TIDES

A Brief Synopsis of Vertical Datums for Pensacola Bay and Vicinity

Height is a relative term [a 6-ft-tall person remains 6-ft when standing on a building], but elevation implies a fixed horizontal plan, or datum, that the measurement is referenced to. Typical datums used to establish vertical elevations in the marine environment are: Mean Higher High Water (MHHW), Mean High Water (MHW), Mean Seal Level, (MSL), Mean Tide Level (MTL), Mean Low Water (MLW), Mean Lower Low Water (MLLW), and the National Geodetic Vertical Datum (NGVD). MHHW, MHW, MSL MTL, MLW, and MLLW are local datums derived from statistical analysis of tidal measurements at that location. For many engineering applications, these local datums can be considered identical over a limited region in one body of water, such as Pensacola Bay, but should not be assumed identical between bodies of water, such as between Pensacola Bay and the Gulf of Mexico or Choctawhatchee Bay.

NGVD (sometimes referred to as Sea Level Datum of 1929) is an attempt to provide a fixed datum, useable over continental scales, that was adopted as a standard geodetic reference for heights. It was derived from a general adjustment of the first order leveling nets of the U.S. and Canada. Mean sea level was held fixed as observed at 26 stations in the U.S. and Canada. Numerous adjustments have been made since originally established in 1929. Another datum, the North American Vertical Datum (NAVD 88), was calculated through simultaneous, least squares, minimum-constraint adjustment of Canadian-Mexican-United States leveling observations. Local mean sea level at Father Point/Rimouski, Canada was held fixed as the single constraint.

The following table shows the official datums provided by the National Ocean Service (NOS), the federal government agency charged with measuring and establishing them throughout the country, at several locations around Pensacola Bay. The datums were established through analysis of tidal records from tide gages placed at these sites. The Pensacola Bay, Port station is a primary, or permanent tide gage. The others were temporary gages established for relatively short times. It includes representative stations in the general area , moving generally from west to east. Each measurement is in feet relative to MLLW as measured at each station, so MLLW is always zero. It may appear at first that NGVD is moving up and down. Since the zero is a local datum it's more accurate to consider NGVD as the fixed vertical plane, and the position of the local datums, including MLLW, as varying from place to place. Thus the last column, the elevation of MHW relative to NGVD, is a useful measure of how much these local datums vary.

Station	MLLW	MLW	NGVD	MHW	MHW-NGVD
Fort Pickens	0	0.03	0.17	1.14	0.97
Pensacola Bay, NAS	0	0.04	0.21	1.17	0.96
Pensacola Bay, Port	0	0.04	0.26	1.23	0.97
Little Sabine	0	0.06	NA	1.27	NA
Santa Rosa Sound, Woodlawn	0	0.04	NA	1.32	NA
Santa Rosa Sound, Hurlbert Field	0	0.09	0.57	1.11	1.08