Please contact these cities directly for this information. A benchmark is a relatively permanent material object, natural or artificial, bearing a marked point whose elevation above or below an adopted datum is known. Common examples of benchmarks are metal disks set in concrete or a corner of a concrete headwall or sidewalk. An elevation is defined as the vertical distance, along a line that follows the direction of gravity as indicated by a plumb line, from a datum to a point or object. NGVD 29 was established by leveling between 26 tidal stations located along the coasts of the United States and Canada.
Leveling is the process of determining elevations of points or their difference in elevation.
A tidal station is the geographic location at which tidal observations are conducted. NGVD 29 is a fixed datum and does not take into account the changing stands of sea level. Because there are many variables affecting sea level, and because this datum is a best fit over such a broad area, the relationship between the NGVD 29 datum and local mean sea level is not consistent from one location to another in either time or space.