Where the numbers come from
Elevations are the USGS 3D Elevation Program's bare-earth lidar for New York City (1-meter source, averaged to 10 m), sampled every 45 m across the island and interpolated to each corner. Streets are NYC's official centerline file, so every corner, park path, stair street and greenway is included.
Grades on short blocks are good to about ±0.5%. Bridges, viaducts and the High Line are graded from their end points, not the ground below. Bike routes mostly respect one-way streets: riding against traffic counts as walking your bike. Check signs before you go.