Sync coordinates from map and .xml file

public class POI {
    public int X { get; set; }
    public int Y { get; set; }
    public string Value { get; set; }
}
public class PoiStore {
    public const int MAP_SIZE_X = 7000;
    public const int MAP_SIZE_Y = 2000;
    public const int DELTA = 100;
    private readonly IList<POI>[,] _array = new IList<POI>[MAP_SIZE_X/DELTA, MAP_SIZE_Y/DELTA];
    /// <summary>
    /// Store in an array that collects all POIs in a DELTA sized grid.
    /// </summary>
    private void Store(POI poi) {
        var x = poi.X / DELTA;
        var y = poi.Y / DELTA;
        var list = _array[x, y];
        if (list == null) {
            list = new List<POI>();
            _array[x, y] = list;
        }
        list.Add(poi);
    }
    /// <summary>
    /// Read the XML and store in an array
    /// </summary>
    public void ReadXml(string url) {
        var doc = XDocument.Load(url);
        foreach (var uriElement in doc.Root.Elements("URI")) {
            Store(new POI {
                X = int.Parse(uriElement.Element("CoordX").Value),
                Y = int.Parse(uriElement.Element("CoordY").Value),
                Value = uriElement.Element("Value").Value
            });
        }
    }
    /// <summary>
    /// Find all POIs that are within the square given by <c>from</c> and <c>to</c>
    /// </summary>
    public IEnumerable<POI> Find(Point from, Point to) {
        for (var xIndex = from.X / DELTA; xIndex <= to.X / DELTA; xIndex++) {
            for (var yIndex = from.Y / DELTA; yIndex <= to.Y / DELTA; yIndex++) {
                var list = _array[xIndex,yIndex];
                if (list == null) continue;
                foreach (var poi in list) {
                    if (from.X <= poi.X && poi.X <= to.X && from.Y <= poi.Y && poi.Y <= to.Y) {
                        yield return poi;
                    }
                }
            }
        }
    }
}