The docs also seem to indicate that 3mx3m is the smallest grid they can address, but at the same they use addressing in slums as a use case...who has that much real estate in the slums?
Wait a minute. How are Kolkatans with (probably) cheap phones getting their hands on coordinates that accurate when phone GPS is usually only good to within ~5m?[1] I assume the postal workers are getting high end equipment?
First guess: Signs, maybe? You only need a few accurate surveyors and they could write coordinates in places. Use the cheap accuracy to get close then follow the signs.
