/// <summary>
        /// Compresses the internal of the graph, freeing new space.
        /// </summary>
        public override void Compress()
        {
            _graph.Compress();

            // rebuild index.
            if (_vertexIndex != null)
            {
                float latitude, longitude;
                _vertexIndex.Clear();
                for (uint idx = 0; idx < _graph.VertexCount; idx++)
                {
                    if (_graph.GetVertex(idx, out latitude, out longitude))
                    {
                        _vertexIndex.Add(new GeoCoordinate(latitude, longitude), idx);
                    }
                }
            }
        }
 /// <summary>
 /// Compresses the internal of the graph, freeing new space.
 /// </summary>
 public override void Compress()
 {
     _graph.Compress();
 }