-define datums – various surfaces from which “zero” is measured. -geoid is a vertical datum tied to MSL. -geoid height is ellipsoid height from specific ellipsoid to geoid. -types of geoid heights: gravimetric versus hybrid.

The present WGS 84 system uses the 1996 Earth Gravitational Model (EGM96) geoid and is the ellipsoid that best fits the geoid model for the selected survey points in the set.

A datum is built on top of the selected spheroid, and can incorporate local variations in elevation. With the spheroid, the rotation of the ellipse creates a totally smooth surface across the world. Since this doesn’t reflect reality very well, a local datum permits local variations in elevation to be incorporated.

A datum transformation is a set of math formulas that converts point coordinates from one datum to another. Datum transformations take place in three-dimensional space, but you can get a sense of how they work from a two-dimensional example. The datum transformation is the math you use to do this.

Earth Gravitational Model 2008
Currently, WGS 84 uses the Earth Gravitational Model 2008. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 2160.

The horizontal datum can be accessed and used through a collection of specific points on the Earth whose latitude and longitude have been accurately determined by NOAA’s National Geodetic Survey. Conversion between these two can be done through geodetic surveys at tide gauges.

Datums represent the shape of the earth and because they are different, there can be “shifts” between data in one datum and data in another datum. The shift is just a few meters between some datums, such as NAD 83 and WGS 84. However, the datum shift can be over 100 meters for other datums.

A datum is simply a foundation and reference for spatial measurements. A system of coordinates is then used to describe those measurements relative to the datum, and a projection is the visual representation of those measurements on a different surface.

EGM96 is a more complex model based on the gravitational force of the Earth (which is not constant) that defines what “sea level” or “up/down” mean, a smooth but irregular shape called “geoid”.

For example, the “WGS84 projection” is a geographic one. A UTM projection is a projected one. Either of these will use only one datum. However, the data on the map could have come from multiple sources, all with unique projections and therefore datums.

Datum shift between ED50 and WGS 84 The longitude and latitude lines on the two datums are the same in the Archangel region of north-west Russia . As one moves westwards across Europe, the longitude lines on ED50 gradually become further west than their WGS 84 equivalents, and are around 100 metres west in Spain and Portugal .

This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution). The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about -105 m to about +85 m.

For satellite geodesy, a global geodetic datum (e.g. WGS 84) is defined by : • Three constants to specify the origin of the coordinate system (x, y, z) • The constants to specify the orientation of the coordinate system • Two constants to specify the dimension of the reference ellipsoid (a and f) 8 Definition of World Geodetic System 1984

The geodetic datum of ED50 was centred at the Helmertturm on the Telegrafenberg in Potsdam, (then East) Germany; the intent was to encourage cooperation with the socialist states during the cold war, which failed. The adjustments for later versions of the datum ( ED77, ED79) used the Munich Frauenkirche as starting point.