Ellipsoidally Referenced Surveys (ERS)

Datums and Hydrographic Surveying

Vertical Datums and Hydrographic Surveying

Coast Survey conducts ellipsoidally referenced surveys, or ERS, to maximize the efficiency, accuracy, and flexibility of hydrographic operations. Hydrography acquired in a geodetic reference frame can be transformed to serve a multitude of coastal mapping and nautical charting products and services that require an expression in various geodetic, orthometric, and [non]tidal water level datums.

Vertical Datums

A vertical datum is a zero-height (or depth) reference surface from which hydrographic features and bathymetry are measured. In mapping and charting, the term ellipsoidally referenced survey (ERS) refers to the fact that positioning is achieved within a geometric, three-dimensional coordinate reference frame (or geodetic datum) using Global Navigation Satellite System (GNSS) technology.

Geodetic Datums

A geodetic datum is said to be realized through the combined specification of:

1. a coordinate system origin, orientation, and reference ellipsoid or geoid, and
2. a self-consistent set of reference coordinates (geodetic control).

The geodetic datum for the U.S. GNSS, Global Positioning System (GPS), is the World Geodetic System 1984 (WGS 84). WGS 84 coordinate frame realizations are regularly aligned with International Terrestrial Reference Frame (ITRF) realizations, with cartesian (X, Y, Z) coordinates of terrestrial points agreeing within a few centimeter (e.g., WGS 84 (G2296) 2024.0 and ITRF2020 2015.0). Expression of location using geodetic coordinates (latitude, longitude, height) are according to a projection on the reference ellipsoid. The WGS 84 reference ellipsoid (also named WGS 84) is slightly different than the Geodetic Reference System 1980 ellipsoid (GRS 80); GRS 80 is commonly used to compute ITRF geodetic coordinates. The North American Datum of 1983 (NAD83) is a geocentric reference system as well, but is what is known as a national reference frame.

The important distinction between the NAD83 reference frame and global reference frames (like WGS 84 and ITRF) is that NAD83 is affixed to specific tectonic plates. The NAD83(2011) 2010.0 realization effectively moves with the North American tectonic plate to keep the coordinate expression of "tectonic-static" points relatively constant over time. Likewise, two other NAD83 2010.0 realizations, NAD83(PA11) and NAD83(MA11), are designed to keep coordinates stable with respect to the Pacific and Mariana tectonic plates, respectively. The NAD83 coordinate origin realization turned out to be approximately 2.2 meters displaced from the (now accurately known) position of the Earth's center of mass.

Geodetic coordinates can be expressed in different geodetic datums, provided the differences in origin, reference ellipsoid, and the national-vs-global frame type are properly taken into account. The National Geodetic Survey's modernized National Spatial Reference System (NSRS) replaces the NAD83 and is aligned with the ITRF at the inception date of the NSRS. The modernized NSRS consists of an expanded family of plate-fixed terrestrial reference frames assigned to the North American, Pacific, Caribbean, and Mariana tectonic regions. Vertical reference surface types important to ERS hydrography include additional datum types, connected to a geodetic datum: geopotential datums, tidal datums, and non-tidal datums.

Geopotential Datums

Geopotential datums employ the Earth's gravity field as a reference surface. A geoid model constitutes a particular geopotential scalar value and is expressed in terms of a geodetic height: Geoid undulation is the outward-normal distance from the reference ellipsoid (aka ellipsoid height) associated with the geodetic datum realization. Orthometric height of some point is a measure of the curved-path direction of gravity (along the plumb line), from the geopotential-turned-orthometric datum. In contrast, a dynamic height value designates a relative geopotential value; converted to units of length, it is known as hydraulic head. Orthometric heights have a clear geometric meaning of "vertical", but dynamic heights are a more accurate expression of potential energy. The North American Vertical Datum of 1988 (NAVD 88) is an example of an orthometric height datum. The modernized NSRS also improves upon NAVD 88, with an internally-consistent geopotential datum called the North American-Pacific Geopotential Datum (NAPGD). The International Great Lakes Datum of 1985 (IGLD 85) is an example of a dynamic height datum. The effect of gravity is fundamental to water level models; depending upon intended use, orthometric or dynamic datum height is an important relation to include in the transformation of ERS height (depth) coordinates into a tidal or non-tidal datum basis.

Tidal Water Level Datums

NOAA tidal datums are relative to a local mean sea level (LMSL), and it includes the effects of both land motion and change in sea level. Note that by definition, a global "mean sea level" is necessarily in reference to a specific geoid model (there is no "the" mean sea level). NOAA's National Ocean Service (NOS) ascertains LMSL using tide stations, computed from either a 19-year [equivalent] or 5-year average of hourly water level observations; the latter being used in areas of rapid, net-LMSL movement. Tidal datum "planes" are computed using a 19-year [equivalent] average of [semi]diurnal water level extrema: Mean High Water (MHW), Mean Lower Low Water (MLLW), etc.

Non-Tidal Water Level Datums

In non-tidal areas, the intermediate LMSL-realization either remains a relevant step of measure, or the topographic environment is such that a direct orthometric- or dynamic-height relation to the non-tidal datum makes sense. Non-tidal datum domains include areas where perennial surface water levels are governed by hydrology or hydraulic effects, as well as regions where other oceanographic and meteorological effects dominate what small influence tides may have in the long-term. Various non-tidal datum examples exist within the scope of NOAA hydrographic responsibility: within coastal embayments (e.g., where NOAA defines a Low Water Datum equal to the LMSL datum minus 0.5 feet), upstream in rivers (e.g., the Low Water Reference Plane (LWRP) in the Mississippi River, and the Columbia River Datum (CRD) in Oregon), above lock and dam systems (e.g., the New York Canal System), and various inland lakes (e.g., Great Lakes Low Water Datum (LWD) IGLD 1985, and the Low Lake Level of Lake Champlain).

ERS Publications

Ellipsoidally Referenced Surveying for Hydrography
Mills, J., & Dodd, D. (2014). Ellipsoidally Referenced Surveying for Hydrography, International Federation of Surveyors (FIG) Publication NO 62

The Ellipsoid-Referenced Zoned Datum: A Poor Man's VDatum for NOAA Hydrography in Alaska
Riley, J. L., Greenaway, S., & Wood, D. A. (2016). The Ellipsoid-Referenced Zoned Datum: A Poor Man's VDatum for NOAA Hydrography in Alaska, Canadian Hydrographic Conference 2016, Halifax, NS

Measuring the Water Level Datum Relative to the Ellipsoid During Hydrographic Survey
Rice, G., & Riley, J. L. (2011). Measuring the Water Level Datum Relative to the Ellipsoid During Hydrographic Survey, Proceedings of U.S. Hydro 2011. April 25-28, 2011. Tampa, FL.

A Derived Model of Alaskan Sea Surface Topography: A Critical Piece of the Vertical Datum Transformation for Ellipsoid Referenced Hydrography
Wood, D. A., Riley, J. L., & Greenaway, S. (2016). A Derived Model of Alaskan Sea Surface Topography: a critical piece of the vertical datum transformation for ellipsoid referenced hydrography, Canadian Hydrographic Conference 2016, Halifax, NS

QA & QC in NOAA ERS Hydrography
Rice, G., & Riley, J. L. (2011). Measuring the Water Level Datum Relative to the Ellipsoid During Hydrographic Survey,
Canadian Hydrographic Conference, April 14-17, 2014, St. John's, NL