OPUS - Understanding the Results

> Problem: I observed two points 320' apart, with a top of the line GPS unit and then with my unit. Both points differ in Northing by 0.22'+/- and Easting by 0.17'+/-

To clarify: you ran 1 static session on each of 2 points with your receiver, then repeated the procedure using a different receiver, and sent all 4 files to OPUS. The OPUS results show a consistent coordinate shift between the files run with your receiver and those run with the other receiver. Is that correct?

Are you sure you selected the correct antenna model for both of the observation sets?

> > Problem: I observed two points 320' apart, with a top of the line GPS unit and then with my unit. Both points differ in Northing by 0.22'+/- and Easting by 0.17'+/-
>
> To clarify: you ran 1 static session on each of 2 points with your receiver, then repeated the procedure using a different receiver, and sent all 4 files to OPUS. The OPUS results show a consistent coordinate shift between the files run with your receiver and those run with the other receiver. Is that correct?

If that's true, it sounds like it may be bad ARP height entered into OPUS prior to processing.

Not exactly.

I observed the two points with the $20,000 unit using Oregon Real-Time Network and got the position on the two points (averaged 5 sets of 30 epochs each) in Real Time.

I then set up my OPUS unit for 18 minutes and sent to OPUS.

Hope that makes sense. 🙂

Good question.

I THINK I did. The OPUS unit is automatic. I can't change the antenna - it is what it is.

The $20k unit we have used for 5 years and checked numerous HARN stations with excellent results, so I am confident it is correct.

thanx

I don't know a thing about Oregon's RTN, so my next question would be are you certain that the RTN is working on the same NAD83 realization as OPUS?

NAD83(CORS96)vs. NAD83(2011)?

No, I do not know.

I will see if I can find out.

thanx

I probably not the only one confused by the term "OPUS unit" but anyway. In the OPUS dialogs you must select an antenna type and ARP height, right? so what are you inputting there? The problem is likely datum related but I am curious about what an OPUS unit is

It is an L1/L2 unit designed to record OPUS compatible static raw data sets for submission to OPUS for the results. No RTK. Static Only.

thanx

1. Could this be the difference between reference frames and EPOCH dates?

I don't know much about the frame of your VRS system (perhaps Gavin will chime in here), but that is about the difference between NAD83 2011 2010.0 and NAD83 CORS96 2001.0.

To visualize this, I took the REDM CORS site (Redmond OR) published coordinates from the NGS website and converted them to Oregon South NAD83 State Plane:

NAD83(2011) EPOCH 2010.0
44 15 35.14667 N 121 08 52.31510 W 920.345 M
4,751,502.964 SFeet E 945,760.465 SFeet N ORS

NAD83(CORS96) EPOCH 2001.0
44 15 35.14513 N 121 08 52.31624 W 920.367 M
4,751,502.880 SFeet E 945,760.309 SFeet N ORS

The difference is 0.084 x 0.156 feet. So it partially could be that the OPUS-RS solution is NAD83(2011)2010.0 and the VRS network solution is NAD83(CORS96)2001.0.

2. It may also be a cumulation of expected accuracy on a 20-minute OPUS RS solution: There might/could be a couple of CM error (2 cm = 0.065') on each OPUS-RS solution. Perhaps a longer OPUS-STATIC session would match better.

3. Were the antenna's aligned to North for each shot? Some antenna have centering errors of a CM. You can check this out for your antenna at the ANTCAL page for your antenna, in the ANTINFO link you will find a table like this:

3.5 -2.7 84.3
0.0 -1.3 -2.0 -2.3 -2.6 -2.8 -3.1 -3.3 -3.7 -3.9
-4.2 -4.4 -4.6 -4.7 -4.2 -3.0 -0.8 0.0 0.0
2.2 -3.6 93.2
0.0 -1.2 -1.6 -1.7 -1.4 -1.1 -0.9 -1.0 -1.2 -1.6
-2.2 -2.7 -3.0 -3.2 -3.3 -2.9 -1.6 0.0 0.0

The first line lists the eccentricity of the electrical center of the antenna within the physical housing (in MM). So for the antenna above, we would expect a X difference of 7 mm by rotating the antenna 180 degrees.

Having said this, if Gavin chimes in, I defer to him 😉 !

Mark

Mark,

It certainly COULD be something like that. It would be great if it was, because then I could account for it and adjust accordingly.

A longer occupation would probably give better results, but I wanted to test the OPUS unit the way "I would like" to be able to use it. IF getting better results requires an hour or so on each point, I am less interested. I can easily deal with 15 minute occupation times, as it gives this Geezer time to drink his coffee, draw diagram in filed book, cogitate on survey plan, etc.

I was careful to align the antenna to North.

I have sent an email to ODOT, hoping they can enlighten me about the epoch/data frame issue. (that stuff is above my pay grade).

I am enjoying ALL of the questions and comments. Even if I don't find out the exact answer, I have already learned a lot.

thanx to all.

Geezer

Ohh, The ORGN transitioned at the beginning of March to the new frame.

There is a big discussion of it [ here ] with lots of color pictures.

For what you are doing, it may make sense to collect observations over a couple of high order control points that have 4-hour+ OPUS results available. ORGN has a check-in Mark program, just for this purpose. See [ here ].

Mark

Well, I just got this from the ODOT advisor.

That shift cited in the blog (Northing by 0.22'+/- and Easting by 0.17'+/-) is almost exactly the difference between NAD83 (CORS96) epoch 2002.00 and the new NAD83 (2011) epoch 2010.00 in the mid-Willamette Valley in Oregon. The shift between NAD83 epochs varies in various regions of Oregon because of differential tectonic plate movement in this state.

A current OPUS solution and a current Oregon Real-time GPS Network (ORGN) real-time position are both referenced to the new NAD83 (2011) epoch 2010.00. Our ORGN users have been reporting that their current ORGN real-time correctors are matching very closely with OPUS positions.

Possibly the ORGN RTK solution referred to in the blog was a solution prior to 8 March 2013, the date we switched the ORGN over to NAD83 (2011) epoch 2010.00.

Please feel free to post this to your blog along with my contact information below. I will be happy to answer questions about the ORGN.

Thanks,

Ken
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Ken Bays
Technical Manager, Oregon Real-time GPS Network (ORGN)
www.TheORGN.net
Lead Geodetic Surveyor, Oregon Department of Transportation
Oregon Professional Land Surveyor: 78507 LS
Alaska Professional Land Surveyor: LS 5861
4040 Fairview Industrial Dr SE, MS 4, Salem, OR 97302
Phone: 503-986-3543
Cell: 503-580-8793
Fax: 503-986-3548
kenneth.bays@odot.state.or.us
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Yes, the first observations were frrom March of 2010.

I was trying to compare those positions with the OPUS results taken yesterday.

My Bad!

I guess I will take the $20k unit back out and reobserve.

thanx to all,

Geezer (ok, ok, I live up to my name!!!! LOL)

Mr. Bays, You are a credit to your organization and our profession!

James

I've always been told to keep OPUS control about a mile apart from each other. I would hold one point as control, hold the OPUS derived bearing from published coordinates, and shoot the distance or traverse with an gun.

> No, I do not know.
It is, as of March.

> Yes, the first observations were frrom March of 2010.
>
> I guess I will take the $20k unit back out and reobserve.
No need to reobserve if you have the 2010 raw data. Re-Send it in to OPUS and you should get results on the new adjustment.

BTW, establishing "control" simply by separately OPUS'ing points that are 320 feet apart is sub-optimal procedure.

Geezer,

What kind of receiver are you using for the "OPUS Only" receiver?

I see that Igage has an OPUS receiver for a pretty reasonable price that I am considering adding to my GPS arsenal as soon as I can work it into my budget.

The website for their receiver is:
X90-OPUS Static GPS Reciever

Thanks,
Jimmy

> I've always been told to keep OPUS control about a mile apart from each other.

Huh? That makes no sense.