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RTK Corrections and Offline Editing

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06-30-2023 06:33 AM
GregCarlino2
Occasional Contributor

Greetings:

I am new to field data collection, so I'm still learning basic workflows and need to clarify my understanding of a seeming contradiction in the data-collection process:

I have a Leica GG04 Plus, with Zeno Connect, which I would like to use for High Accuracy data collection within Field Maps.  Being in Ohio, we have access to the ODOT Real Time Network (NTRIP), which can provide the necessary RTK corrections to achieve the high accuracy (1-2cm horizontal) advertised with the GG04.  This connection, however, requires a Wi-Fi connection.  If wi-fi is required for, then I would need to take a mobile hot-spot (or possibly a tablet with cellular plan) to acquire this connection in the field.  (I'm trying to avoid an using L-Band correction service subscription).  However, a wi-fi connection would make offline data collection a moot point.  Is this a Catch-22, or is my understanding off?  Do I have to choose between High Accuracy and Offline collection capability?

Thank you,

Greg

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MichaelLohr
Occasional Contributor II

Greg your setup is very similar to what we have been running for several years, using Trimble Mobile Manager (TMM), R1 and R2 GPS and using the cm level high accuracy via internet connection when using the R2. Our mobile field units all have cellular capability, and this enables access to the RTK corrections via internet. Our connection to the correction signal is similar to your ODOT connection. 

To use it without a subscription to a satellite-based correction signal, you must provide an internet connection somehow, either through the cellular enabled tablet, a smartphone hotspot, or a pocket style cellular hotspot. In addition to the correction signal, TMM needs to login check your credentials while starting up, though Field Maps now does this behind the scenes. The R1 unit doesn't need the correction signal or TMM at all, but you still need the internet connection to display your maps on the mobile device.

For disconnected editing in areas where there is marginal or no cellular signal, you will be limited to sub-meter accuracy via the R1 unit, if you don't use a satellite-based subscription service for corrections. Yes, this is the catch-22. Preparing for disconnected editing means downloading the needed data to the mobile device while you do have good internet connection, then resynching only when back in good cell service area or risk losing your collected data. Surveyors are able to overcome the "no cell service" limitation to high accuracy work by setting up a separate GPS base station at a known fixed location, then transmitting correction signals from the base station to the rover station via radio signals. More complicated and more expensive equipment but doesn't rely on internet-based corrections. 

Hope that helps.

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8 Replies
ColinLawrence
Esri Regular Contributor

Hi Greg,

You can still utilize "offline" data collection in Field Maps even if your device has a data/wifi connection. If you download the map offline in Field Maps all of the edits will be stored locally and synced back to the server at your convenience or on the default sync interval. This can be beneficial if you have a spotty connection or will be moving between areas of connectivity.

Is there are reason you wouldnt want to use the "online" version of the map given you need to have connectivity to access RTK?

Regards,
Colin
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GregCarlino2
Occasional Contributor

Hi Colin,

Yes, this is the 'catch-22' I was referring to, and why I'm confused: 

If I need a wi-fi connection to leverage the RTK connection, then doesn't that make being able to go offline a moot point?

Perhaps I can reword my question this way: 

Is offline data collection only feasible in use-cases where High Accuracy (non-RTK) is not a requirement?

Thanks,

Greg

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ColinLawrence
Esri Regular Contributor

RTK would be the limiting factor here given it would only be available where you have connectivity. To answer your question more directly; offline data collection is not only feasible in cases where high accuracy is not a requirement. Offline maps should be used in any scenario where you will have limited to no connectivity in the field. However, they can also still be used where you expect connectivity but maybe dont need to push edits immediately in the field or want to limit data usage which was the point to my original comment. Whether or not your GPS receiver (or correction service) is available in the same area is really a separate question.  I hope this helps!

Regards,
Colin
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JoelCusick
New Contributor III

Colin, correct me if I'm wrong, but the basic premise is Field Maps must have a basemap in order for any data collection to occur in the field, with or without realtime access to a NTRIP stream.  NTRIP is second-by-second realtime positions being stored in the map.   Intermittent cellular broadband connectivity is the achilles heel in field collection.  Therefore, going into the field without an offline map ahead of time and losing NTRIP also means losing the map.  Data collection ceases.  Going into the field with an offline map allows Greg to continue to store positions offline (but without the precision of NTRIP, and the resultant storage of positions in two different datums...an issue that can only be corrected in a post-process environment).  No offline map = No data being collected when internet in the field breaks down.

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ColinLawrence
Esri Regular Contributor

Hi @JoelCusick , thanks for the reply. The point I was trying to make is that taking a map "offline" in Field Maps does not mean that the data/wifi connection on your device stops or that we will not attempt to access it; it simply means the basemap is downloaded and edits made in this map will be stored on a local geodatabase and synced back to there server if/when a connection is available. You can use the "offline version" of a map with a data connection and with an RTK/correction service connection. I may be going off on a tangent a bit from the original question but I just wanted to try and clarify that.

As RTK requires a data connection it might make sense to not have to bother preparing your maps for offline use. There are other services, as others have pointed out in this thread, that would allow high accuracy data collection in areas of no/low connectivity. I hope this helps clarify things!

Regards,
Colin
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MichaelLohr
Occasional Contributor II

Greg your setup is very similar to what we have been running for several years, using Trimble Mobile Manager (TMM), R1 and R2 GPS and using the cm level high accuracy via internet connection when using the R2. Our mobile field units all have cellular capability, and this enables access to the RTK corrections via internet. Our connection to the correction signal is similar to your ODOT connection. 

To use it without a subscription to a satellite-based correction signal, you must provide an internet connection somehow, either through the cellular enabled tablet, a smartphone hotspot, or a pocket style cellular hotspot. In addition to the correction signal, TMM needs to login check your credentials while starting up, though Field Maps now does this behind the scenes. The R1 unit doesn't need the correction signal or TMM at all, but you still need the internet connection to display your maps on the mobile device.

For disconnected editing in areas where there is marginal or no cellular signal, you will be limited to sub-meter accuracy via the R1 unit, if you don't use a satellite-based subscription service for corrections. Yes, this is the catch-22. Preparing for disconnected editing means downloading the needed data to the mobile device while you do have good internet connection, then resynching only when back in good cell service area or risk losing your collected data. Surveyors are able to overcome the "no cell service" limitation to high accuracy work by setting up a separate GPS base station at a known fixed location, then transmitting correction signals from the base station to the rover station via radio signals. More complicated and more expensive equipment but doesn't rely on internet-based corrections. 

Hope that helps.

GregCarlino2
Occasional Contributor

Michael, Colin, and Joel.  I am learning the field data collection process, as I can, and this is certainly a point of confusion.  Though the wifi/cellular requirement for RTK is the assumption I was left with, I could not find it stated explicitly, as such, anywhere.  This answers my original question, and gives me an explanation that I can take back to management to justify a request for a mobile wifi hotspot.  Thank you all for your feedback, I appreciate the assistance.

Regards,

Greg

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MichaelLohr
Occasional Contributor II
You are welcome, sir. Good luck.
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