Ellipsoids are used to represent the shape of the Earth. For many applications, and especially in small-scale mapping (typically world maps), the Earth can be represented as a sphere.

Most of the projections supplied in the ilog.views.maps.projection package assume by default that the Earth is a sphere with a radius of approximately 6371 kilometers. However, because the Earth rotates on its axis, it is slightly flattened at the poles, and is therefore better approximated by an ellipsoid rotating on the polar axis. Ellipsoidal projections are used for accurate, large-scale maps and flat coordinate systems. However, in very large scale maps, representing a continent or the whole planet, it is recommended that you use spherical projections. Indeed, the elliptical form of most of the projections in the projection package is accurate only for a few degrees of latitude or longitude around the projection center.

Ellipsoids can be described by many parameters. The ellipsoids provided in the JViews Maps package are defined by two parameters:

If only one parameter is provided, the ellipsoid is assumed to be a sphere. The following example defines a sphere with a radius of 6 000 kilometers (6000000 meters).

Most of the mapping applications use the ellipsoid IlvEllipsoid.SPHERE that defines a sphere having dimensions very close to those of the Earth. Generally, the selected ellipsoid should be as close as possible to the actual shape of the Earth as far as the region to be represented is concerned. The radius of the sphere is expressed in meters.

The following example defines an ellipsoid with an equatorial radius of 6000 kilometers and an eccentricity squared of 0.0067:

If you prefer to provide some other parameter than the eccentricity squared, you can use the conversion methods provided by the IlvEllipsoid class.

The following example defines an ellipsoid with an equatorial radius of 6000 kilometers and a polar radius of 5900 kilometers:

The polar radius is converted to an eccentricity squared value with the ESFromPolarRadius() method.

The class IlvEllipsoid provides the following conversion methods for polar radius and flattening:

The IlvEllipsoidCollection class manages lists of predefined ellipsoids. A list of predefined ellipsoids, or kernel collection, can be retrieved using the method GetKernelCollection.

Ellipsoid collections are read from XML files containing the ellipsoid definitions. The Document Type Definition (DTD) for these definition files is as follows:

In an ellipsoid definition file, you can find:

The following XML file defines the Clarke 1880 ellipsoid, modified for IGN. This ellipsoid will be available as “Clarke 1880 (IGN)”. Then you must set “WGS 1984” as an alias for the kernel “WGS 84” ellipsoid: