/*$************************************************************************************************
 **
 ** $Id: Ellipsoid.java 831 2006-05-01 00:27:23Z Desruisseaux $
 **
 ** $URL: http://svn.code.sf.net/p/geoapi/code/branches/2.0.x/geoapi/src/main/java/org/opengis/referencing/datum/Ellipsoid.java $
 **
 ** Copyright (C) 2003-2005 Open GIS Consortium, Inc.
 ** All Rights Reserved. http://www.opengis.org/legal/
 **
 *************************************************************************************************/
package org.opengis.referencing.datum;

// J2SE extensions
import javax.units.Unit;

// OpenGIS direct dependencies
import org.opengis.referencing.IdentifiedObject;

// Annotations
import org.opengis.annotation.UML;
import org.opengis.annotation.Extension;
import static org.opengis.annotation.Obligation.*;
import static org.opengis.annotation.Specification.*;


/**
 * Geometric figure that can be used to describe the approximate shape of the earth.
 * In mathematical terms, it is a surface formed by the rotation of an ellipse about
 * its minor axis. An ellipsoid requires two defining parameters:
 * 

 * 

 *   
{@linkplain #getSemiMajorAxis semi-major axis} and
 *       {@linkplain #getInverseFlattening inverse flattening}, or
 *   
{@linkplain #getSemiMajorAxis semi-major axis} and
 *       {@linkplain #getSemiMinorAxis semi-minor axis}.
 * 
 * 

 * There is not just one ellipsoid. An ellipsoid is a matter of choice, and therefore many
 * choices are possible. The size and shape of an ellipsoid was traditionally chosen such
 * that the surface of the geoid is matched as closely as possible locally, e.g. in a country.
 * A number of global best-fit ellipsoids are now available. An association of an ellipsoid with
 * the earth is made through the definition of the size and shape of the ellipsoid and the position
 * and orientation of this ellipsoid with respect to the earth. Collectively this choice is captured
 * by the concept of “{@linkplain GeodeticDatum geodetic datum}”. A change of size, shape, position
 * or orientation of an ellipsoid will result in a change of geographic coordinates of a point and
 * be described as a different geodetic datum. Conversely geographic coordinates are unambiguous
 * only when associated with a geodetic datum.
 *
 * @version Abstract specification 2.0
 * @author ISO/DIS 19111
 * @author Martin Desruisseaux (IRD)
 * @since GeoAPI 1.0
 */
@UML(identifier="CD_Ellipsoid", specification=ISO_19111)
public interface Ellipsoid extends IdentifiedObject {
    /**
     * Returns the linear unit of the {@linkplain #getSemiMajorAxis semi-major}
     * and {@linkplain #getSemiMinorAxis semi-minor} axis values.
     *
     * @return The axis linear unit.
     */
    @UML(identifier="getAxisUnit", specification=OGC_01009)
    Unit getAxisUnit();

    /**
     * Length of the semi-major axis of the ellipsoid. This is the
     * equatorial radius in {@linkplain #getAxisUnit axis linear unit}.
     *
     * @return Length of semi-major axis.
     * @unitof Length
     */
    @UML(identifier="semiMajorAxis", obligation=MANDATORY, specification=ISO_19111)
    double getSemiMajorAxis();

    /**
     * Length of the semi-minor axis of the ellipsoid. This is the
     * polar radius in {@linkplain #getAxisUnit axis linear unit}.
     *
     * @return Length of semi-minor axis.
     * @unitof Length
     */
    @UML(identifier="secondDefiningParameter.semiMinorAxis", obligation=CONDITIONAL, specification=ISO_19111)
    double getSemiMinorAxis();

    /**
     * Returns the value of the inverse of the flattening constant. The inverse
     * flattening is related to the equatorial/polar radius by the formula
     *
     * ivf = re/(re-rp).
     *
     * For perfect spheres (i.e. if {@link #isSphere} returns {@code true}),
     * the {@link Double#POSITIVE_INFINITY} value is used.
     *
     * @return The inverse flattening value.
     * @unitof Scale
     */
    @UML(identifier="secondDefiningParameter.inverseFlattening", obligation=CONDITIONAL, specification=ISO_19111)
    double getInverseFlattening();

    /**
     * Indicates if the {@linkplain #getInverseFlattening inverse flattening} is definitive for
     * this ellipsoid. Some ellipsoids use the IVF as the defining value, and calculate the polar
     * radius whenever asked. Other ellipsoids use the polar radius to calculate the IVF whenever
     * asked. This distinction can be important to avoid floating-point rounding errors.
     *
     * @return {@code true} if the {@linkplain #getInverseFlattening inverse flattening} is
     *         definitive, or {@code false} if the {@linkplain #getSemiMinorAxis polar radius}
     *         is definitive.
     */
    @UML(identifier="CS_Ellipsoid.isIvfDefinitive", obligation=CONDITIONAL, specification=OGC_01009)
    boolean isIvfDefinitive();

    /**
     * {@code true} if the ellipsoid is degenerate and is actually a sphere. The sphere is
     * completely defined by the {@linkplain #getSemiMajorAxis semi-major axis}, which is the
     * radius of the sphere.
     *
     * @return {@code true} if the ellipsoid is degenerate and is actually a sphere.
     */
    @UML(identifier="secondDefiningParameter.isSphere", obligation=CONDITIONAL, specification=ISO_19111)
    boolean isSphere();
}