects or elevation critical projects is to dial up and find nearby NGS monument. If elevation fits set up base on it and get hopefully reach project to set more local control. Then move base to our new local control.
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@mark-mayer all sounds interesting and I will see if I can try to work this process into a future job.
Just wondering though what kind of projects are you investing this additional time into and how much does it correct your positions as opposed to let??s say just a dial up shot from a CORS (if available to you)
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Posted by: @ryancj31
Maybe this leveling and adjustment in 2012 is what is giving us great results?
MN and WI are among the most densly covered states for height modernization and GPSonBM, which has resulted in excellent geoid modeling of the area. Accurate geoid is what ties leveling and GPS together.
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@bill93 wow I do work in a sweet spot. I??ve never looked into that before. Thanks for finding that map. Quite the stark contrast to a vast majority of the states.
We typically can find a NGS monument within a few miles of a majority of our projects. In the back of my mind I always wondered if other regions were blessed with that as well.
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My projects are typically sites for development and road development ranging in size from a portion of a city block to several city blocks.
The time for each iteration of this process, after you become familiar with it, is perhaps 10 minutes. I use StarNet to do this shifting, but it can easily be done in CAD. If you need multiple days of field work to complete you can, by setting the base over the same point on each visit, average multiple OPUS sessions and improve precision.
Doing this puts your job on a correct geodetic basis to a degree of accuracy sufficient to coordinate it with GIS data that the designers may employ. It also forms a basis for recovering your control when it comes time to return to the site to pick up more detail months later, or to perform stakeout of design. At such time that 10 minutes may pay back many times over
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The last project where I tied to a CORS then sent in my .TO data files into OPUS changed the Coordinates of the point thusly:
CORS: Lat 05.94945″ LONG 17.27720″
OPUS: LAT 05.94950″ LONG 17.27716″
I would have to look again but I believe that was a 2 hour session.
OPUS is no help for the other static sessions I run so using it for the day is mostly pointless. But the CORS point worked amazing well to reduce those static sessions so I wanted a full day file from it and processed remote disconnected points using it.
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@mightymoe seems like a shift in coordinates around 7 feet if you are working in the neighborhood of 45 degrees latitude. That??s substantial. 7 feet from what though?
I guess I am still a bit confused about the purpose or intent of using OPUS. Is often used for more accurately outputting into other coordinate systems or lat/long?
I??m used to large highway projects being in State Plane coordinates or maybe county coordinates. Others are seeking that info in a very accurate lat/long or UTC?
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I’m showing the seconds only, the two numbers are less than 0.01 feet apart. My point being that OPUS isn’t all that useful since I use the CORS point’s data during my daily workflow.I will download the entire day of CORS data and use it to supplement my survey. It’s a powerful tool to have available. OPUS can’t do that.
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After my recent experiences using OPUS, I find myself asking that question a lot
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Posted by: @ryancj31
I guess I am still a bit confused about the purpose or intent of using OPUS. Is often used for more accurately outputting into other coordinate systems or lat/long?
I??m used to large highway projects being in State Plane coordinates or maybe county coordinates. Others are seeking that info in a very accurate lat/long or UTC?
OPUS-S uses 3 CORS that it thinks are best and averages the results for your point. OPUS-RS uses 9. If you are concerned with where you are in the larger world, that is probably better than relying on a single CORS.
If your concern is only positions within a project, processing against a single base or nearby CORS is fine for relative position accuracy and perhaps more convenient.
This is independent of how you choose to present the coordinates, because they can be exactly converted between lat-lon, XYZ, SPC, UTM, etc. at will.
Keep track of the datum and realization; any of the coordinate types can be used with any datum (some combinations are uncommon). Most of your work is probably in a recent realization of NAD83. GNSS data is native to IGS, which WGS84 resembles. OPUS and probably your software have a conversion to NAD83. Mixing realizations can foul things up. If you have old data in NAD83(9x) you need to convert it to NAD83(2011) to use it with current data. Likewise for vertical data, know which geoid model it came from.
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Posted by: @ryancj31
I guess I am still a bit confused about the purpose or intent of using OPUS. Is often used for more accurately outputting into other coordinate systems or lat/long?
I??m used to large highway projects being in State Plane coordinates or maybe county coordinates. Others are seeking that info in a very accurate lat/long or UTC?
State plane and county coordinates are projected coordinates, and thus have a geodetic basis, and are almost always based upon the NSRS. OPUS is the simplest – or the least labor-intensive – method to tie projects to the NSRS during post-processing, and because it is an NGS tool it has become the standard for a lot of organizations.
Like any other post-processing tool, it has its pros and cons. I prefer to post-process my own data, and use OPUS solely as a check, in conjunction with another tool such as Trimble’s RTX-PP or CSRS-PPP. It is a great tool, but has limitations such as not utilizing all constellations.
After seeing how often folks who should know better screw up post-processing, it’s not surprising to me that so many public sector projects only accept OPUS-derived values. It’s not fool-proof, but harder to screw up than doing it yourself.
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil Postman -
I’m somewhat confused about just what you are saying, and the workflow you use. It seems that you prefer to download CORS data to resolve your vectors. By which you arrive at results for your base that equal those of OPUS within 0.01′. That means you prefer not to use OPUS. It doesn’t make OPUS worthless. Quite the contrary.
I would agree that trying to resolve the (results of the) many vectors you might collect in a day radiating from your base with OPUS would be a loser. OPUS Projects maybe. That is not what OPUS is intended to be used for.
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