The National Geodetic Survey is a part of NOAA, chartered with defining and maintaining the coordinate system used by charts and maps in the US. They collaborated with the Canadian federal and provincial geodetic agencies to define
NAD 83, which remains the current reference.
The Sand Point Calibration Baseline is a set of surveyed points along a 1058m line, with line of sight visibility allowing precise calibration of distance measuring devices. It was established by the National Geodetic Survey, and is maintained by Seattle Public Utilities (who is a frequent user of the amenity).
The erect stones are public art, not calibration points.
Straight Shot by Perri Lynch succeeded at making the survey visible to me, and now to you.