Nosy Neighbor: What’s This Thing Embedded in the Sidewalk?

I hope it’s new or else I really need to work on my powers of observation. What is it? —D.

I was at the intersection of Church and Leonard when I first noticed one: a marker, about the diameter of a tennis ball, embedded in the sidewalk. I was pretty sure I’d never seen one before. Poking around online, I found someone to email about it: Hector Rivera, Topographical Bureau Associate at the Manhattan Borough President’s Office. Here’s what he wrote back:

“This is a Manhattan Monument, used by surveyors to identify property lines. I have attached a PDF of the Manhattan Borough Survey Map, which shows the coordinates for that monument. There are two types of markers throughout the city. You have your Monuments that have longitude and latitude. Then you have the Bench Marks, also throughout the city, that will give you the elevation about sea level based on Manhattan Datum. Theses items have been in use for years and are still in use even with GPS equipment, especially in Manhattan (because of all the tall buildings, GPS equipment can only provide a correct read at certain times of the day when the GPS equipment has a straight uninterrupted signal to the satellite in space).”

Pictured below is a detail from the Survey Map (of the Church/Leonard intersection); I added the arrow to indicate where the Monument I noticed is. The map has black dots—which Rivera said indicate Monuments—all over. So how come I don’t see them everywhere? “Some of the markers are below grade and have square covers,” said Rivera.

Survey mapNote: D.’s question is recent, but the answer is mainly the same as from this 2011 post.

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