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/*
 *    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.crs;

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

import org.opengis.annotation.Extension;
import org.opengis.annotation.UML;
import org.opengis.referencing.ReferenceSystem;
import org.opengis.referencing.cs.CoordinateSystem;

/**
 * Abstract coordinate reference system, usually defined by a {@linkplain
 * org.opengis.referencing.cs.CoordinateSystem coordinate system} and a {@linkplain
 * org.opengis.referencing.datum.Datum datum}. The concept of a coordinate reference system (CRS)
 * captures the choice of values for the parameters that constitute the degrees of freedom of the
 * coordinate space. The fact that such a choice has to be made, either arbitrarily or by adopting
 * values from survey measurements, leads to the large number of coordinate reference systems in use
 * around the world. It is also the cause of the little understood fact that the latitude and
 * longitude of a point are not unique. Without the full specification of the coordinate reference
 * system, coordinates are ambiguous at best and meaningless at worst. However for some interchange
 * purposes it is sufficient to confirm the {@linkplain #getName identity of the system} without
 * necessarily having the full system definition.
 *
 * The concept of coordinates may be expanded from a strictly spatial context to include time.
 * Time is then added as another coordinate to the coordinate tuple. It is even possible to add two
 * time-coordinates, provided the two coordinates describe different independent quantities. An
 * example of the latter is the time/space position of a subsurface point of which the vertical
 * coordinate is expressed as the two-way travel time of a sound signal in milliseconds, as is
 * common in seismic imaging. A second time-coordinate indicates the time of observation, usually
 * expressed in whole years.
 *
 * @version Abstract
 *     specification 2.0
 * @author Martin Desruisseaux (IRD)
 * @since GeoAPI 1.0
 */
@UML(identifier = "SC_CRS", specification = ISO_19111)
public interface CoordinateReferenceSystem extends ReferenceSystem {
    /**
     * Returns a relevant coordinate system instance. Special cases:
     *
     * 

     *   
     *       If the CRS instance on which this method is invoked is an instance of the
     *       {@linkplain SingleCRS single CRS} interface, then the CS instance which is returned
     *       shall be one of the defined sub-interfaces of {@linkplain CoordinateSystem coordinate
     *       system}.
     *   

     *       If the CRS instance on which this method is invoked is an instance of the
     *       {@linkplain CompoundCRS compound CRS} interface, then the CS instance which is returned
     *       shall have dimension and axis components obtained from different {@linkplain
     *       CompoundCRS#getCoordinateReferenceSystems components} of the instance CRS.
     * 
     *
     * @departure Strictly speaking, this method is defined by ISO 19111 for {@linkplain SingleCRS
     *     single CRS} only. GeoAPI declares this method in this parent interface for user
     *     convenience, since CS {@linkplain CoordinateSystem#getDimension dimension} and
     *     {@linkplain CoordinateSystem#getAxis axis} are commonly requested information and shall
     *     be available, directly or indirectly, in all cases (including {@linkplain CompoundCRS
     *     compound CRS}).
     * @return The coordinate system.
     */
    @Extension
    CoordinateSystem getCoordinateSystem();
}