[Qgis-user] Trimble GeoXT 2005 Accuracy

[Springfield Harrison]

Thanks Dan.  See my relies to Kirk and Greg.  The Emlid sounds 
interesting, will have a look.

Thanks again . . . .

-----
Cheers, Spring



On 06/Mar/2021 08:03, Dan wrote:
> Kirk is spot on. That unit is for GIS use and cannot receive RTK GNSS 
> corrections. You will need a survey grade receiver, with RTK 
> corrections (or post processed) for better accuracy.
>
> Budget option for cm accuracy is the Emlid Reach RS or RS2
>
> On Sat, 6 Mar 2021 at 23:53, Greg Troxel <gdt at lexort.com 
> <mailto:gdt at lexort.com>> wrote:
>
>
>     Springfield Harrison <stellargps at gmail.com
>     <mailto:stellargps at gmail.com>> writes:
>
>     > I recently acquired a Trimble GeoXT 2005 Series and am puzzled
>     by the
>     > results it produces:
>     >
>     > 1. Compared to a variety of "known" points, it consistently records
>     >    positions that appear to be in error by 1.2 - 1.5 m NW from the
>     >    known point.
>     > 2. Points are collected and then mapped in QGIS as NAD83, UTM
>     Zone 10 N.
>     > 3. The known points include property survey pins, Government control
>     >    survey monuments, Total Station survey points derived from the
>     >    above, other GPS results (Trimble ProXRS) and identifiable
>     points on
>     >    orthophotos.
>     > 4. I'm using SBAS correction in the GeoXT.
>     >
>     > It appears to be adding a consistent offset to the GPS result
>     although
>     > no offset has been set in TerraSync.
>     >
>     > Many thanks for any thoughts on this situation . . . . .
>
>     I'm really not clear on what this particular receiver is purporting to
>     do, but a consistent meter-ish offset smells like an incorrect datum.
>
>     If you are using SBAS and in the US, that means WAAS.  So you are
>     getting results that in some CRS that the list hasn't figured out what
>     it is, but "ITRF2008 current epoch" is my best guess.  That's
>     essentially equal to "WGS84(G1762) current epoch".
>
>     Those frames are definitely not equal to any flavor of NAD83.
>
>     qgis, via proj, will treat "WGS84"  and "NAD83" both as datum
>     ensembles
>     and because each ensemble has a low-accuracy member treat them as
>     equal,
>     and thus choose a null transform.  IMHO this is the wrong thing to do,
>     as WGS84(G1762) and NAD83(2011) are both datums with high intrinsic
>     accuracy and are definitely not  equivalent.
>
>     Converting from ITRF2014 to NAD83(2011) will apply a datum shift.
>
>     Advice 1 is to shift your project CRS from NAD83 to ITRF2014 and
>     see if
>     the relative position of the observations and controls changes. 
>      If so,
>     you have datum transform trouble.
>
>     My real advice 2 is to take the data file from the unit and label
>     it as
>     ITRF2014, and then see how things line up.  If you are talking about a
>     meter you need to really pay very close attention not only to datum
>     labeling but also in understanding the transformations your
>     software is
>     making.
>
>     Greg
>     _______________________________________________
>     Qgis-user mailing list
>     Qgis-user at lists.osgeo.org <mailto:Qgis-user at lists.osgeo.org>
>     List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
>     Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20210306/ce088d96/attachment.html>