Primary Reference Data

Ellipsoids

An ellipsoid is a mathematical approximation of the spheroidal shape of the earth. As such, ellipsoid definitions are at the heart of all coordinate system definitions.

The ellipsoid property editor is used to add or modify an ellipsoid definition

The name of the ellipsoid is a code that is taken from an internationally recognised list of standardised ellipsoid definitions. An ellipsoid is often linked to a specific region or area of use. The GRS80 ellipsoid happens to be the one on which the geodetic datum of Australia is based.

The mathematical properties of the ellipsoid are best explained by way of a diagram.

The two axes specify the size of the ellipsoid, while the flattening represents the slight "bulging" of the earth at the equator.

The semi-major axis and inverse flattening (1/f) are the parameters that are most widely used in the coordinate conversion formulae.

The EPSG ID number is an optional reference number that is used to identify the ellipsoid in the online EPSG register. This register is a list of coordinate systems used throughout the world and is usually the best source of information in this regard.

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Datums

A datum definition is always based on a given ellipsoid. The datum definition includes parameters such as the prime meridian that define the positioning and orientation of a three-dimensional (X,Y,Z) space according to which the longitude and latitude of a point on the ellipsoidal surface can be determined.

The datum property editor is used to add or modify a datum definition.

Field	Description
Name	A code that is usually taken from an internationally recognised list of standardised datum definitions.
Description	A short description of the datum
Ellipsoid	The ellipsoid on which the datum is based
Prime meridian	The longitude origin (0°) of the coordinate space
Meridian name	The official name of the primary meridian
Area of coverage	A datum is usually linked to a specific region or area of use
Information source	Optional reference to the source of the data

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Projections

A projection is used to represent part of the curved surface of the earth on a flat two-dimensional plane that is more suitable for mapping purposes. A projection is always based on a specific datum. The projection essentially converts latitude and longitude (in degrees) to northing and easting values (in metres) relative to the origin point of the projection.

There are many different types of projections, including cylindrical, conic and azimuthal projections. Each of these have their own characteristics and mathematical representation.

The OCRIS Database Toolbox supports only Transverse Mercator projections, and in particular the Universal Transverse Mercator (UTM) system.

The UTM system divides the globe into sixty zones of six degrees (6°) longitude each, starting at -180° and ending at +180°. Each zone is projected onto a flat plane along a line of longitude that is referred to as the central meridian of the zone.

The equator divides the zone into a northern and southern part.

The following example shows the properties of a typical UTM projection zone as displayed in the projection editor.

Field	Description
Name	A code used to uniquely identify the projection
Description	A short description of the projection
Central meridian	Central line of longitude for the projected zone
Zone width	Width (in degrees) of each zone  (e.g. 6° for UTM)
Longitude origin	Longitude of the origin for the projection (e.g. -180° for UTM, being the western edge of zone 1)
Latitude origin	Latitude origin (usually the equator)
Central scale factor	The central scale factor is measured along the central meridian
False easting	False easting (added to prevent negative numbers for easting)
False northing	False northing (added to prevent negative numbers for northing)
Hemisphere	Specifies whether the zone falls in the northern or southern hemisphere

For detailed discussions and illustrations explaining the workings of a UTM projection, please refer to one of the many books and articles on geodesy that are available on the internet.

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Height Datums 

In addition to latitude and longitude or easting and northing, all survey points also have a value for the third dimension, namely height.

For any given point on the earth's surface, the height (also referred to as elevation, or relative level) is usually measured relative to an imaginary surface called the geoid, which is determined by the gravitational field of the earth and is approximated by mean sea level. This type of height measurement is known as orthometric height.

Height datums are often named according to the area of use, in much the same way as geodetic datums.

When working on a global scale (e.g. with global positioning satellites), height is usually measured relative to the surface of a specific ellipsoid. This is known as ellipsoidal height.

Unlike the ellipsoid, which is a smooth mathematical surface, the geoid is an equipotential surface that can be very irregular, especially in areas of varying topography. In some locations, the geoid may extend above the surface of the ellipsoid, while dipping below the ellipsoid in other areas. The difference in height between the two surfaces is the geoid height, also known as the geoid-spheroid separation.

The relationship is defined as

where  h is the ellipsoidal height, H is the orthometric height and N is the geoid height.

The OCRIS Database Toolbox does not deal directly with height datums as such; however, these parameters are of some importance in datum transformations, as will be discussed in later sections.

Numerous articles on this subject are available on the internet, including calculators for determining the geoid height at any given location. Simply search for "geoid-spheroid separation" and find the tools and information applicable to your location.