Specific conductance of water is in microsiemens per centimeter at 25 degrees Celsius (µS/cm). VERTICAL DATUM Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.