Spatial Search

To support the ability to retrieve the data based on spatial coordinates, the spatial search have been introduced.

Creating Indexes

To take an advantage of spatial search, first we need to create an index with spatial field. To mark field as a spatial field, we need to use SpatialGenerate method

object SpatialGenerate(double lat, double lng);

object SpatialGenerate(string fieldName, double lat, double lng);

object SpatialGenerate(string fieldName, string shapeWKT);

object SpatialGenerate(string fieldName, string shapeWKT, SpatialSearchStrategy strategy);

object SpatialGenerate(string fieldName, string shapeWKT, SpatialSearchStrategy strategy, int maxTreeLevel);

public enum SpatialSearchStrategy
{
	GeohashPrefixTree,
	QuadPrefixTree,
}

where:

• fieldName is a name of the field containing the shape to use for filtering (if the overload with no fieldName is used, then the name is set to default value: __spatial)
• lat/lng are latitude/longitude coordinates
• shapeWKT is a shape in WKT format
• strategy is a spatial search strategy (default: GeohashPrefixTree)
• maxTreeLevel is a integer that indicates the maximum number of levels to be used in PrefixTree and controls the precision of shape representation (9 for GeohashPrefixTree and 23 for QuadPrefixTree)

In our example we will use Event class and very simple index defined below.

public class Event
{
	public string Id { get; set; }

	public string Name { get; set; }

	public double Latitude { get; set; }

	public double Longitude { get; set; }
}

public class Events_SpatialIndex : AbstractIndexCreationTask<Event>
{
	public Events_SpatialIndex()
	{
		this.Map = events => from e in events
							 select new
								 {
									 Name = e.Name,
									 __ = SpatialGenerate("Coordinates", e.Latitude, e.Longitude)
								 };
	}
}

Spatial search strategies

1. GeohashPrefixTree

Geohash is a latitude/longitude representation system that describes earth as a grid with 32 cells, assigning to each grid cell an alphanumeric character. Each grid cell is divided further into 32 smaller chunks and each chunk has also an alphanumeric character assigned and so on.

E.g. The location of a 'New York' in United States is represented by following geohash: DR5REGY6R and it represents the 40.7144 -74.0060 coordinates. Removing characters from the end of geohash will decrease the precision level.

More information about geohash uses, decoding algorithm and limitations can be found here.

1. QuadPrefixTree

QuadTree represents earth as a grid with exactly four cells and similarly to geohash, each grid cell (sometimes called bucket) has a letter assigned and is divided further into 4 more cells and so on.

More information about QuadTree can be found here.

Precision

By default the precision level (maxTreeLevel) for GeohashPrefixTree is set to 9 and for QuadPrefixTree the value is 23, which means that the coordinates are represented by a 9 or 23 character string. The difference exists, because the QuadTree representation would be much less precise if the level would be the same.

1. Geohash precision values (from unterbahn.com).

1. Quadtree precision values

Radius search

The most basic usage and probably most common one is to search for all points or shapes within provided distance from the given center point. To perform this search we will use WithinRadiusOf method that is a part of query customizations.

session.Query<Event, Events_SpatialIndex>()
       .Customize(x => x.WithinRadiusOf(
	       fieldName: "Coordinates",
	       radius: 10,
	       latitude: 32.1234,
	       longitude: 23.4321))
       .ToList();

The method can be used also when using LuceneQuery.

session.Advanced.LuceneQuery<Event, Events_SpatialIndex>()
	   .WithinRadiusOf(fieldName: "Coordinates", radius: 10, latitude: 32.1234, longitude: 23.4321)
	   .ToList();

Advanced search

The WithinRadiusOf method is a wrapper around RelatesToShape method.

IDocumentQueryCustomization RelatesToShape(string fieldName, string shapeWKT, SpatialRelation rel);

public enum SpatialRelation
{
	Within,
	Contains,
	Disjoint,
	Intersects,

	/// <summary>
	/// Does not filter the query, merely sort by the distance
	/// </summary>
	Nearby
}

where first parameter is a name of the field containing the shape to use for filtering, next one is a shape in WKT format and the last one is a spatial relation type.

So to perform a radius search from the above example and use RelatesToShape method, we do as follows

session.Query<Event, Events_SpatialIndex>()
		.Customize(x => x.RelatesToShape("Coordinates", "Circle(32.1234, 23.4321, d=10.0000)", SpatialRelation.Within))
		.ToList();

or when we want to use LuceneQuery then

session.Advanced.LuceneQuery<Event, Events_SpatialIndex>()
	   .RelatesToShape("Coordinates", "Circle(32.1234, 23.4321, d=10.0000)", SpatialRelation.Within)
	   .ToList();