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Alaska Tidal Datum Portal - FAQs

Common Questions

Q: What is a datum and why do I need to care about tidal datums?

A: A vertical datum is a reference surface from which we can measure elevations on the earth's surface. The North American Vertical Datum of 1988 (NAVD88) is currently the official geodetic vertical datum for the United States. A NAVD88 elevation is an orthometric height, which means a height above the geoid (an equipotential gravitational reference surface that approximates an idealized global sea surface). Orthometric heights may be determined by combining a measured height above the ellipsoid surface (a simplified, smooth 3D model of the earth's shape) with a model of the geoid. NAVD88 elevations are obtained in this manner by combining the vertical component of the NAD83 ellipsoid, which can be measured using a GPS receiver or though differential leveling, with GEOID12A (the best available geoid model).

Consider the following example:

You own a home on the coast that has a foundation elevation of 2 m, referenced to NAVD88

One day you hear a report from the National Weather Service that a large storm system will be delivering a surge to your area with a forecast water level elevation of approximately 1 meter above MSL (Mean Sea Level)

What does this mean to you?

surge example

Note that your home elevation and the forecast surge have not been reported using the same datum. To compare the forecasted water elevation to your foundation elevation you will need to know how your local MSL relates to NAVD88 (the red value in the image above). Your local MSL datum may be higher (as shown) or lower than the NAVD88 datum depending on your location along the coast.

If the local MSL in your community is 1.5 m above NAVD88 then the forecasted surge elevation of 1 m above MSL is 2.5 m referenced to NAVD88. Since your house is only at 2 m referenced to NAVD88 you will need to take action in this example.

Tidal datums are specific to the location of measurement and they may become considerably different with distance from the original tide station, particularly in shallow water areas. Therefore, a tidal datum applies only to the immediate area for which it was determined.

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Q: What types of tidal datums exist?

A: A complete list of definitions for tidal datums maintained by NOAA is available at the NOAA Tides & Currents website. Tidal datums typically refer to a 19-year National Tidal Datum Epoch (currently 1983-2001), a period of time that encompasses full variation in the path of the moon relative to the sun. A Modified 5-Year Epoch is used in certain regions with anomalous sea level changes. Approximately 12% of Alaska's local tidal datums are calculated with a Modified 5-Year Epoch (currently 2002-2006).

The datums most commonly used in Alaska are listed below:

Datum Name Definition
Mean Higher High Water (MHHW) Average of the higher high water height of each tidal day observed over a 19-year National Tidal Datum Epoch or a Modified 5-Year Epoch.
For stations with shorter series (1-3 months is common in Alaska), comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the Modified or National Tidal Datum Epoch.
Mean High Water (MHW) Average of all the high water heights observed over a 19-year National Tidal Datum Epoch or a Modified 5-Year Epoch.
For stations with shorter series (1-3 months is common in Alaska), comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the Modified or National Tidal Datum Epoch.
Mean Sea Level (MSL) The arithmetic mean of hourly heights observed over a 19-year National Tidal Datum Epoch or a Modified 5-Year Epoch.
Shorter series (1-3 months is common in Alaska) are specified in the name; e.g. monthly mean sea level and yearly mean sea level.
Mean Lower Low Water (MLLW) Average of the lower low water height of each tidal day observed over a 19-year National Tidal Datum Epoch or a Modified 5-Year Epoch.
For stations with shorter series (1-3 months is common in Alaska), comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the Modified or National Tidal Datum Epoch.
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Q: I thought that NAVD88 elevations are referenced to Mean Sea Level, aren't they the same thing?
Rimouski, Quebec

Do you live in Rimouski?
If you answered no, you need to pay attention to your tidal datums.

A: Elevations referenced to NAVD88 (North American Vertical Datum of 1988) should never be used as Mean Sea Level.

This is one of the most common mistakes made by someone who is new to working with elevations in the coastal zone and you are not alone - confusion about how local MSL relates to published geodetic elevations has existed as long as we have had national geodetic datums.

The first standardized vertical datum used in the United States was the National Geodetic Vertical Datum of 1929 (NGVD29), previously named the "Sea Level Datum of 1929." NGVD29 utilized the 19-year water level records from a network of 26 tide gauge stations in the United States and Canada to establish a fixed MSL (Mean Sea Level) surface to which all geodetic benchmarks in the nation could be adjusted. In 1988 a more accurate vertical datum (NAVD88) was adopted as the national standard. NAVD88 is referenced to the Mean Sea Level at one tide station located at Father Point in Rimouski, Quebec.

tidal differences

NAVD88 "MSL" ≠ Local MSL
Position of Local MSL relative to NAVD88 (GEOID12A) in meters.

On an idealized earth without ocean mixing or atmospheric variation the surface of the ocean would coincide with an equipotential surface in the gravity field (the geoid) and orthometric heights would correspond to the height above mean sea level. However, there are two primary reasons that the orthometric NAVD88 elevations do NOT correspond to local mean sea level:

  1. Our map of the earth's gravity field is not complete so we need to use a model of the geoid (GEOID12A for example). Geoid models are a good approximation of the true geoid but they are not perfect, particularly in many parts of Alaska.
  2. Oceanographic and atmospheric effects are constantly changing the ocean surface and causing it to deviate from where we would expect it to be on an idealized earth. Local mean sea level is not driven by the earth's gravity field alone.
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Q: What are some causes of variation in my local tidal datums?

A: You may notice that tidal datums in your area of interest might vary slightly in publications and from what you calculate. This variation can usually be attributed to one of the following factors:

Different Tidal Benchmark

For some tide stations, published geodetic elevations may be available for more than one tidal benchmark. If tidal datums are connected to the geodetic datum via a different benchmark there may be minor differences in the calculated adjustment due to allowable vertical measurement errors.

Orthometric Height Computation

NAVD88 elevations are orthometric heights that are calculated by combining a geoid height (height above the surface of equal gravitational potential) with the measured height above the reference ellipsoid at a given point of interest. As geoid models are improved, the orthometric height of a tidal benchmark that is used to relate the benchmark to the tidal datum will change.

Re-establishment of Tide Station

Reactivation of a tidal station, particularly for a prolonged period of time, may result in an improved measurement of the water levels in a particular area. For example, many tidal datums in Alaska are currently based on <1 month of water level measurements. A reoccupation for a duration of 3 months or more can greatly improve the accuracy of the calibrated tidal datums.

Tidal Epoch

A tidal epoch is a specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values for tidal datums. The current National Tidal Datum Epoch is 1983-2001 and the Modified 5-year Epoch is 2002-2006. Tidal datums calculated in different epochs may vary, particularly in regions with positive or negative trends in sea level change.

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Q: I don't see a location listed in the calculator; where are the rest of the tidal datums?

A: A tidal datum is only applicable to the area immediately surrounding the same tide station. Tidal datums are referenced to local water levels and should not be applied to coastal areas of differing oceanographic characteristics. Likewise, the elevation relationship between a tidal and geodetic datum is valid only at the location of the datum measurements. Due to variations in tidal range, bathymetry, topography, the geoid, and vertical land subsidence or uplift, the extrapolation of this unique relationship beyond the location of measurement requires careful corrections and mathematical interpolation, and is not always possible.

Important: If your area of interest is not listed in the tidal datum calculator, you cannot use the values from another nearby location for the reasons described above.

Until the network of tidal datum measurements is expanded in Alaska, the ability to link offshore measurements to onshore measurements is limited. The following options are available to obtain a surrogate tidal datum estimate:

Install a temporary tide gauge

A temporary tide gauge, installed by a government agency, university or private consultant, may be used to monitor water levels for a period of several days to several months. The measured values can be tied to the local geodetic datum using traditional differential leveling techniques or a static GPS survey occupation of at least three benchmarks.

Measure the geodetic elevation of an identifiable foreshore feature

Appropriate foreshore features will vary by shore type and tidal range. Examples include the wrack line on a beach or a biological band, such as barnacles, on a rocky coastline corresponding to the approximate mean higher high water. This measurement may be accomplished using traditional differential leveling techniques or with the use of a static GPS survey occupation.

Use a modeled value

Hydrodynamic circulation numerical models are capable of simulating typical water level conditions over large computational domains. These models are limited by the quality of bathymetric data, particularly in coastal areas, but are capable of producing an estimate of tidal datums in areas where no direct measurements are available. Refined gravimetric geoid models may also be used to approximate local MSL values. The USGG2012 geoid model, when used with the GRS80 ellipsoid, will produce a reasonably good approximation of local MSL values in most of Alaska.

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Q: What is VDatum?

A: VDatum is a software tool maintained by NOAA. It allows users to easily transform elevation values between a range of tidal and geodetic (both orthometric and ellipsoidal) datums. VDatum is a valuable tool in the creation of seamless topographic-bathymetric elevation grids that are necessary for coastal mapping applications, including storm surge, tsunami and sea level change vulnerability and ecosystem management.

VDatum uses a continuous sea surface grid (the TSS, Topography of Sea Surface) to make transformations between tidal and geodetic datums. This method is preferable to methods that rely on direct local transformations alone because it allows for interpolation across areas between known tidal datums. Because VDatum requires this TSS, a robust network of established measured tidal values, geodetic measurements and tide model predictions is a prerequisite to the availability of the VDatum software in any region.

VDatum is currently available for all coastal states in the contiguous United States (including the Great Lakes) as well as in Puerto Rico and the Virgin Islands.

For additional information on VDatum and its status in Alaska, visit the NOAA VDatum website.

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Q: Where can I find additional information about tidal datums?

A: You will find links to tidal datum overview documents and technical publications under the 'More Resources' links of this web page.

If you have additional questions or comments about the content of this tidal datum portal please contact Nora Nieminski at DGGS. We would value any feedback about how you have used this site or how we could improve it.

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