The user may define subclasses of this class.
Projection is an abstract superclass. Should the Projection subclasses found below be inadequate, the user may define new subclasses of Projection.
category: indicates whether the projection is based on an ellipsoid or sphere. This is meaningful with respect to the following projection subclasses: LatitudeAndLongitude, TransverseMercator, Mercator, Stereographic, Polyconic, and LambertConformal. See the comment under ProjectionCategory for further clarification.
xUnits: defines the type of units applicable to x coordinate values.
yUnits: defines the type of units applicable to y coordinate values.