Learn About Datums and Transformations
A datum is any reference system against which measurements may be made. Horizontal datums measure positions on the surface of the earth, while vertical datums are used to measure land elevations and water depths. There are numerous horizontal and vertical datums in use for a variety of geospatial applications. Thus, to integrate data from multiple sources that are referenced to different datums requires appropriate datum transformation tools. NOAA’s National Ocean Service provides two unique tools, NADCON and VDatum, for transformation across horizontal and vertical datums, respectively.
Horizontal Datum Transformations
Horizontal datums differ in the choice of spheroid and its point of origin that are used to approximate the surface of the earth. Local horizontal datums position a selected spheroid so as to closely fit the surface of the earth in a local area, and their origin is fixed to a location on the earth’s surface. The North American Datum of 1927 (NAD27) is an example of a local horizontal datum. It uses the Clarke spheroid of 1866 with an origin located on the surface of the earth at Meades Ranch, Kansas. Advances in surveying and geodesy eventually led to the need to establish the North American Datum of 1983 (NAD83). NAD83 uses the ellipsoid from the Geodetic Reference System of 1980 (GRS 80) with an origin located at the earth’s center of mass. To convert data between NAD27 and NAD83, the National Geodetic Survey (NGS) developed a program called NADCON which uses minimum curvature to relate coordinate differences between the two systems. Visit the NGS NADCON web page.
Types of Vertical Datums
Vertical datums have traditionally come in two categories: those based on a form of mean sea level (MSL), called orthometric datums, and those based on tidally derived surfaces of high or low water, called tidal datums. In addition, there is a recently added third category, consisting of 3dimensional or ellipsoid datums realized through spacebased systems such as the Global Positioning System (GPS). A vertical datum transformation tool, VDatum, has been developed by the National Ocean Service which allows the easy transformation of elevation data between any two vertical datums, among a choice of 28 orthometric, tidal and ellipsoid vertical datums.
Topographic maps (e.g., from USGS) generally have elevations referenced to orthometric datums, either the North American Vertical Datum 1988 (NAVD 88) or to the older National Geodetic Vertical Datum 1929 (NGVD 29). All GPS positioning data are referenced to one of many 3D/ellipsoid datums. NOAA’s nautical charts have depths referenced to mean lower low water (MLLW), and bridge clearances are referenced to mean high water (MHW). The legal shoreline in the U.S., which is the shoreline represented on NOAA’s nautical charts, is the MHW shoreline; that is, the landwater interface when the water level is at an elevation equal to the MHW datum. The MLLW line is also depicted on NOAA’s charts.

Figure showing VDatum software and a list of datums included. 
Orthometric datums are essentially equipotential (gravitational) surfaces of the earth with one or more tide stations used as control points. They have often been viewed as being based on a form of MSL. The National Geodetic Vertical Datum of 1929 (NGVD 29), which was originally called the “Sea Level Datum of 1929", has 21 tide station control points in the U.S. and 5 in Canada. MSL, however, departs from an equipotential surface through the effects of winds, atmospheric pressure, water temperature, salinity, and currents. (Local MSL also changed since 1929 due to vertical land movement.)* Unacceptable inconsistencies in NGVD 29 led to the establishment of a new national orthometric datum, the North American Vertical Datum of 1988 (NAVD 88), which has only one control point (Father Point, Quebec, Canada). The differences between these two orthometric datums can be up to 2.2 meters.
*The use of multiple control points introduced error into the datum by forcing the equipotential surface to match MSL in numerous locations.
3D or ellipsoid datums, which have become so important since the development of GPS, are based on a geometric model, an ellipsoid that approximates the earth’s surface (without the topography). There can be many different 3D datums depending on how the origin of the ellipsoid is defined. For example, there is a 2 meter difference between two of the most frequently used 3D datums, the North American Datum of 1983 (NAD 83) and the World Geodetic System of 1984 (WGS 84). VDatum uses only the vertical component of the 3D datum, which, as the name implies, is a complete 3dimensional coordinate system.
The geoid is a specific gravitational equipotential surface which best fits (in the least squares sense) global sea level. Since this equipotential surface includes the effects of topography, it will differ significantly (by as much as 100 meters) from a geocentric ellipsoid because of the Earth’s irregular mass distribution, being higher than the ellipsoid where there is a greater mass. GEOID99, the latest geoid model developed by NOS, specifically relates NAD83 ellipsoid heights to NAVD 88 orthometric heights. It was calibrated against GPS ellipsoid heights on leveled benchmarks throughout the conterminous United States.
Tidal datums are based on averaged stages of the tide, such as MHW and MLLW. To minimize all the significant tidal daily, monthly, and yearly variations, a tidal datum such as MHW is defined as the average of all the high water elevations over an 18.6year period (often rounded to 19 years). This also averages out most meteorological effects on water level, which could bias a tidal datum computed from a shorter length data time series. Tidal datum elevations vary significantly with horizontal (geographic) distance, especially in shallower waters, and they usually vary more rapidly than the horizontal variation in orthometric or 3D/ellipsoid vertical datums. In Tampa Bay the separations between the tidal surfaces and the NAD 83 (and other 3D/ellipsoid datums) are in excess of 24 meters. The relationship of NAVD 88 to local mean sea level is calibrated from tide model comparisons with leveled tidal benchmarks, and is approximately a constant 0.163 meters in Tampa Bay.
