Implement selector for altitude output: Ellipsoidal vs. Geoidal (MSL) height

[mkrupczak3]

Currently, OpenAthena for Android only outputs height of the target as height above the WGS84 reference ellipsoid, A.K.A "hae".

Some applications, such as military use, prefer to use Geoidal height (such as mean sea level) that take the shape and rotation of the earth into consideration and using the expected sea level as the base.

OpenAthena for Android should be able to output height of the target in either format

Related issue:

63

More info: https://www.reddit.com/r/askscience/comments/13t44me/how_do_modern_navigation_aids_account_for/jltapm4/

https://enterprise-insights.dji.com/blog/geoid-vs-ellipsoid

[mkrupczak3]

@bsolon524 It seems that EGM96 above mean sea level (AMSL) is drastically more common than WGS84 ellipsoidal for most use cases.

I'm concerned actually that selection between height above ellipsoid and EGM96 geoidal height will be confusing and irrelevant for most users.

I'm almost done with the implementation of this feature. What I'll do instead is:

• for WGS84 and NATO MGRS modes, the vertical datum of the target output will be converted from hae to AMSL
• for the Cursor on Target output, the vertical datum will be hae as per the specification documents
• for CK-42 output modes, the vertical datum will be hae, but for the Krassowsky 1940 ellipsoid rather than the WGS84 reference ellipsoid

[mkrupczak3]

~I'm also going to deprecate the instructions for using the Python Elevation library with the eio clip command to obtain GeoTIFF files. That library uses EGM96 geoidal altitude for geotiff's produced with it, though it doesn't store this in any metadata which can be accessed programmatically.~ EDIT: no longer need to deprecate

OpenAthena for Android will now assume that the vertical datum for GeoTIFF input files are in EGM96 AMSL. This means that standard SRTM or NASADEM should be used with OpenAthena for Android going forwards, not any converted version with height above ellipsoid as the vertical datum.

This will also help with support of military standard DTED2 and DTED3 files in #52, as the vertical datum for these will always be EGM96 AMSL in meters

[mkrupczak3]

@rdkgit