Carlsbad, NM has a altitude of 3295 feet. I’m gonna suggest your lowest elevation was Boston (again).
North: Inwood, U.S.A.
South: Cape Bruny, Australia
East: Suva, Fiji (longitudinally)/ Long Island Airport, NY, USA (relative)
West: Nuku’alofa, Tonga/Geraldton, Australia
Deleted for a double post.
Addendum: Up/down - unknown, but not far from sea level. Ours is a coastal people.
Furthest north: Arlanda Airport, Arlanda, Sweden
Furthest east (of Greenwich): Stadion tunnelbana station, Stockholm, Sweden
Furthest west (of Greenwich): Desolation Sound, British Columbia
Furthest south: San Diego, California
The furthest north I’ve ever been in this country is Thompson, Manitoba, although I don’t remember it real well.