What datum is used when streaming GGA via UART?

[b.p]

I’m trying to ascertain which vertical datum is used for the elevations given in the GGA strings when streaming positions via UART/Serial.

For reference, I’m in Australia and the CORS network is in GDA2020. Our company uses Australian Height Datum (AHD) for most cases.

Everything I’ve read (example) suggests that the elevations in a NMEA GGA string should be in ellipsoidal heights with another column containing the geoid separation.

I just did a quick test that included recording the same point with the ReachView app as well as streaming GGA strings via UART to a logger.

The elevations shown in the GGA strings are the same value as the ‘Elevation’ column from the exported ReachView data - i.e., in AHD and not Ellipsoidal heights.

I also tried creating a new ReachView survey project using ellipsoidal heights for the vertical datum. I recorded a couple of points and then checked the GGA strings again and they were still showing the AHD height.

Could someone please provide some info about whether this is expected behaviour and if I’ve got myself confused.

Cheers!

[kseniia.suzdaltseva]

Hi @b.p,

It can be a bit tricky indeed

In the NMEA standard, ellipsoidal heights are divided into orthometric height and geoid separation. These are the 9th and 11th fields in the GGA message. The vertical datum chosen in ReachView 3 doesn’t affect the format.

Software that receives the messages often sums up these values to provide ellipsoidal heights. You can sum up them yourself too.