At the edge of a quiet residential street in Brooklyn, Moses Gates stood atop a large pile of rocks, surrounded by nearly two dozen people.

Some took pictures with their iPhones; others clapped and encouraged Mr. Gates to seek a biblical influence to commemorate the moment. Mr. Gates obliged, gamely picking up a rock with each hand.

“The first commandment is: ‘Thou shall walk all 2,217 census tracts in New York City,’ ” Mr. Gates said, grinning.

On that particular Sunday in March, Mr. Gates, an author and urban planner fascinated by the city’s lesser-known corners and ever-shifting demographics, became the first person believed to have done exactly that: Over the last decade, he walked all of New York City’s census landscape, tract by tract, with Rikers Island the only understandable exception. (When Mr. Gates began his journey, the city had 2,217 tracts, or county subdivisions, according to the 2000 census; the number had fallen to 2,168 tracts in the 2010 census.)