Tidal DatumsText and Figures Source: NOAA/NOS CO-OPS Tidal Datums related publications

What is Tidal Datum ? For marine applications, a base elevation used as a reference from which to reckon heights or depths. It is called a tidal datum when defined in terms of a certain phase of the tide. Tidal datums are local datums and should not be extended into areas which have differing hydrographic characteristics without substantiating measurements. In order that they may be recovered when needed, such datums are referenced to fixed points known as bench marks.

What is Chart Datum ? The datum to which soundings on a chart are referred. It is usually taken to correspond to a low-water elevation, and its depression below mean sea level is represented by the symbol Z;. Since 1980, chart datum has been implemented to mean lower low water (MLLW) for all marine waters of the United States, its territories, Commonwealth of Puerto Rico, and Trust Territory of the Pacific Islands. See datum and National Tidal Datum Convention of 1980.

What is a Bench Mark ? A fixed physical object or mark used as reference for a horizontal or vertical datum. A tidal bench mark is one near a tide station to which the tidal datums are referenced. A primary bench mark is the principal mark of a group of tidal bench marks to which the tidal datums are referenced. The overall quality of datums is dependent on both the quality of the bench mark and the quality of the leveling between the bench marks and the tidal gauges.

Types of TidesMixed TideSemidiurnal TideDiurnal Tide

MHHW

Principal tidal datums related to a beach profile

Datum Definitions Mean Sea Level (MSL) is a tidal datum determined over a 19-year National Tidal Datum Epoch, a time period that approximates the variation in the path of the moon about the sun (regression of the moons nodes). MSL pertains to local mean sea level and should not be confused with the datums of North American Vertical Datum of 1988 (NAVD 88) which are a fixed reference for elevations determined by geodetic leveling.

Datum definitions contd.. National Geodetic Vertical Datum of 1929 (NGVD 29) is a fixed datum adopted as a national standard geodetic reference for heights, but is now considered superceded. NGVD 29 is sometimes referred to as Sea Level Datum of 1929 or as Mean Sea Level on some early issues of Geological Survey Topographic Quads. NGVD 29 was originally derived from a general adjustment of the first-order leveling networks of the U.S. and Canada after holding mean sea level observed at 26 long term tide stations as fixed. Numerous local and wide-spread adjustments have been made since its establishment in 1929. Bench mark elevations relative to NGVD 29 are available from the National Geodetic Survey (NGS) data base via the World Wide Web.

Datum definitions contd.. North American Vertical Datum of 1988 (NAVD 88) is a fixed datum derived from a simultaneous, least squares, minimum constraint adjustment of Canadian/Mexican/United States leveling observations. Local mean sea level observed at Father Point/Rimouski, Canada was held fixed as the single initial constraint. NAVD 88 replaces NGVD 29 as the national standard geodetic reference for heights. Bench mark elevations relative to NAVD 88 are available from NGS through the World Wide Web.

Datum definitions contd..

Principal Tidal datums and their relationship to geodetic datum NGVD 1929 for a typical mixed curve (should this read typical mixed tide?

Significance of Datums in Marine ApplicationsI would also add that efforts are currently focused on defining a global vertical datum and that the ellipsoid has been suggested because vertical coordinates provided by GPS can be translated into a coordinate referenced to a tidal datum plane using a set of vertical transformation functions.

Applications of Tidal DatumsTidal datums are chiefly used to determine horizontal boundaries and for estimating heights or depths. They are created to serve the needs of a variety of users, including mariners, engineers, scientists, and the general public. While a vertical datum is the fixed base elevation used as a reference from which relative heights or depths are determined, tidal datums are defined by a certain phase of the tide. Different user groups use tidal datums defined by different phases of the tide. Examples of applications of tidal datums by the following users are provided below. Boaters/ShippingThe depths on nautical charts in U.S. coastal waters are referenced to mean lower low water (MLLW). The reference base of the heights of structures on nautical charts (e.g. bridge clearance) is mean high water (MHW). Using MLLW provides pilots with a margin of safety consistent with average meteorological conditions. Storm Surge/Emergency Management Planners (EMP)Storm surge, measured as the difference between observed water levels and the elevation of the astronomically predicted tide, can cause injury, death, property damage, damage to structures, avulsion, and erosion. EMP use all tidal datums at their disposal (e.g. MLLW, mean higher high water (MHHW), and National Geodetic Vertical Datum of 1929 (NGVD 29), as well as historical data in their planning activities. ModelersPhysical, chemical, and biological properties of the ocean-atmosphere-terrestrial system may be predicted by advanced hydrodynamical numerical models. When these models are used to predict things like tides, water levels and storm surges, the accuracy of their sea surface height fields are evaluated and calibrated by data from multiple vertical tidal and geodetic datums.

Website linksNOAA/NOS CO-OPS Tide Datums and Their Applications Tide and Currents Glossary

Computational Techniques for Tidal Datums

These types of tides are what makes up a datum. For instance, if you were to take the all the HHWs for a month and calculate their mean you would get the MHHW monthly mean [once referenced to a corresponding control stn with a 19 yr mean you will get the datum.] Say you want to know what the inequality is for the high waters: subtract the MHHW from the MHW. The next slde will show these kinds of relationships.

Todds stuff:Semi Diurnal Two high and two low tides per day. Heights of the highs andlows are relatively equal. Occurs along the US East Coast

Diurnal One high and one low tide per day.

Mixed A combination of the other two types of tides. Will show a significantdifference in the heights of successive highs and lows.

US West Coast is Mixed (Primarily Semi-Diurnal) - Normally two highs andlows per day, but occasionally can be only one.US Gulf Coast is Mixed (Primarily Diurnal) Normally one high and low perday. During Spring tides will become Semi-Diurnal.Highest and Lowest Observed are not a tidal datum or a computation.The acronyms can be found in our reference Glossary

Standard Method (MTL, MSL, Mn, DHQ & DLG are known):MLW= MTL-(0.5*Mn)MHW= MLW+MnMLLW= MLW-DLQMHHW= MHW-DHQDTL= 0.5*(MHHW+MLLW)GT= MHHW-MLLW

MRR Method (MTL, DTL, MSL, Mn & GT are known):MLW= MTL-(0.5*Mn)MHW= MLW+MnMLLW= MLW-0.5*GTMHHW= MHW+GTDHQ= MHHW-MHWDLQ= MLW-MLLW

GT is known as the spring or diurnal range depending on what coast it is on.Boundaries to talk about in order of graphic enhancements: #MLW #MHWA coastal home owner would have more land if living in VA compared to MD owners#MLLW (chart datum)#territorial sea related to MLLW (12 n. mi.)#exclusive economic zone related to MLLW (200 n. mi.)