[Solved] Anyone know what this is? They are embedded in the sidewalks around the city

Seattle

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.