[TopCat]

For instance, in the AIP, for White Waltham, it says:

Elevation/Reference temperature: 127ft / 19C

Geoid undulation at AD ELEV PSN: 152 FT

Normally I'd subtract elevation from altitude to get height AAL, but
I'm curious whether it's similar with GPS altitudes.

What does the Geoid undulation refer to ? Is it useful for anything?

[riverrock]

My understanding (happy to be corrected) is roughly :
The earth isn't quite round, but GPS satellites go round the earth in a standard orbit. So GPS systems calculate your height based on a standard shape (an ellipsoid - a squished sphere) rather than mean sea level.
The undulation is how far sea level is vs that standard shape.

GPS devices have maps of how much that undulation is, so they can accurately report height from sea level. That can be used with a terrain map to work out your height above ground.
Most of the time, this doesn't need to be too accurate - perhaps accurate to 1 km squared. However, landing, if using GPS based height, is one of those times. Therefore you need and airport height from sea level, and the distance sea level is from the ellipsoid (the undulation) to be able to use GPS for height.

Certified devices, used for GPS approaches, will do all of this maths entirely hidden from you. This sort of thing is one of the reasons that Sky Demon disable the height part of their virtual ILS below 500 feet.

[TopCat]

Thanks, very helpful.

Do you happen to know whether in a GPX file exported from SkyDemon, the figure inside the <ele></ele> tag is the height above the ellipsoid, or the height above MSL?

I can believe that the SkyDemon app might make the correction on the screen display, but I'm wondering if the GPX file is uncorrected?

[riverrock]

I believe the standard is it should be based on the WGS84 datum .
However - you'd need to check with Sky Demon, and it could be dependant on which hardware it is running on, or externally linked GPS.