Its called a benchmark, used by surveyors as a known point from which to take readings. We had a benchmark on the edge of of our property in the town where In grew up. It was a square, white marble column, like a truncated obelisk with the point cut off to make a 4" square flat top, buried at the roadside and standing a couple inches proud of the grass. It had a cross with a dot in the middle, and a geodetic ID number engraved on the top.
Nailed it with the lawnmower once or twice. That'd really put the Fear in you.
My grandma has a little concrete thing like that that marks where her property ends (or where 1 part of it ends, legally it's 2 rectangles sharing a side). Once after not mowing for like a month and a half I forgot it was there and totally ruined the freshly replaced blades 🥲
A marker point for geodetic marking. Also known as triangulation station or trigonometrical point, it's fixed to the ground with its known coordinates.
This is a survey monument vs the original post being a control point. Survey monuments are established on a larger scale.
When you need to reference a known elevation or XY coordinate, there is a network of survey monuments with known coordinates, usually installed and documented by governments. You reference a known location’s elevation and north x west coordinates and then use instrumentation to determine angle changes to transfer the known elevation to a series of other locations.
When you section off property or build things, like roads or buildings, you use this network of points and triangulate paths off of them to the job site by calculating angles and distances. Since you have to maintain line of sight while traversing the distance, you add points you can use to pivot on and travel great distances. These points are called ‘control points’ or ‘benchmarks’.
The main post pic is a control point. Your picture is a survey monument.
once you start to notice them, you'll never stop seeing them everywhere.
this one is a magnetic nail, so it can be found more easily with a metal detector, which is pretty handy when there's a foot or two of snow on the ground.
you'll also see crosses, squares, and triangles carved out in stone, as well as the super common drill holes in concrete, which are typically only for temporary points.
I've found 100+ year old control points drilled into old stone walls, and they can still check within like ¼" of the new maps.