Accuracy Issue

Question

I encountered accuracy issue utilizing ArcPad 10.0R4 with Trimble GeoExplorer GeoXH 6000 Series.   The issue is the GeoXH unit is supposed to be less than 1ft accuracy, after post process, however, I consistently getting >=3ft.   My GeoXH 6000 setups are:
-ArcPad 10.0 R4
-Utilizing QuickProject default
-GPS Datum is default by GPS unit (D_WGS_1984)
-Projection from GPS unit (GCS_WGS_1984, Geographic, Degree, Datum D_WGS_1984 (6326))
-Protocol:  Positions ArcPad extension
-Port:  COM3: GNSS Serial Port
-Enable Averaging at 20 points
When 'Import ArcPad QuickProject' from Trimble Postions Desktop Administration for Check-in and Post Process, the collected point feature shows >=3ft, when compared with our monument data.
Our monument data is NAD 1983 StatePlane Texas North Central FIPS 4202 US Feet.
Any assistance are greatly appreciated,
Richard

AlisonWalker · Answer

I encountered accuracy issue utilizing ArcPad 10.0R4 with Trimble GeoExplorer GeoXH 6000 Series.   The issue is the GeoXH unit is supposed to be less than 1ft accuracy, after post process, however, I consistently getting >=3ft.   My GeoXH 6000 setups are:
-ArcPad 10.0 R4
-Utilizing QuickProject default
-GPS Datum is default by GPS unit (D_WGS_1984)
-Projection from GPS unit (GCS_WGS_1984, Geographic, Degree, Datum D_WGS_1984 (6326))
-Protocol:  Positions ArcPad extension
-Port:  COM3: GNSS Serial Port
-Enable Averaging at 20 points
When 'Import ArcPad QuickProject' from Trimble Postions Desktop Administration for Check-in and Post Process, the collected point feature shows >=3ft, when compared with our monument data.
Our monument data is NAD 1983 StatePlane Texas North Central FIPS 4202 US Feet.
Any assistance are greatly appreciated,
Richard

Hi Richard,

The 3' you are seeing is a classic datum shift between the WGS 1984 datum and the NAD 1983 datum that your monument is in.  Convert your GPS data to NAD 1983 or the monument data to WGS 1984, and I bet it will all line up.  The accuracy of your Geo is likely meeting the spec.  It's just the datum shift that is causing it to not line up.

MarcoBoeringa · Answer

Richard,

Please note I am not a GPS expert, but from what I read about it all, I doubt if you will be getting 1 ft accuracy without correcting against a fixed reference GPS station like in RTK/DGPS.

Stating the projection in which your "monument data" is stored (State Plane... etc.) doesn't tell anything about the accuracy of the monument data itself you compare the GPS readings too, it could be off by dozens to hundreds of feet for all we know... Projections are not a measure of accuracy.

You may find this thread of some use:

"Nightmare on GIS Street" by Eric Gakstatter