Tide elevations are measured relative to a vertical datum, a reference system that allows one to locate a point on the Earth’s surface. Without a common datum from which measurements are referenced, surveyors would calculate different elevation values for the same location. There are two main types of vertical datums for coastal applications: orthometric and tidal, defined below. Many cities also establish a city-specific vertical datum relative to a local point of reference (for example, high water line or mean sea level). Most coastal water level data is collected in reference to a local tidal datum, while most transportation projects are designed based on an orthometric datum. As a result, it is common to require conversion between datums to successfully evaluate coastal conditions.

Tidal datums are based on tidally-derived surfaces of high or low water elevations defined by phases of the tide ( ). They are used to describe the average location where the water and land intersect for each major tidal phase. The hydrodynamics of tidal fluctuations are controlled by local processes; thus, it is important to remember that tidal datums provide a local tide characterization and will vary along the coastline.

Commonly Used Tidal Datums

Mean High Water (MHW): Mean of all high-water elevations observed during NTDE Mean Sea Level (MSL): Mean of hourly water levels observed during NTDE Mean Low Water (MLW): Mean of all low water elevations observed during NTDE In areas with mixed semi-diurnal tides, such as the Northern Texas coast, two additional datums are defined: Mean Higher High Water (MHHW): Mean of the higher of the two daily high-water elevations observed during the NTDE Mean Lower Low Water (MLLW): Mean of the lower of the two daily low water elevations observed during NTDE

All tidal datums are referenced to a 19-year averaging period known as the National Tidal Datum Epoch (NTDE). This 19-year period is significant, as it encompasses the length of time necessary for variations in lunar cycles (which influence tide levels) to occur. The current NTDE spans from 1983-2001 and is actively considered for revision every 20-25 years. Use of NTDE allows tidal datums throughout the U.S. to have a common reference. Tidal datums are commonly reported relative to the Mean Lower Low Water (MLLW), which is the lowest reported tidal datum. For example, when a report states the Mean High Water (MHW) at the Galveston Pier 21 station (NOAA #8771450) is 1.32ft, it means that, on average, over the 19-year period of 1983-2001, the average high tide was about 1.32ft above MLLW ( , 2019). However, tidal elevations can also be easily expressed relative to other datums reported at the same tide station. Refer to the

Relationships Among Datums

subsection for more information and common conversions.

Orthometric datums use the Earth’s gravity field to reference heights. The North American Vertical Datum of 1988 (NAVD88) is the current national standard vertical datum. It replaced the National Geodetic Vertical Datum of 1929 (NGVD29), which was the previous national standard for 60 years. Refer to NOAA for the most current .

The relationship among datums often varies from one area of the shoreline to another. To make a comparison with land surveyed data, all data must be converted to a standard reference, such as NAVD88. As an example, Table 15-4 presents the relationship between tide heights referenced to NAVD88 and MLLW datums at the Galveston Pier 21 tide station.

To evaluate water surface elevations relative to the various datums reported for a tide station, visit the NOAA or TCOON tidal datum section of the tide station’s website. A list of tidal datums, similar to those listed in Table 15-4 provided as an example for Galveston Pier 21 gauge, will be provided. Conversions between the datums can be calculated by making the datums relative to each other through subtraction. For example, to convert MHHW referenced to MLLW to be relative to NAVD88, subtract 0.14 feet from 1.41 feet in the right-hand column to obtain 1.27 feet in the left-hand column. This comparison indicates that MHHW is 1.27 feet above NAVD88 and 1.41 feet above the MLLW.

Table 15-4: Galveston Pier 21 (NOAA #8771450) Vertical Datums (NOAA Tides and Currents, 2019)

	Relative To
Datum	NAVD88 (feet)	MLLW (feet)
MHHW	1.27	1.41
MHW	1.18	1.32
MSL	0.69	0.83
MLW	0.16	0.30
MLLW	-0.14	0.00
NAVD88	0.00	0.14

The relationship between NAVD88 and tidal datums has been calculated by NOAA for many of the tide gages along the Texas coastline. The conversion is different for every tide station. Therefore, investigating the relationship between tidal datums, orthometric datums, and site-specific upland surveys used for project design is very important. If a conversion for NAVD88 has not been calculated for a tide station of interest, refer to a professional surveyor or the USACE District office to obtain the necessary offset.