well, no secret, it is in the elevation tutorial:
Aaah ok you were talking about the USGS portal, but sadly it's for USA only !
I've tested successfully a mesh creation based on IMG filed exported from that site, I haven't seen a export directly in TIFF was possible, good to know.
Basically I was very surprised and excited because I understood that there was info on mesh creation in the official AFS wiki because you had talked about "IPACS-wikipages". But unfortunately I think that IPACS has never published official information about mesh creation, except in the forum to say that it could be "dangerous".