[PROJ] Altitude

[Roger Oberholtzer]

We have usually been focused on latitude/longitude when using proj. We
would now like to address altitude. I have looked in the proj docs,
but I cannot say that I am much the wiser in this respect.

We currently use the old C API (proj_api.h). But we are moving to the
new C API. That's a work in progress. And perhaps the subject of other
questions.... Here I am considering height.

Our typical scenario is that we collect data from a receiver (high-end
dual antenna - so good quality) in a moving vehicle. We collect
LAT/LONG in WGS84, and then project it using proj. For example to
SWEREF99. So it's basically EPSG:4326 ->  EPSG:3006, or the inverse.
Of course we don't only use SWEREF99. It's just an example.

The receivers are configured to collect altitude as the geoid in
EGM96. I think it is in fact the default for our receivers. So I guess
it is a common altitude.

To date, we have just let the altitude go along for the ride when
using proj. Our users can select if they want the altitude as supplied
by the receiver, or as proj calculates. There is a difference in the
values.

I have never understood how altitude is described in proj. For
example, using the example of EPSG:3006, the corresponding altitude
could be SWEN17_RH2000 (See
https://www.lantmateriet.se/en/maps-and-geographic-information/gps-geodesi-och-swepos/Om-geodesi/Transformationer/Geoidmodeller/).

Can I use proj to get SWEN17_RH2000 from EGM96? Perhaps I just didn't
recognize the information in the docs when I looked through it.

Or should I be looking elsewhere?

Sorry for being a noob here. But that's the state of my understanding
with respect to altitude.

-- 
Roger Oberholtzer