A few days ago Kent McMillian asked me to comment on the need to take into account ITRF/NAD 83 coordinate system differences in OPIE/ORGI coordinate computations. Kent is right these differences do need to be taken into account, so here are my comments.

A GPS vector is referenced to the coordinate system of the satellite coordinates used in its computation. This means the vector's differential coordinates are referenced to the axes orientation and scale of the satellite coordinate system. A vector's differential coordinates do not contain information on the origin of the axes of any coordinate system. Although when computing the GPS vector the coordinates of one of the two stations involved has been held fixed to its NAD 83 value, it is easy to understand that the coordinates of this single station cannot control the orientation and scale of the GPS vector.

The satellite coordinates used by surveyors are in almost all cases referenced to the ITRF97, ITRF 00, or the WGS 84 coordinate systems. For practical purposes the orientation and scale of these coordinate systems can be considered identical. So we will simply say that GPS vectors computed in OPIE/ORGI are referenced to the ITRF97 coordinate system.

Normally when surveyors use vectors to compute coordinates of stations of unknown position, they hold fixed coordinates of one or more stations to their NAD 83 coordinates. The resulting computed coordinates of the unknown stations are then treated as NAD 83 coordinates. However, an error has been introduced because of mixing coordinate systems -- ITRF97 for the vectors and NAD 83 for the stations held fixed.

For the line lengths involved in most survey applications the error is so small it can safely be ignored. The error introduced is about 0.015 cm per 1 km of line length. So in dealing with survey areas no more than 20 km in extent you are talking about errors that are, at most, a few millimeters. But for the OPIE/ORGI situation where you are talking about line lengths of several hundred kilometers you are talking about errors of several centimeters.

The approach to satisfactorily eliminating the errors due to mixing coordinates is not too difficult. First, no new vectors need be computed. The vectors are already referenced to ITRF 97. To get ITRF 97 coordinates for the stations whose coordinates you are computing all that needs to be done is to hold fixed in the coordinate computations the ITRF97 (1997.0) coordinates given on the NGS data sheets, rather than the NAD 83 coordinates, as has been done previously. The result will be ITRF 97 (1997.0) coordinates for POST or other stations being solved for. To get NAD 83 coordinates one needs to use the transformation parameters between ITRF 97 (1997.0) given on the NGS web site to convert from ITRF to NAD 83 coordinates. The above approach ignores changes in ITRF and NAD 83 coordinate axes orientation differences between 1997 and the current date. But again we are back to the few millimeter level of error.

Using an NGS computer program with the parameters built in is best if you are like me. Whenever I have to input to a program something like transformation parameters, I always get the sign wrong somewhere.

I would be more than glad to reprocess any/all of the three sessions using the ITRF97 values for the CORS stations. That would not be a serious problem. Taking the resulting ITRF97 related coordinates for POST to NAD83 is my problem. Is it possible for me to have access to this software to make the coordinate transformation possible?

J.D.

Now I have seven books on my desk and no closer to the truth...

Guess I'll have to re-read all the stuff on NGS again....and on a holiday weekend to boot....

TM

my dumb question above. I had a pre-senior moment and forgot where I left the transformation software. Found it again.

j.d.

Did i just miss it in the settings or do you have to create the ITRF system in Solutions ??

Jimbo

Jimbo,

The ITRF97 coords are in the published data sheets. I would imagine its just a matter of processing geodetic/elipsoid in settings.

I've tried (including 3 or 4 weeks ago) today to use the HTDP programs to get "known" results for idiot testing. Me being the idiot. In order to convince myself I could follow the procedures properly I was trying to convert the published ITRF97 coords to NAD83 values as published. Close, but no cigar. I have no problem with inputing the ITRF geo coords into the processor, and getting ITRF results. I do need some provable examples though on converting from ITRF to NAD83.

j.d.

by the way, I tried to email you last night but got a "Return to Sender". I suppose Elvis is running a bait shop on you side of the river now and intercepting emails on the side.

I did some testing last night using the HTDP program to convert several HARNS (that we had done OPUS sessions on..
I input the OPUS ITRF and computed the NAD-83 value.It agreed pretty good but not exact. The difference would be in the fact that the OPUS is a calculated position based on the data processed and not an absolute coordinate.

I did notice on looking at a couple of the OPUS reports that they showed the residuals to be lower on the NAD83 than the ITRF positions (?)..

I posted some questions on the main board about ITRF (didn't want to ask Trimble software questions on this side), but some of the answers are generic and may apply to your software as well...

Didn't try it, but the published ITRF at one of the CORS should calculate (using HTDP) pretty exact to their published NAD83..Or should, since this is probably the method the NGS used to arrive at a NAD83 station position..

Don't know about you, but I'm not going to spend as much time on the ITRF system as we have on other issues. At this point, all my points (PID Sheets) only have published NAD83 values, so this is my ultimate goal to match..

TM

PS- Merry Christmas to all you Blue Box Guys!

TM,

Thanks. I'm still not doubting the future need of ITRF shift/rotation, or some other means of updating control points. I think it's a valid point. I'm going to continue to concentrate my limited mental resources and time on my original mission plan.

First, to show that repeatable results are possible with L1 only from "extreme" distances, ie 100k and above, with the following in mind:

1. Lengthy sessions of at least 8 hours

2. Stable ionospheric conditions exist, including kP index of 2 or below, no solar flairs within the past 36 hours prior to the session (determined from x-ray flux), with observation session beginning at or about local midnight in hopes of the lowest TEC possible.

3. Use 4 or more CORS stations (or other L1 signals) distributed around the point to be determined. I still think geometry comes into play.

4. Use all vectors in the processor, including CORS to CORS. I still have a hunch this adds necessary data for the least squares solution.

5. Constrain to ALL CORS stations for final adjustment. See 4 above.

No matter what reference frame we chose to use, whether it be ITRF or NAD83, I think I'm close to proving the validity of my above mission goals. I am a dirt surveyor, not a geodisist. My goal is to find out if I can put this stuff to work for me. If this stuff hasn't been thouroughly researched by now, it should have been. And if it has been, send me the book, preferably the "dirt surveyor translation".

j.d.