The '?xyz' form is used by android devices (and according to apple
mailing list archives, also by older iOS devices). The 'loci' field
(defined in 3GPP 26.244) is used by recent iOS devices.

Even though the loci field can contain an altitude, it was plain
0 in my sample. Just export longitude and latitude, in a string
format matching the one used by the '?xyz' metadata field.
Signed-off-by: Martin Storsjö <martin@martin.st>