Calculate linear distances between 3D landmarks in 3D array

[CatherineSulzmann]

How can I calculate the linear distances between 3D landmark coordinates in a 3D array?
I've found interlmkdist function in Geomorph that calculates linear distances between 2D landmark coordinates in a 3D array - but not found one for 3D landmark coordinates.

Thanks for your help

[technocrat]

Should be doable if you have the three landmarks in common lon/lat units, such as meters, for both points relative to the reference geoid in meters. If not too far apart, the difference between the Euclidean distance and the surface trace should be negligible, unless there’s a lot of changing relief along the way.

d = \sqrt{(x_2 - x_1)^2 + (y_2-y_1)^2 + (z_2 -z_1)}