Just got an upvote on my first answer here..... wow, massive wave of nostalgia...

@DoubleAA Yeah, but when you draw the box of Teaneck/Bergenfield to its Northeastern extremities (e.g. Sparkill), and draw the line down to the Southeastern corner, you're in Manhattan already

Ostensibly (by the Google satellite maps I've been looking at), the Techum Shabbos of Manhattan, Brooklyn, Queens, the Bronx (each), and maybe even Staten Island would include all of the city as well as NJ and NY suburbs and Long Island

@ephraimhelfgot I haven't checked the maps, but it's still possible that'd only works for Teaneckians (?) entering Manhattan, but not for Manhattanites (?) entering Teaneck.

Just checked it out-- the upper edge of Manhattan is at the same latitude as Teaneck

Also there might be gaps like highways or something which are <266 ft which actually split things up.

Also, more simply, probably the position on the website you linked to isn't precise and/or universal.

That's the main concern I have

I should probably look it up in Shulchan Aruch/Rama

Also, maybe all the responsa about NY Techumin date from before it became a sprawling metropolis.

@HodofHod and, thus, a Nice Answer. :-) And I get nostalgic when that happens to me, too.

(Your first answer was loftier than mine. :-) )