-brendan



*Message sent from a system outside of UConn.*

 

Yes, definitely, a double transformation is happening, but where? In which step(s)? I would expect position #1 or position #4 to be the "correct" workflow, so I don't understand why position #3 is lining up with my benchmark. For my own sanity, I need to know exactly what PFO is transforming/double transforming, and why when I use the reference position provided from the base files does my data not align with the benchmark?

 

<gnss_positions.png>

 

 

 

 

On Thu, Sep 30, 2021 at 1:58 PM Stacy Deming <[log in to unmask]> wrote:

*Message sent from a system outside of UConn.*

 

Short answer is you are transforming your data twice, or this is what I believe is happening. If you do the export using NADCON ( all 0 parameters in the transformation equation) you should get positions that correlate with control. When you differentially correct the positions they are already transfomed to the base station Datum. The only way you will know is to give it a try but I think this is what is going on.

 

A 1 meter shift is a sure sign of a Datum shift.

 

 

On Thursday, September 30, 2021, 12:53:40 PM EDT, Justin Richardson <[log in to unmask]> wrote:

 

 

*Message sent from a system outside of UConn.*

 

I Had the exact same problem with a Trimble 7x and Zephyr antenna and using Pathfinder.  I tried everything for years and couldn’t correct the data.  I finally gave up, purchased an R2 with RTK subscription and collect via iPad running Collector and Survey 123 and Voila!  No more shift!  I know that doesn’t answer your question, but I just wanted you to know that I struggled with the same shift.

 

From: Northeast Arc Users Group <[log in to unmask]> On Behalf Of Christina Herrick
Sent: Thursday, September 30, 2021 12:29 PM
To: [log in to unmask]
Subject: questions about GNSS position shifts

 

*Message sent from a system outside of UConn.*

 

I am struggling to understand how Trimble’s Pathfinder Office transforms and exports GNSS data and I’m hoping someone here can explain.

 

We collect our GNSS positions in autonomous mode using a Trimble 7X Centimeter Edition rover unit and then use Trimble Pathfinder Office to correct these positions against a CORS base station. Our unit collects in the WGS84 datum (we confirmed this by checking the settings). Our correction process has always used the reference position provided from the downloaded base files, which in this case is in the NAD 1983 (2011) datum. We do not choose to use the reference position provided from Trimble’s base station list, which provided the reference position in terms of ITRF00 (1997) (which I believe is synonymous with WGS 1984 (G730)). 

 

After differential correction, we provide a coordinate system and datum to export features. We’ve always exported in latitude/longitude in the WGS 1984 datum. However, for a recent project we’ve changed our procedures and exported data in latitude/longitude, but in the “ITRF to NAD 1983 (2011)” datum provided by PFO. We’ve noticed that these positions have shifted about one meter when compared to our original procedures of exporting in WGS84. This generally makes sense to me because I know that WGS84 and NAD83 have about a 1m shift, but I get into the weeds when I process/export our data in other ways.

 

We collected a position over a NHDOT benchmark. We’re less than one mile from the CORS base station. Our unit delivers +/- 1.0cm horizontal accuracy after post-processing. I’ve exported our data four different ways: (1) using the downloaded reference position and exporting in WGS84 datum, (2) using the downloaded reference position and exporting in ITRF to NAD 1983 (2011), (3) using Trimble’s reference position and exporting in WGS84 datum, (4) using Trimble’s reference position and exporting in ITRF to NAD 1983 (2011) datum. I also downloaded from NH GRANIT a list of benchmarks that includes the one we’ve used here, so I can compare our outputs.

 

Position #3 from above, that used the Trimble-provided reference position (in ITRF00 1997) and was exported in the WGS84 datum, aligns with the benchmark shapefile (there’s a 1cm shift, which is within the margin of error). 

 

The positions that were corrected according to #1 and #4 above align with each other

(+/- 3cm), but are about 1m south of the benchmark. The position that was corrected using #3 above is about 2m south of the benchmark.

 

Can someone please explain to me what’s going on behind the scenes with PFO to cause the points to change position, and what is the “correct” workflow?

 

Thank you!

Christina

 

------------------------------------------------------------------------- This list (NEARC-L) is an unmoderated discussion list for all NEARC Users.

If you no longer wish to receive e-mail from this list, you can remove yourself by going to http://s.uconn.edu/nearcsubscribe.

---

The information contained in this e-mail is intended for the addressed recipient only. If you have received this e-mail in error, please contact us at (207) 985-3385. This e-mail does not reflect an intention by the sender to conduct a transaction or make any agreement by electronic means or provide an electronic signature unless otherwise directly stated herein. We assume no liability for any damage claimed to have been sustained as a result of transmittal of malware.

------------------------------------------------------------------------- This list (NEARC-L) is an unmoderated discussion list for all NEARC Users.

If you no longer wish to receive e-mail from this list, you can remove yourself by going to http://s.uconn.edu/nearcsubscribe.

------------------------------------------------------------------------- This list (NEARC-L) is an unmoderated discussion list for all NEARC Users.

If you no longer wish to receive e-mail from this list, you can remove yourself by going to http://s.uconn.edu/nearcsubscribe.

------------------------------------------------------------------------- This list (NEARC-L) is an unmoderated discussion list for all NEARC Users.

If you no longer wish to receive e-mail from this list, you can remove yourself by going to http://s.uconn.edu/nearcsubscribe.