Nomad (with ArcPad and GPScorrect) horizontal accuracy question

[YukunXing]

It is probably off-topic, but I don't know a good place to post Trimble related questions. My email to the Trimble vendor went unanswered. Folks on this forum helped me to set up GPScorrect so someone might be able to shed some light on this. Thanks in advance!

I???ve collected some test points using ArcPad on an open parking lot and post-processed using Pathfinder. The PDOP value reported was always below 5 during the collection. The post-processing summary reported a very limited number of readings to be within 5m accuracy, the majority was worse.

ArcPad settings used:
Protocol: Trimble GPScorrect
Port: COM2:GPS Serial port
Baud: 9600

Number of positions to average: 10 (both points and vertices)

Datum: D_WGS_1984


Trimble GPScorrect setting (I guess these doesn???t matter much since I???m post-processing):
DOP Type: PDOP
Max PDOP: 20.0
Min SNR: 33.0
Min Elevation: 5 degree
Velocity Filter: Off
Use GLONASS: Auto


The feature class was created in ???NAD_1983_HARN_StatePlane_Virginia_North_FIPS_4501???, as suggested by a Trimble training I attended last year. For post-processing, the selected base station was ???CORS, STERLING (LWX1),  VIRGINIA???, which is less than 20km from where the data was collected. For Reference Position, the ???Use reference position from base provider??? was selected. The base file downloaded covered 100% of the collected points.

Is there any improper settings based on my description? If not, what???s the likely cause(s) for the bad accuracy?


On a separate note, I???ve heard that using an external antenna may increase the horizontal accuracy in addition to boosting signal reception. Is this true with the NOMAD?

[YukunXing]

Eric:

I assume you'd like the uncorrected AXF which unfortunately I no longer have. So I went out and collected 2 positions really quick. This time it is better than my previous attempts. Pathfinder Differential Correction untility reports 50% of the points are within 5m. The base station I chose is "CORS,STERLING (LWX1), VIRGINIA". Files are attached.

I'm running GPScorrect 3.1 and ArcPad 8.0 on the NOMAD, and Pathfinder 4.2, ArcGIS 9.3 (SP3 I believe) on my desktop.

Jason B.:

I see. Yeah I totally agree either is a pretty bad accuracy nowadays. I am just trying to make sure I'm not the main contributor to the bad accuracy. I guess as far as that is concerned, I get my answer, since no one commented like "oh NO NO, you shouldn't do that". Actually last week I indeed was trying to download NGS benchmark and wanted to see if there was one really close. The server had some glitch though so I couldn't get it. If I test it against a benchmark in the future, I'd sure share the results. Thanks for sharing yours.

Graham:

Thanks for your comments. I assumed your point is that the GPS antenna is under the power button. With that in mind today I hold the unit as far from my body as possible.

[TomEngelsma]

We typically use 30-40 positions to average which may help your accuracy. We use an XRT receiver with the Nomad and can achieve 4 inch accuracy with real time corrections in the field using a hot spot for a Internet connection.

It is probably off-topic, but I don't know a good place to post Trimble related questions. My email to the Trimble vendor went unanswered. Folks on this forum helped me to set up GPScorrect so someone might be able to shed some light on this. Thanks in advance!

I�??ve collected some test points using ArcPad on an open parking lot and post-processed using Pathfinder. The PDOP value reported was always below 5 during the collection. The post-processing summary reported a very limited number of readings to be within 5m accuracy, the majority was worse.

ArcPad settings used:
Protocol: Trimble GPScorrect
Port: COM2:GPS Serial port
Baud: 9600

Number of positions to average: 10 (both points and vertices)

Datum: D_WGS_1984


Trimble GPScorrect setting (I guess these doesn�??t matter much since I�??m post-processing):
DOP Type: PDOP
Max PDOP: 20.0
Min SNR: 33.0
Min Elevation: 5 degree
Velocity Filter: Off
Use GLONASS: Auto


The feature class was created in �??NAD_1983_HARN_StatePlane_Virginia_North_FIPS_4501�?�, as suggested by a Trimble training I attended last year. For post-processing, the selected base station was �??CORS, STERLING (LWX1),  VIRGINIA�?�, which is less than 20km from where the data was collected. For Reference Position, the �??Use reference position from base provider�?� was selected. The base file downloaded covered 100% of the collected points.

Is there any improper settings based on my description? If not, what�??s the likely cause(s) for the bad accuracy?


On a separate note, I�??ve heard that using an external antenna may increase the horizontal accuracy in addition to boosting signal reception. Is this true with the NOMAD?

[JasonTipton]

We use an XRT receiver with the Nomad and can achieve 4 inch accuracy with real time corrections in the field using a hot spot for a Internet connection.

But, you are using a ProXRt that uses H-Star and carrier not just code. You just happen to be using a Nomad as the interface.

I do agree, though, on increasing the number of positions that you take.

[ericbock]

I processed the file with another base station and everything fell within 2-5 meters, which is the accuracy stated for the 800 series nomad.  You may want to update your base station list and always try a few different stations out to see which ones give you the best results.

Another thing to watch out for when comparing your GPS data to NGS control is that you are matching apples to apples.  There are several version of NAD83 datum, so you need to make sure you are choosing the correct one.  in PFO or GPS Analyst differential correction wizard, Using refence position from base files typically puts your data in NAD83 CORS96 datum(always verify this).  Using reference position from base provider puts the data in the WGS84 datum (similar to the NAD83 1986 datum...a no transformation datum in the trimble software).  However, since your unit is 2-5 meters it may be hard to see this datum shift.  It is more noticeable when you are using subfoot - decimeter units.

Good luck guys!

--------------------------------------------------
Searching for base files...

File C:\GPS Projects\ESRI User Forum\Base\CORS, LOYOLA 4 COOP (LOY4),  VIRGINIA\loy413011151.zip downloaded.
Successfully found or downloaded 1 of 1 files.
Search complete.

--------Base Data Details:--------------------
Using reference position from base provider:
  Name: CORS, LOYOLA 4 COOP (LOY4),  VIRGINIA
  Position: 39°06'46.32585"N,  77°33'49.79433"W,  74.10 m
Source: C:\GPS Projects\ESRI User Forum\Base\CORS, LOYOLA 4 COOP (LOY4),  VIRGINIA
  loy413011151.zip
    Local time: 5/10/2011 9:59:45 AM to 5/10/2011 10:59:45 AM
    Position: 39°06'46.29766"N, 77°33'49.78549"W, 75.27 m, 0.00 m Antenna height
    Distance from base provider: 0.89m

--------Coverage Details:--------------------
Rover file: GPSCorrect.ssf
  Local time: 5/10/2011 10:11:37 AM to 5/10/2011 10:15:35 AM
100% total coverage
  100% coverage by loy413011151.zip

--------------------------------------------------
Differentially correcting...
  Differential correction settings:
    Use smart automatic filtering: On
    Re-correct real-time positions: On
    Output positions: Corrected and uncorrected

--------------------------------------------------
Processing rover file, GPSCorrect.ssf ...
...to output file, C:\GPS Projects\ESRI User Forum\NOMAD\GPSCorrect_1.cor
    Carrier processing...
        No carrier processing performed as file has no carrier data
        Corrected 0 positions
    Code processing...
        Selected 20 positions for post-processing
        Corrected 20 positions

--------------------------------------------------
Differential Correction Summary:
    1 file processed.  In this file:
        20 (100.0%) of 20 selected positions were code corrected by post-processing
        0 (0.0%) of 0 selected positions were carrier corrected by post-processing

    Estimated accuracies for 20 corrected positions are as follows:
            Range Percentage
            ---------- ----------
             0-15cm      -
            15-30cm      -
            30-50cm      -
             0.5-1m      -
               1-2m      -
               2-5m 100.0%
                >5m      -

Differential correction complete.