Your response below in the GPS Test thread begs a coupla questions for us O.R.G.I. guys also !!

What accuracy is the point position of an OPUS calculated position ?? ± .1 or 1: 1,000,000 or ??? Are you suggesting that with two intervisible points, with coordinates corrected for combined scale factor, the OPUS positions may not calculate to a vector length extremely close to the EDM measured ground distance ??

Second, what is the maximum effective range of CORS when using OPUS ??

Does OPUS use more than 3 sites to determine the point position ??

I'm gonna go look at the OPUS site but if somebody already knows handily....

Jimbo

CONTD:

After doing a little reading it seems OPUS expects the accuracy to be 5 cm peak to peak as a general maximum.

I didn't find anything that limited the range of OPUS. It just says it will use 3 CORS sites. Which answers my 3rd question.

Also JD it seems you are right about the inclusion of the vectors between CORS Stations being used in the solution.

From what I read, or my understanding of such and MR.G will hopefully correct me if I'm wrong, OPUS coordinates are "more correct" in relation to CORS sites (NAD 83 coordinates) than they are HARN sites (NAD 83 coordinates). Thi is simply because they do include the CORS site to site vectors which "updates" the weighted site coordinates when processed in the adjustment.

which makes me wonder if they aren't included in the adjustment, then the published coordinates would be used exclusively, thereby leading to a better accuracy toward published NAD 83 Harn coordinates....or am I jumping to a conclusion here ???
Modified By James Webb on 12/16/2001 at 12:07 PM

The results you get from OPUS include an accuracy estimate from OPUS and a disclaimer that says they did not review your field work. If you set up on the edge of the cap you will get a fine set of coordinates for the edge of the cap. Same story for setting up on the top of the access cover.

Randy Mosley had his RTK base station collecting data on an NGS station while doing some mapping work on Altus Air Force Base, Oklahoma. I have been pestering him to give OPUS a try and he decided that would be a good test. The coordinates OPUS gave him were a very good match for the control point.

I am going to send Randy a note and ask him to get in on this to get more specific about the results. If you look at the CORS map you will see that southwest Oklahoma is not in an ideal location for CORS use but the results were very impressive.

Test OPUS on a HARN (HPGN) station, preferably one that is also a bench mark, in your own neighborhood and you will know how well it works.

Do a test with two receivers. Use OPUS to compute both ends and then use your vector to compare results. What the heck, do one in the back yard where you can check your vector with your pocket tape. Nothing builds confidence in a new idea like doing something where you already know the answer.

Our problem is we are single frequency. JD and I have been wondering if the results we are calculating....say .1' or so horizontal, with a .1' residual...are in the same ballpark as an OPUS solution that uses dual frequency ??? And these are at the 125± mile range.

In Lake Charles, I have 1 Harn monment...if it hasn't been destroyed and 2 PAC's.....I think one is gone. And since OPUs doesn't use L1 only input......

Jimbo

I was under the impression that PAC's were a part of the HARN. I've been saying that my original homegrwon coordinates for POST were derived from HARN monuments (and local monuments).

Correct me now so I can revise any future statements regarding the origin of POST.

J.D.

p.s. If PAC's aren't part of the HARN, we have no HARN around here that I'm aware of.

pss Jimbo, Do a radial search for control on the NGS site and ask for "Horizontal - B Order or Better (HARN)". All my PAC's come up under this identification format.

Modified By J.D. Billings on 12/16/2001 at 4:55 PM

Jimbo,

I just ran the NGS search at 20 miles for N301219/W0931300 (is that close)and came up with "zilch". Except the one and only you must be referring to.

CIVIC - PID BK3291 (a CBN station)

I didn't even find a PAC within the 20 miles. But since the NAVD 88 of CIVIC is published as 6.99 feet (just behind the seawall), I suppose your airport is of the hydro style. (lol)

Man, I thought I had it bad with nothing but 3 PACS from 12-18 miles away.

We need more control, huh?

j.d.

(small case initials - embarressed because i leaped before i looked)

JD,

No reason for small case.

Yeah, CIVIC is the one and it might be gone. Gotta check...seems something was done near it recently........

I said 2 PACS but maybe not. I know of 2 monuments at the regional airport...not hydro LOL....... Just assumed they were PACS...might be SACS or lower.

I think some HARNS are PACS but not alll PACS are HARNS.....confused yet ???

Jimbo
Modified By James Webb on 12/16/2001 at 7:45 PM

Here is the note that Randy sent to me. This is based on his RTK base station set up on ORD Azimuth Mark on Altus Air Force Base, Oklahoma. Randy is using a data sheet from a survey project done for AAFB for the coordinates.

I set the base on Ord AZ and had it running RTK and at the same time had it collecting 30 second epochs. I had about 4 to 5 hours of data I think.

Ord AZ is 2nd order (established by GPS and Invar Rods). The results missed the published Altus AFB data sheets by .001m Northing, .011m Easting, and .044m Vertical. The distances to the three CORS that OPUS used was 240,987m, 157202m, and 163906m.

Not bad for a test and getting the results back in less than 30 minutes.

Did Randy say whether or not station ORD AzMk coordinates were NAD83 or not?

Results in 30 minutes? How many days did he wait before submission of the data file?

And 30 second epochs? I know absolutely zero about RTK (and a few other things ... o.k. a lot of other things), but isn't 30 seconds a bit of a stretch for RTK positioning?

Distances of 149.74, 97.68 miles and 101.84 miles. Sounds like megavectors to me.

J.D. wrote:

isn't 30 seconds a bit of a stretch for RTK positioning?

I suspect that he was transmitting at a much higher rate than he was logging.

Perhaps I submitted my post just at the same time as someone else, because the "...Bob..." subject line above didn't come from me.

Randy uses a script of PASH commands with Survey Control I in a Husky FS/2 to start his RTK base station so that he can record data at one interval and do RTK work at another. It is only a little bit tricky to make it all come together and he is getting ready to find out if it will work with a Ranger.

As I recall he submitted his file to OPUS the day after his RTK field work and then again about a week later just to see what difference it made. Randy told me the difference was very small between the two OPUS processing sessions. Not enough difference to justify waiting for the better ephemeris to come along.

He is just wanting to find out if OPUS will provide a way to put his RTK projects on the NSRS without having to kick out a bunch of control. Randy would rather do an RTK traverse than a legitimate control survey. I have to get out my big stick to get him to post process GPS data.