Usage of local geoid with RS2, NTRIP correction and NMEA

Dear List

I am using a RS2-receiver with RTK getting the correction by NTRIP over the mobile-connection. The output I am consuming automatically over the RS232-port as NMEA-GGA string.

The horizontal coordinates are fantastic. Better than 0.05 meters in both directions.

The height has a constant offset of about 0.8 meters.

Personally, I guess this is related to the geoid.

Question:

  • Is there an internal geoid in the RS2?
  • If yes, is there a possibility to ignore it or to replace it with a local geoid.

The conversation has to happen in the receiver, because the data are not going the ReachView-app.

Looking forward to any ideas and hints,
Yvo

Hi, @Yvo_Weidmann, welcome back to Emlid Community forum!

The altitude that is embedded in the NMEA output is ellipsoidal. To use geoid models you must be in a Survey in ReachView 3.

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Hi @pedroaugustofaria

Thank you very much for your input. That is very helpful.

Perhaps I will switch in my code from NMEA-output to XYZ-output. With this output I will get geocentric-coordinates and the later transformation works perfectly. Now, I have readings better than 0.05m horizontally and better than 0.10m vertically.

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Excellent!

Hi @Yvo_Weidmann,

The NMEA GGA message includes 2 fields related to elevation data. These fields contain orthometric height and geoid-ellipsoid separation that are related to the EGM 2008 geoid.

However, apps receiving position output in NMEA format usually automatically calculate ellipsoidal heights in WGS84. If you don’t use any app and output NMEA data, you need to sum up 2 values from the NMEA message to calculate ellipsoidal heights.