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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?

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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.

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.

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?

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.

NOMAD.zip

toolbox2.jpg

GrahamW

Back in the days of me being a GPS salesman.... we tested Juno vs nomad and found that the juno allways had better results.. and a Trimble rep informed us that TDS put the GPS chip on the nomad under the power button(close to your body and hand!) where the juno has its chip top and centre under the Trimble m.

the test we used was to map car park lines in and out of cover at the same time to get as close as possible to the same ephemeris results...

jasonblocker1

xynewtry,

We had the same issue, reported accuracy did not match results for post processed data. We did not use gps autocorrect, instead collected with terrasync (post processed) and compared against arcpad only with SBAS. We collected over surveyed control points (6) in a vareity of environments
Results are attached, I'd guess that your picking up some multipath...somewhere. See attached image below referencing post processed sirf chip data.

I am suggesting that post-processing NOMAD data will not make or break your work. If your comfortable with 1-3M reported, 2-5M reported isn't that big of a jump. Our expereince with ArcPad and no correction we achieve <3M accuracy and have field verified over known control and in far worse terrain/environment than your average unit is going to come across. If your super interested, chase down some ngs bench marks and go out- test your unit with GPS correct and without, write it up and share with the world.

Jason

error.JPG

Doc1.pdf

ericbock

Can you send me your data (AXF and GPScorrect.ssf) file and I will take a look at it. What version of GPS Analyst are you running, as well as ArcGIS and service pack.

eric_bock@neigps.com

YukunXing

Thank you for all your reply guys. I understand NOMAD is no match for a Geo, but that's not the point here. The Post-processed accuracy I'm getting is not as good as even the real-time accuracy from the Trimble spec sheet, that's what puzzles me and why I want to figure out if I was doing anything wrong.

To answer some of your questions.

It is a 900 G series;

My claim that the majority of my test points were worse than 5m is from the Pathfinder post-processing summary screen. Based on that out of the 20 locations collected, 2 were between 3~5m, 18 were worse than 5m;

The points were collected on an open parking lot, so there shouldn't be much effects from trees or buildings etc;

The unit had been on for a good 15 mintues (I waited until the WAAS real-time correction kicked in) before I started collecting. Most of the time during the collection, the PDOP is around 2.2, but I did notice it jumped to around 5 at times;

Yes I was standing still during the collection;

"Running gps correct on a receiver that isn't collecting code and carrier is a total waste of time."

I don't fully understand this statement. I'm using ArcPad to collect data, and the purpose of GPScorrect is so that I can post-process. Jason B., are you suggesting that post-processing is meaningless for a NOMAD?

Thanks!

jasonblocker1

Moral of the story: Nomad is a Mapping grade receiver. It makes pretty pictures on a map, and that is all I expect from it.

Exactly, it depends on what your using it for. If you need the accuracy, buy the equipment and get it. For me and our workflow, we've found that the Juno fits our general state wide inventory very very well. Running gps correct on a receiver that isn't collecting code and carrier is a total waste of time.

Jason B.

JasonTipton

I'm also assuming that the GPS chip is similar in Juno and Nomad.

I believe they have the same chip, but Trimble specs the Juno and the new Nomad 900's at 1-3m where the older Nomads are at 2-5m. Both of those are assuming that you postprocess. They blame the Nomad's powerful processor chip for the interference.

Also, it is true that the elevation mask is lower, but you can throw out those calculations in Pathfinder and apply a mask on the back-end before postprocessing.

Moral of the story: Nomad is a Mapping grade receiver. It makes pretty pictures on a map, and that is all I expect from it.

JoshWhite

Wow, that is bad accuracy. Some car GPS models can do that well. I consider a PDOP value over 3 to be unacceptable for our data collection. We use Nomads for the field computer but have ProXH receivers mounted on a pole and can routinely get sub meter and often sub foot accuracy. Most results I would say are sub 2 ft.

jasonblocker1

We went through a similar test when developing a plan to deploy 15 Juno's for a state wide inventory. Ultimately we decided that the assumed accuracy of post processing didn't warrant the cost of time and money. We tested Arcpad/Juno without any correction against Terrasync/Juno accuracy wise and the Arcpad/Juno beat accuracy of processed data 4/5 points collected. I later found a write up by someone from National Parks and found the excerpt in attached jpg. All of our testing was done away from the city, wooded or pasture land.

If your only collecing code, does it really matter if it's 2-5 M accuracy as oppposed to 1-3 M? I'm also assuming that the GPS chip is similar in Juno and Nomad.

Jason

SiRF Chips.jpg

Untitled.jpg

JasonTipton

what�??s the likely cause(s) for the bad accuracy?

A:You're using a Nomad.

What Nomad are you using? 800 or 900 series? The 800 series is only accurate 2-5m, 900 1-3m.

You said very limited number under 5m. What was the actual breakdown? What do you consider worse than 5m? That is a pretty broad range.

I wouldn't waste your money on an antenna. It may help with reducing multipath and it would get the signal above your head, but you are still using a Nomad.

Everything degrades your positional accuracy. Were you around trees, buildings, any source of radio-interference? Did you wait until the points were collected before you started filling out any attributes? Were you looking down at the screen watching it collect the points, or did you have the device above your head or at least head level? Were you standing perfectly still while collecting? How long did you have the device on before you started collecting? If you have a 900 with Windows 6.1 than it gets a "Hotfix" at first. Then, after a few minutes it gets a real fix. The "hotfix" is handy and quick, but not as accurate as a real fix. Did you have a fresh almanac?

5 isn't that great of PDOP. Try for something a little closer to 2.

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