A vertical datum is technically, a surface of zero elevation to which heights of various points are referred in order that those heights be in a consistent system. More broadly, a vertical datum is the entire system of the zero elevation surface and methods of determining heights relative to that surface. Over the years, many different types of vertical datums have been used. The most dominant types today are tidal datums and geodetic datums.

Tidal datums – are determined by averaging the level of water at a tide gauge over time. Some simple examples of these are Mean Sea Level (MSL), Mean Low Water (MLW) and Mean Higher High Water (MHHW).

Mean Sea Level (MSL) is a tidal datum which is computed by the Center for Operational Oceanographic Products and Services (CO-OPS) as part of the National Tidal Datum Epoch (NTDE) based on data collected over a 19-year tide cycle. It pertains to Local Mean Sea Level (LMSL) at the tide station at which it was observed and should not be confused with any other vertical datum, including LMSL at other tide stations. The current NTDE for the United States is 1983-2001.

Geodetic datums – are predominantly determined through a process of surveying known as geodetic leveling, determining the height differences between points in the ground known as bench marks. These height differences can only yield actual heights at the benchmarks if at least one datum origin point is chosen to serve as the absolute level of the vertical datum. It is frequently the practice of those responsible for defining a geodetic datum, to choose a datum origin point that is also at a tide gauge so a relationship between the tidal and geodetic datums exists, though this is by no means a requirement.

In the United States and its territories, NOAA’s National Geodetic Survey (NGS) responsibilities include defining and providing access to and maintenance of geodetic vertical data. These datums are part of the overall National Spatial Reference System (NSRS). North American Vertical Datum of 1988 (NAVD 88), American Samoa Vertical Datum of 2002 (ASVD02), Guam Vertical Datum of 2004 (GUVD04), Northern Marianas Vertical Datum of 2003 (NMVD03) and Puerto Rico Vertical Datum of 2002 (PRVD02). Additionally, Virgin Island Vertical Datum of 2009 (VIVD09) will soon be published and a new Hawaii vertical datum is in the planning stage. Each of these datums has either a single datum origin point, otherwise known as a primary bench mark, or one such point per island in its coverage area. Every datum origin point, in these five datums, is at a tide gauge. To determine the heights of the respective origin points, first local mean sea level relative to the respective NTDE was determined. Next, a nearby bench mark was chosen as the datum origin point for a particular continuous land area (which could be an island or an entire continent). The height above LMSL for those origin bench marks was determined by leveling between the tide station reference mark and the bench mark that is serving as the datum origin point. This height above LMSL was held fixed as the single initial constraint for that particular land area.

The Vertical Mark Number (VM) and Permanent Identifier (PID) shown on the bench mark sheet are unique identifiers for bench marks in the CO-OPS NGS databases, respectively. For bench marks where both VM and PID are indicated, both tidal and geodetic vertical datum elevations are available. Contemporary geodetic vertical datums established by NGS are realized through the publication of heights derived from a simultaneous, least squares minimally constrained adjustment of leveling observations between passive geodetic controls spanning the region for which the datum is defined

North American Vertical Datum of 1988 (NAVD 88)

North American Vertical Datum of 1988 (NAVD 88) consists of a leveling network on the North American Continent, ranging from Alaska, through Canada, across the United States, affixed to a single origin point on the continent:

  • Tide Station & Location = Pointe-au-Pere,Rimouski, Quebec, Canada
  • PID = TY5255
  • GSD* Designation = 54L071
  • Bench Mark = 1250 G
  • Height above LMSL(meters) = 6.271

* Geodetic Survey of Canada = GSD

In 1993 NAVD 88 was affirmed as the official vertical datum in the National Spatial Reference System (NSRS) for the Conterminous United States and Alaska (see Federal Register Notice (FRN)). Although many papers on NAVD 88 exist, no single document serves as the official definitional document for that datum. Detailed information about NAVD 88 is available here.

American Samoa Vertical Datum of 2002 (ASVD02)

American Samoa Vertical Datum of 2002 (ASVD02) consists of a leveling network on the island of Tutuila (in the territory of American Samoa) affixed to a single origin point on the island:

  • Tide Station & Location = 1770000 — Pago Pago Harbor
  • PID = DE8786
  • VM = 12710
  • Bench Mark = 177 0000 S TIDAL
  • Height above LMSL(meters) = 1.364

Per FRN (2009), ASVD02 was affirmed as the official vertical datum in the National Spatial Reference System for the island of Tutuila in American Samoa, replacing all previous height systems for this region. The official definitional document for that datum was published in 2009 and is available here.

Guam Vertical Datum of 2004 (GUVD04)

Guam Vertical Datum of 2004 (GUVD04) consists of a leveling network on the island of Guam affixed to a single origin point on the island:

  • Tide Station & Location = 1630000 — Apra Harbor, Guam
  • PID = TW0041
  • VM = 1684
  • Bench Mark = 163 0000 TIDAL 4
  • Height above LMSL(meters) = 0.419

Per FRN (2009), GUVD04 was affirmed as the official vertical datum in the National Spatial Reference System for the island of Guam, replacing all previous height systems for this region. The official definitional document for that datum was published in 2009 and is available here.

Northern Marianas Vertical Datum of 2003 (NMVD03)

Northern Marianas Vertical Datum of 2003 (NMVD03) consists of a set of three independent leveling networks on the islands of Rota, Saipan and Tinian (in the Commonwealth of the Northern Mariana Islands). Each of these leveling networks is affixed to a single origin point on their respective islands reference to the Saipan tide station.

Rota:

  • Tide Station & Location = Rota East Harbor, Rota
  • PID = DG4014
  • VM = None
  • Bench Mark = TIDAL 3
  • Height above LMSL(meters) = 1.482

Saipan:

  • Tide Station & Location = 1633227 — Tanapag Harbor, Saipan
  • PID = DG3988
  • VM = 16316
  • Bench Mark = 163 3227 TIDAL UH-2C
  • Height above LMSL(meters) = 1.657

Tinian:

  • Tide Station & Location = Tinian Harbor, Tinian
  • PID =AA4407
  • VM = None
  • Bench Mark = TIDAL 1
  • Height above LMSL(meters) = 2.361

Per FRN (2009), NMVD03 was affirmed as the official vertical datum in the National Spatial Reference System for the islands of Rota, Saipan and Tinian in the CNMI, replacing all previous height systems for this region. The official definitional document for that datum was published in 2009 and is available here.

Puerto Rico Vertical Datum of 2002 (PRVD02)

Puerto Rico Vertical Datum of 2002 (PRVD02) consists of a leveling network on the island of Puerto Rico affixed to a single origin point on the island:

  • Tide Station & Location = 9755371 — San Juan
  • PID = TV1513
  • VM = 1386
  • Bench Mark = 975 5371 A TIDAL
  • Height above LMSL(meters) = 1.334

Per FRN (2012), PRVD02 was affirmed as the official vertical datum in the National Spatial Reference System for the islands of Puerto Rico replacing all previous height systems for this region. Currently, there is no official defining document.

Virgin Island Vertical Datum of 2009 (VIVD09)

Virgin Island Vertical Datum of 2009 (VIVD09) consists of a set of three independent leveling networks on the islands of St. Croix, St. John and St. Thomas (in the United States Virgin Islands). Each of these leveling networks is affixed to a single origin point on their respective island.

St. Croix:

  • Tide Station & Location = 9751401–Lime Tree Bay
  • PID = DK7165
  • VM = 1636
  • Bench Mark = 975 1401 M
  • Height above LMSL(meters) = 3.111

St. John:

  • Tide Station & Location = 9751381 –Lameshur Bay
  • PID = DL3636
  • VM = 18179
  • Bench Mark = 975 1381 A
  • Height above LMSL(meters) = 1.079

St. Thomas:

  • Tide Station & Location = 9751639— Charlotte Amalie
  • PID = DL3908
  • VM = 1372
  • Bench Mark = 975 1639 F
  • Height above LMSL(meters) = 1.552

The FRN (2011), VIVD09 was affirmed as the official vertical datum in the National Spatial Reference System for the islands of St. Croix, St. John and St. Thomas in the U.S. Virgin Islands, replacing all previous height systems for this region. Currently, there is no official defining document for VIVD09.

Jim McLefresh

Jim retired from Spokane County as a GPS Survey Technician in May 2015 and is a member of and the GPS Committee Co-Chair for the Land Surveyors Association of Washington State. His primary function, before retirement, was to research and draft Records of Survey and Right-of-Way drawings for construction projects at Spokane County and assemble the survey crew’s field data into “Design Ready Drawings”. He also lectures on GPS, AutoCAD and Field Survey Procedures and has been a regular Speaker at Trimble’s Dimensions conventions.

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One comment on “A Primer In Vertical Datums

  1. Howdy,

    Enjoyed your article. Good summary and tabulation. In some ways, the hardest part of modern surveying is keeping track of what each number means and how it is related to others. Looking at CO-OPS data and NGS data for points in both databases is revealing.

    I had hoped there would be some discussion about the linkage of the geoid to these datums. After all, the new vertical datum will be based on GNSS-derived ellipsoid heights and a high-resolution geoid model.

    The relationship: ellipsoid height (h) – orthometric height (H) – geoid-ellipsoid separation (N) should sum to zero (h-H-N should equal zero (0). While some want to rely on the legacy network of monumented points, benchmarks are increasingly endangered by man-made and natural forces and have not been revisited to verify their accuracy, in most cases, in decades.  There are fundamental problems with NAVD88 including a cross-continent tilt. 

    Remember the NGS definition of the geoid: "The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level."  The equipotential part is especially important because it captures the essential task we seek to achieve with a system of heights "the determination of fluid flow."  

    The NGS site has a number of presentations on their web site discussing and illustrating deficiencies in the current and past vertical networks.

    Annoyingly,

    DMM

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