# Geodetic Datasource

The Geographic Calculator contains an extensive geodetic parameter database which includes many objects that are neatly organized and may be easily located.  Over 3500 coordinate systems are available for use out of the box.  The contents of the Geodetic Datasource can be browsed or searched by using the menus available under the Datasource menu.

You can extend the Geographic Calculator to support an unlimited number of custom linear and angular units, ellipsoids, datums, datum transformations, and coordinate systems. The parameters for each master definition are stored within an XML database file referred to as the Geodetic Datasource, a file named geodata.xml.  All user-defined parameters will be stored in a file named custom.xml

Individual entries in the Geodetic Datasource are known as "Objects".  There  are different types of objects for different types of definitions.  Objects contained in the Geodetic Datasource are:

• Units of Measure

• Linear Units - Units for measuring straight line, Cartesian distances.
• Angular Units - Type of units for measuring rotation.
• Scalar Units - Unit of measure for scale factor.
• Prime Meridians - The Prime Meridian represents the line of longitude that is designated as 0 degrees.  All other lines of longitude are expressed by the angle by which they deviate from the Prime Meridian.  Together with the equator of an Ellipsoid, the Prime Meridian defines the origin of a Geodetic Coordinate System.  A Prime Meridian is defined by the angle that separates it from the Greenwich Meridian.
• Ellipsoids - An Ellipsoid gives a Datum its size and shape.  An Ellipsoid does not have an origin and cannot be used as a base model for coordinates on its own.
• Datums - A Datum is the base model that maps are built on.  All coordinate systems must have a datum associated with them to be related to any other map.  Without a known datum, coordinates are meaningless.
• Coordinate Transformations - Coordinate Transformations (Concatenated, Datum, Parametric or Vertical) - Parameters to transform coordinates from one Coordinate System to another.
• Coordinate Systems (Coordinate Reference Systems) - A Coordinate System is a complete definition needed to express the context of a set of map data.
• Point Styles (Coordinate Points) - A Point Style is used to define the orientation of axes used and the type of units used in the system.
• Areas of Use - Define a geographic extent within which a particular object is intended to be used.
• Usage - Defines how and where a coordinate system, datum or transformation is intended to be used. A usage includes both scope and extent. Area of use represents the geographic extent. A coordinate system or transformation can be tied to multiple usages.
• Scope - Description of the primary purpose to which a coordinate reference system, datum or coordinate operation is applied.