[Qgis-user] Trimble GeoXT 2005 Accuracy

[Dan]

 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> wrote:

>
> Springfield Harrison <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
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