/*
 *    GeoTools - The Open Source Java GIS Toolkit
 *    http://geotools.org
 *
 *    (C) 2011, Open Source Geospatial Foundation (OSGeo)
 *    (C) 2003-2005, Open Geospatial Consortium Inc.
 *    
 *    All Rights Reserved. http://www.opengis.org/legal/
 */
package org.opengis.referencing.datum;

import javax.measure.unit.Unit;
import javax.measure.quantity.Angle;
import org.opengis.referencing.IdentifiedObject;
import org.opengis.annotation.UML;

import static org.opengis.annotation.Obligation.*;
import static org.opengis.annotation.Specification.*;


/**
 * A prime meridian defines the origin from which longitude values are determined.
 * The {@link #getName name} initial value is "Greenwich", and that value shall be
 * used when the {@linkplain #getGreenwichLongitude greenwich longitude} value is
 * zero.
 *
 *
 * @source $URL: http://svn.osgeo.org/geotools/trunk/modules/library/opengis/src/main/java/org/opengis/referencing/datum/PrimeMeridian.java $
 * @version <A HREF="http://portal.opengeospatial.org/files/?artifact_id=6716">Abstract specification 2.0</A>
 * @author  Martin Desruisseaux (IRD)
 * @since   GeoAPI 1.0
 */
@UML(identifier="CD_PrimeMeridian", specification=ISO_19111)
public interface PrimeMeridian extends IdentifiedObject {
    /**
     * Longitude of the prime meridian measured from the Greenwich meridian, positive eastward.
     * The {@code greenwichLongitude} initial value is zero, and that value shall be used
     * when the {@linkplain #getName meridian name} value is "Greenwich".
     *
     * @return The prime meridian Greenwich longitude, in {@linkplain #getAngularUnit angular unit}.
     */
    @UML(identifier="greenwichLongitude", obligation=CONDITIONAL, specification=ISO_19111)
    double getGreenwichLongitude();

    /**
     * Returns the angular unit of the {@linkplain #getGreenwichLongitude Greenwich longitude}.
     *
     * @return The angular unit of greenwich longitude.
     */
    @UML(identifier="getAngularUnit", specification=OGC_01009)
    Unit<Angle> getAngularUnit();
}