[Qgis-user] Trimble GeoXT 2005 Accuracy

[Nicolas Cadieux]

Yes the Emlid unit is quite interesting for the price...

Nicolas Cadieux
https://gitlab.com/njacadieux

> Le 6 mars 2021 à 11:04, Dan <19dmb83 at gmail.com> a écrit :
> 
> 
> 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
>> _______________________________________________
>> Qgis-user mailing list
>> 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
> _______________________________________________
> Qgis-user mailing list
> 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/20210306/61b269b0/attachment.html>