ArLocationDataSource: Update to include options for reporting height as MSL

[nCastle1]

Background:

• Android's location library reports altitude in WGS84/height above ellipsoid (HAE)
• iOS's location library and many of Esri's data products use height relative to Mean Sea Level (MSL)
• The difference between WGS84/HAE and MSL can be significant and varies by location, so it is not trivial for the user to correct.

Options:

1. Listen for and parse NMEA messages - GGA and GNS in particular - to read the altitude provided in MSL by the GNSS device
   • This is what the popular gpsTest app does to report MSL values - source and source
   • This is somewhat finicky as it depends on behavior that varies considerably between devices, but appears simplest in terms of toolkit implementation; otherwise we'd need to ship an elevation model.
2. Use a geoid model to correct elevation values before passing them on.
   • EGM2008 is available in a GIS format; perhaps we could use that
3. Use GeometryEngine doesn't currently support projectZ, so not an option
4. Use a web service
   • Relatively simple to implement, but do we want to hard-code projection service into the location data source?
   • Is it acceptable to wait on a web service for getting the altitude as often as once a second (or more often)? probably not

Additional Requirements:

• There needs to be an option to enable or disable this; customers may still want to use the more accurate HAE values rather than MSL
  • Proposed: returnAltitudeAsMeanSeaLevel

[nCastle1]

Interesting ArcUser article

[TADraeseke]

This might be relevant when it comes time to implement this in the Android toolkit: https://stackoverflow.com/questions/57975969/accessing-nmea-on-android-api-level-24-when-compiled-for-target-api-level-29