Hey Paul!

Yep, as it currently stands, Turf doesn't support the elevation parameter in GeoJSON in any meaningful way. This was a combination of two factors:

1. It's kind of hard to implement
2. The GeoJSON spec didn't specify a unit or meaning for the altitude coordinate of a position

Luckily, #2 is no longer the case! The IETF draft in section 4 specifies the meaning of altitude: https://tools.ietf.org/html/draft-ietf-geojson-03#section-4

An OPTIONAL third
position element SHALL be the height in meters above or below the WGS
84 reference ellipsoid. In the absence of elevation values,
applications sensitive to height or depth SHOULD interpret positions
as being at local ground or sea level.

So that's cleared up. Happy to accept any PRs that interpolate/include elevation when it's present!

👍 Support having elevation in all computations, reference #628.

I too am interested in the addition of the altitude element and this becoming significant in the calculation of distance.

To that end, I did find the following useful response along the same lines, which demonstrates how to add the altitude distance calculation:

https://stackoverflow.com/questions/64877775/calculate-distance-between-two-coordinates-with-short-distances-considering-alt