Why are my data collector points shifted southwest of my survey point?

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08-23-2019 04:18 PM
LauraMawhinney
New Contributor III

Hello,

Our current process is to use high accuracy Trimble R2 GPS units with a NTRIP connection to local CORS stations here in Tucson, AZ.  The horizontal accuracy is falling anywhere between 1 and 5 inches. We had our Survey team collect a set of data points so that we use this for comparison.  When comparing the Trimble points to the survey points, they are within a range of 1 to 5 inches.  Anything less than 6 inches is acceptable for us.  However, we are also experience lost connections from time to time. 

So, I am testing out Trimble's RTX Field Point product as an alternative.  The fix time is anywhere from 2 to 5 minutes.  The horizontal accuracy reported back is generally 3.9 inches. The mobile device is a samsung galaxy S8.   My web map layers have a WKID projection of 3857.  So I set the collector for ArcGIS (classic) location profile as:

GNSS Receiver Coordinate System: GCS ITRF 2008

Map Coordinate System: WGS 1984 Web Mercator Auxiliary Sphere

Datum Transformation: itrf_2008_to_nad_1983_2011 + wgs_1984_(itrf08)_to_nad_1983-2011

When I bring the points collected while using Trimble's RTX field point product into ArcPro they (green pts) are all consistently shifted west and south to the survey points (yellow).  This leads me to think that I am not doing something right rather than an issue with the unit or the RTX.  Attached is a screen shot of the location profile and of the points compared to the surveyed location with a one foot buffer.  I am thinking it may be the profile, but I am not sure what else to use.

If anyone has an idea of what is going on, I would really appreciate your thoughts, ideas and suggestions.

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10 Replies
LanceCole
MVP Regular Contributor

Laura, 

Oh the fun of differing coordinate systems.  If all your points are shifted about the same distance and direction this is typically the result of a transformation error between differing coordinate systems.

  1. check with your surveyor and confirm exactly which coordinate system your “control” points were collected.  
  2. confirm the coordinate system your GPS is utilizing for your raw data.  This raw data should be stored in your point data as latitude, longitude and altitude if you have enabled high accuracy GPS data in your feature service.  
  3. use the latitude, longitude and altitude values to project your survey data to the same coordinate system as your control data.  This can be completed in Pro using ProjectZ as outlined in Is it possible to collect accurate Z coordinate values with Collector and Trimble R2?  if you do not need the elevation data you and just use Project—Data Management toolbox.  The the raw data from the GPS eliminates the potential of introducing additional transformation error between your GPS and published map.  
  4. If needed, try different transformations when using Project or ProjectZ if your points are still off and see if the one works better than another

If you can attach your control points and sample data I would be glad to take a closer look. 

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LauraMawhinney
New Contributor III

Hi Lance,

 

I appreciate your thoughts. Yes, dealing with coordinate systems and getting them correct is tricky and challenging.  Here is some history.  The surveyor provided the point information in degrees, minutes and seconds.  I converted them to the NAD 1983 HARN StatePlane Arizona Central FIPS 0202 Feet Intl proj coordinate system (WKID 2868).  I projected the points taken with with RTX using the project tool from WKID 3857 to 2868 with a geographic transormation of NAD_1983_HARN_To_WGS_1984_2. 

 

I am going to take you up on the offer of supplying data if you want to look at it.  Attached is a zipped FGDB with three feature classes.  SurveyPts_2868 is the survey points.  RTX_testpts_3857 is the original "raw" data from the hosted feature service.  RTX_testpt_2868 is the result of the projection.

I am curious what you find.

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LanceCole
MVP Regular Contributor

Laura,‌,

I will take a look. 

A couple of red flags went off when reading your response, mainly pertaining to the initial surveyors data.  Even though they gave you latitude and longitude, what system was it in?  WGS_1984, GSC_North_America_1983_HARN or HARN_Corrected, NAD_1983?  Was it in US_Feet or International_Feet?

Also, can you please confirm your GPS RTX is using GCS ITRF 2008 and what datum and unit for the altitude values?  The Trimble site indicates that ITRF2014 is used after March 23rd, 2017 and references other ESRI GNSS Receiver Coordinate Systems to use for your configuration.

LauraMawhinney
New Contributor III

Lance,

Thank you for looking at the data I supplied.

The surveyors use WKID 2223.  Our GIS department uses WKID 2868.  I reached out to my Trimble contact last Friday and asked about the offset issue.  I am hoping to get a reply back this Monday.  I will confirm the datum and units for altitude values.  I am attaching another FGDB with more layers.  I went out this morning and took points at the same locations, but with the NTRIP connection instead of the RTX.  The are close to the survey pts again.  Additionally, I included points from a loaner Leica and Trimble unit taken back in 2017 with a NTRIP connection.  These also cluster all in the same areas.

You mentioned that Trimble site indicates that ITRF2017 is used after March 23rd. 2017.  I found this tidbit online here: Configuring the correct GNSS Receiver Coordinat... | Trimble   There is an asterisk *ESRI Collector does not yet support ITRF2014(current) so this is the best approximation.

I confirmed that data collector doesn't have an option for ITRF for 2014.  The highest I see is 2008.  I will try the EPSG 4326 and see what happens.  

Thank you for your interest.

Laura

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LanceCole
MVP Regular Contributor

Laura,

I looked through both your datasets and cannot determine a specific issue.  It defiantly looks like a transformation error or some type of offset in the system as all the points are off about 1.15 feet at a bearing of 240 degrees.  Since the points from the NTRIP and other points from 2017 align with your surveyors data, that further points to something with the RTX.  I am not very versed with RTX as I am with RTK nor Trimble products.  Unfortunately, Trimble's web site is being temperamental the past couple of days so I could not dig into it learn a little more.  Hopefully your rep reaches out to you on Monday.

We had a similar issue getting our GIS data zeroed in using Collector with a Leica GG04+.  There are so many places an error can be introduced with all the transformations that are utilized between the GPS, Collector, AGOL, then bringing it all back into Desktop typically with other data on different coordinate system.  It took us a couple months to work out all the small errors the last being that you could not directly transform between NAD83(86) which our surveyor uses and NAD83(2011) which our GIS uses without introducing an error of about 1.5 feet.  We found we have to go from NAD83(86) then to NAD83(HARN) and then NAD83(2011).  The data is then within the accuracy of the instruments. It took us a month to figure that one out.

Try setting your GNSS Cordinate system to EPSG 4326 and confirm your surveyors coordinate system.  I would be glad to take a look again once you have that info.

Sorry, I could not be of further assistance.

LauraMawhinney
New Contributor III

Hi Lance,

Thank you for looking at the data and providing feedback.  I also found your journey to resolve issues with your collections effort informative and hopeful.  It gives me hope I can do this too!

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DrewHomola
New Contributor II

I am experiencing the same issue. After talking to a Trimble rep, I have learned that this is a known issue with RTX. The transformation comes from the RTX using ITRF 2014. The rep said ESRI is working on the issue but I have not confirmed this.

MattBrady
New Contributor III

Anyone know if ESRI is making any progress on an ITRF2014 to WGS 1984 Web Mercator Auxiliary Sphere transformation that would allow the Trimble R2 with the RTX satellite correction service to work correctly?  If you set up a new location profile in Collector, there is now an ITRF2014 coordinate system available (EPSG 104019), but when you select WGS 1984 Web Mercator Auxiliary Sphere (EPSG 3857) as the projected coordinate system, it says "No transformation is required, or no valid transformation is available."  It would be nice to know which of the 2 is true - i.e. "No transformation is required", or "no valid transformation is available".  I'm guessing the latter, because when I collect data in ITRF2014 and the default GCS WGS 1984 profile, the points land within 1 cm of each other; so it doesn't seem to have a transformation effect.  I'm guessing a lot of other Collector users are using the Trimble R2 with the RTX correction service, so I hope ESRI gets this solved ASAP.

TinaSlunt
New Contributor

Also interested in the progress for this!  Please respond if you're heard anything.  

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