/// <summary>
        /// Adds a new vertex.
        /// </summary>
        /// <param name="latitude"></param>
        /// <param name="longitude"></param>
        /// <param name="id"></param>
        /// <returns></returns>
        public void AddVertex(long id, float latitude, float longitude)
        {
            var vertex = new RouterResolvedGraphVertex();
            vertex.Id = id;
            vertex.Latitude = latitude;
            vertex.Longitude = longitude;

            // create vertex.
            _vertices.Add(id, vertex);
        }
        /// <summary>
        /// Adds a new vertex.
        /// </summary>
        /// <param name="latitude"></param>
        /// <param name="longitude"></param>
        /// <param name="id"></param>
        /// <returns></returns>
        public void AddVertex(long id, float latitude, float longitude)
        {
            var vertex = new RouterResolvedGraphVertex();
            vertex.Id = id;
            vertex.Latitude = latitude;
            vertex.Longitude = longitude;

            // create vertex.
            _vertices.Add(id, vertex);
        }