More Water Level Information

Outline

• Background
• Data Gathering and Quality Control
• Layer Descriptions
• Vertical Datums and Water Level Observation
• Tidal Predictions vs. Water Level Forecasts
• Other Water Level Observation Resources

Background

Water level changes in response to forcing factors on time scales that range from minutes to thousands of years. The most familiar water level changes we observe are the daily rise and fall of coastal ocean waters due to astronomical tides. However, even on short time scales (minutes, hours, days), wind stress can cause the extent of water level change to deviate significantly from astronomically-predicted levels. The coastal geology of an area, bay morphology, and bathymetry are factors that influence the periodicity and magnitude of water level change. Also, major storms such as tropical waves, tropical depressions, and hurricanes, as well as seismic-generated waves (tsunamis) can cause water level changes far in excess of those caused by monthly variations in the lunar phase.

Water Level measurements are made to meet the needs of mariners, engineers, resource managers, researchers, and the general public. Some of the most important purposes for which they are measured are: for the mariners to estimate draft under keel while in transit, data and datum reference for storm surge monitoring, data for production of tide and tidal current predictions, data for estuarine studies and numerical hydrodynamic models, for determination of mean sea level and other tidal datums for surveying and engineering purposes. SEACOOS water level data product provides a comprehensive view of nearshore and offshore water levels. The water level observations are reported in near real-time referenced to Mean Sea Level (MSL), Mean Lower Low Water (MLLW), and North American Vertical Datum (NAVD 88). Monitoring water level and other variables such as winds and air pressure gives us valuable information about how the changes in water level are caused by weather effects and tides.

Data Gathering and Quality Control

Near real-time measurements of water level observations, collected from a variety of platforms, are displayed. Each hourly display shows the measurement from all reporting stations taken closest to the top of each hour. Many measurements get displayed within one to two hours from the time the measurement was taken. Observations are available for display up to two weeks after it is posted on this site. SEACOOS water level data contributors are University of South Florida (COMPS), Skidaway Institute of Oceanography (SABSOON), University of South Carolina (Caro-COOPS), and University of North Carolina (NC-COOS). The federal data sources for water level observations whether on land or near-shore or offshore are NOAA National Ocean Services and United States Geological Survey. Each contributing institution has its own set of data quality control procedures and assurances.

The water level observations on this website are presented via a map layer on the SEACOOS interactive map. Users are encouraged to read the descriptions of each available vertical datum to gain a better understanding of the utility of each reference level.

Layer Descriptions

The following is a list of vertical datums available for the water level layer on the SEACOOS interactive map.

Mean Sea Level (MSL)

MSL is the arithmetic mean of hourly observations over the National Tidal Datum Epoch, a 19-year period that covers all variations in the path of the moon about the sun (regression of the moon’s nodes). As a tidal datum, MSL pertains to local mean sea level and should not be confused with geodetic datums (NGVD29 or NAVD88) which are a fixed reference for elevations determined by geodetic leveling.

Mean Lower Low Water (MLLW)

MLLW is defined as the arithmetic mean of the lower low water heights of the tide as observed over the National Tidal Datum Epoch. Only the lower low water of each pair of low waters during a tidal day is included in this mean calculation.

North American Vertical Datum of 1988 (NAVD 88)

NAVD 88 is a geodetic 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.

Vertical Datums and Water Level Observation

The surface of the ocean is often used as a reference plane from which to measure height and depth. However, the surface of the ocean is itself variable both through time and across space – it is not “flat”. Temporal variability in ocean water level is driven mainly by tidal cycles and meteorological conditions. Spatial variability in water level is controlled by variations in the Earth’s gravitational field that can deform the surface of the ocean by as much as 100 meters. This variability complicates efforts to use water level as a standard reference level.

To quantify these variations, several methods are used to normalize water levels into a standard vertical reference level or vertical datum. Temporal variability is dealt with by averaging local water levels over a period of years, smoothing variations into local tidal base levels (tidal datums). Spatial variability is dealt with utilizing a single initial base elevation and referencing that level throughout a national network (geodetic datum). More detail on tidal and geodetic datums is provided below.

Standardization of water level into a vertical datum quantifies known variability, enabling the measurement of water level variation as driven by other phenomena of interest – global sea level change, meteorological forcings, or coastal subsidence for example.

Tidal Datums establish local tidal phase averages as reference levels from which to reckon height or depth observations. These datums are averages of observations made over a 19-year National Tidal Datum Epoch, a time period that includes all variations in the path of the moon about the sun. Tidal datums are used to determine many jurisdictional and property boundaries and in nautical charts and navigation. Tidal datums are locally derived and should not be extended into areas which have differing hydrographic characteristics without substantiating measurements. Commonly used tidal datums include Mean Higher High Water (MHHW), Mean High Water (MHW), Mean Sea Level (MSL), and Mean Lower Low Water (MLLW).

Geodetic Datums (also referred to as Orthometric Datums) are vertical datums that reference mean sea level from a select set of initial locations. This initial reference level is then established across a national network using differential leveling procedures and the placement of reference benchmarks. Many terrestrial elevation datasets (e.g. topography) are referenced to these datums. The most commonly used geodetic datums are National Geodetic Vertical Datum of 1929 (NGVD29, now superseded) and the North American Vertical Datum of 1988 (NAVD88).

Conversion between geodetic and local tidal datums is crucial to many coastal water level applications. This conversion requires that a geodetic elevation be established for every water level station/benchmark and local vertical offsets to each tidal datum calculated. This was initially done using conventional leveling methods, and is now increasing done using Global Positioning Systems (GPS) to occupy tidal benchmarks. While GPS observations do not directly provide geodetic elevation, they can be used in calculations to find the vertical offsets to common geodetic datums. A joint project between the National Geodetic Survey (NGS) and NOAA's National Ocean Service (NOS) is developing a vertical datum conversion utility, which will generate tidal-to-geodetic offsets for most coastal locations in the US (VDatum Project).

A Chart Datum is 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.

A Benchmark is a fixed physical object or mark used as reference for a horizontal or vertical datum. A tidal benchmark is one near a tide station to which the tidal datums are referenced. A primary benchmark is the principal mark of a group of tidal benchmarks to which the tidal datums are referenced. Geodetic benchmarks are marks that have been established with an elevation in a national geodetic datum. The overall quality of datums is dependent on both the quality of the benchmark and the quality of the leveling between the benchmarks and the tidal gauges.

Tidal Predictions vs. Water Level Forecasts

SEACOOS water level maps contain integrated, near real-time water levels, measured at different locations over time. The observed fluctuations in water level are mainly caused by tidal cycles and meteorological effects. Observing water level change can aid in projecting water level variability in the future. Two separate products rely on observations to help understand expected water level. A brief outline of tidal predictions and water level forecasts are presented below.

Tide Predictions of water level are locally derived from historical tidal water level responses to regular astronomical cycles. A main driver of local water level in most coastal areas, tides occur as a result of the gravitational attraction and centrifugal pull of the Sun and Moon on the Earth’s surface. These astronomical forces and their patterns of reoccurrence are well understood and can be predicted mathematically. Local geography (coastline geometry and bathymetry for example) moderates how these astronomical forces produce local water level. Understanding how local geography affects tidal levels is derived from an empirical examination of a 19 year window (epoch) of previously observed water level. A tidal epoch spans an 18.6 year astronomical cycle that includes all significant variations of the Moon and Sun that cause slowly varying changes in the range of tide. Thus, local water level predictions take into account how known astronomical forces have operated historically over local geography.

Tide Predictions are useful as they can be projected well into the future (years in advance), but are less accurate than observations or Water level Forecasts since they do not explicitly account for all of the inputs and local variability that drive actual water level. These inputs can be driven by short-term and long term meteorology (wind stress and barometric pressure), hydrology (river run-off), and oceanography (for instance, the Loop Current in the Gulf of Mexico). Note that in some shallow estuaries such as Chesapeake Bay and Galveston Bay, the tidal forcing is less than or equal to the meteorological forcing resulting in inaccurate Tide Prediction products.

Water Level Forecasts model the known forcings on water level - tidal fluctuations, hydrologic conditions, and oceanographic and meteorological conditions as they operate over local coastline geometry and bathymetry (treated explicitly). Forecasts are generated by mathematical models that assimilate current hydrologic and meteorological conditions that impact water level, factors that ignored by water level predictions. SEACOOS modelers use several different models to combine this information to forecast future water level across the region. Because these models rely on current conditions and an explicit treatment of geography, localized event-specific forecasting is possible (hurricane storm surges for example).

Forecasts are useful as they more accurately model future water level, but cannot be projected into the future beyond the available hydrologic and meteorological data available (+84 hours for SEACOOS models). Water Level Forecast models also typically provide forecast information over a model grid, not just at discrete locations that the Tide Prediction products generally provide.

Other Water Level Resources

Below is a short list of sites with relevant information:

• NOAA/NOS CO-OPS: Tide and Current Glossary (PDF)
  http://www.co-ops.nos.noaa.gov/publications/glossary2.pdf
• NOAA/NOS CO-OPS: Tidal Datums and Their Applications (PDF)
  http://www.coops.nos.noaa.gov/publications/tidal_datums_and_their_applications.pdf
• NOAA/NOS CO-OPS: Sea Level Trends
  http://www.co-ops.nos.noaa.gov/sltrends/sltrends.shtml
• NGS/NOS VDatum Project
  http://www.nauticalcharts.noaa.gov/csdl/Vdatum.htm
• National Geodetic Survey: Orthometric/Tidal Datum conversions
  http://www.ngs.noaa.gov/cgi-bin/ngs_opsd.prl