How close you can park to a hydrant
In New York City the curb within 15 feet of a fire hydrant is not a parking space. The hydrant can sit on the sidewalk just past a gap that looks empty. A driver who noses into that gap can still be ticketed.
What you see on the map
When you look up an address, Park Map draws the hydrants the city lists near that curb. The mark is a warning for those 15 feet. It is not a measurement you should pace off from a photo. If you are already at the curb, look at the hydrant itself.
A hydrant and a street-cleaning sign
Both can be true on one block. Street cleaning may end at noon, and the hydrant rule does not. Moving the car back into the same gap after the sweeper passes does not make the hydrant legal.
Also: street cleaning and live cameras.