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Queries and Feature Collections

Geospatial applications typically work with subsets of features in a library, such as buildings in a town, or waterways in a particular region. These subsets are represented as feature collections, which are the result of queries.

All feature collections implement the Features interface, which provides methods to iterate the member features or constrain them further.

  • FeatureLibrary itself is a feature collection, representing all features in the library.

  • Feature collections are lightweight objects that merely described what should be returned; they don’t actually contain any objects and take up minimal space. In other words, query execution is lazy: Features are fetched only once they are needed, in response to iteration or a call to toList().

    You can assign queries to variables and pass them around, but be aware that once the underlying library has been closed, you must not call any of their methods (or iterate over them).

  • Feature collections can be ordered or unordered. Only the nodes of a way and the members of a relation are ordered; all other query results are returned in arbitrary order.

Feature collections behave like Java Collection classes, and hence implement size(), isEmpty(), contains() and toArray(), as well as the ability to iterate. Features also offers these methods:

  • toList() creates an ArrayList containing all features in the collection.

  • first() returns the first feature in the collection, or null if it is empty.

Bounding-box queries

A bounding box (or bbox) describes an axis-aligned rectangle. Bounding-box queries are the most common type of spatial queries.

This type of query returns all features whose bounding box intersects with the bounding box of the query. Note that the result set may include features whose geometry itself does not fall inside the query bbox. Bounding-box queries are designed as a fast primary filter, intended to narrow down candidates from millions to a few hundred. To eliminate the false positives, you can then apply a second, stricter (but more computationally expensive) filter.

  • A bounding box may straddle the Antimeridian (+/- 180 degrees longitude). Features that cross the Antimeridian are returned as multiple Feature objects representing separate parts to the east and to the west.

A bounding box is represented by the Box class, which offers multiple static factory methods. To create a Box from coordinates (longitude and latitude), use ofWSEN():

Box bbox = Box.ofWSEN(8.42, 53.75, 9.07, 53.98);   // West, South, East, North

To obtain the features in a given bbox, use in():

Features subset = features.in(bbox);

Instead of explicitly creating a bounding box, you can also use the bounds() of a Feature:

// All features that may be within 100 meters of the river
return features.in(river.bounds().bufferMeters(100));

Note: If you need to determine which features definitely lie within 100 meters, use maxMetersFrom().

Filtering by type and tags

Features in a collection can be filtered by type:

Features nodes()
Features ways()
Features relations()

There are also four methods that take a query string:

Features select(String query)
Features nodes(String query)
Features ways(String query)
Features relations(String query)

For example:

nodes("[emergency=fire_hydrant]") // nodes that represent fire hydrants
select("na[amenity=pub,cafe]")    // pubs and cafes (nodes and areas)
relations("[route=bicycle]")      // cycling routes

The next chapter covers the GeoDesk query language in detail.

  • Some queries always produce an empty collection. For example, nodes("a") is always empty: areas can be of type Way or Relation, but never Node.

Retrieving features

To process all features in a set, simple iterate:

for(Feature street : streets) ...

To obtain a List:

List<Feature> list = streets.toList();

To obtain the first feature in a set:

Feature city = france("n[place=city][name=Paris]").first();

Note that only the nodes of ways and members of relations are ordered collections; all others are unordered sets, which means you’ll receive a random feature if there are more than one. If the collection is empty, first() returns null.

Spatial filters

Features can be filtered by their spatial relationship to other geometric objects (typically a Geometry, PreparedGeometry or another Feature).

containing

Selects features whose geometry contains A:

  • Every point of A is a point of the candidate feature, and the interiors of the two geometries have at least one point in common.
Features containing(Feature)
Features containing(Geometry)
Features containing(PreparedGeometry)
Features containingXY(int, int)
Features containingLonLat(double, double)

For example:

// In which park (if any) is this statue of Claude Monet?
return features.select("a[leisure=park]")
    .containing(statueOfMonet).first();

// The county, state and country for this point -- should return 
// San Diego County, California, USA (in no particular order)  
return features.select("a[boundary=administrative][admin_level <= 6]")
    .containingLonLat(-117.25, lat=32.99);

coveredBy

Selects features whose geometry is covered by A:

  • No point of the candidate feature’s geometry lies outside of A.
Features coveredBy(Feature)
Features coveredBy(Geometry)
Features coveredBy(PreparedGeometry)

crossing

Selects features whose geometry crosses A:

  • The geometries of A and the candidate feature have some (but not all) interior points in common
  • The dimension of the intersection must be less than the maximum dimension of the candidate and A.
Features crossing(Feature)
Features crossing(Geometry)
Features crossing(PreparedGeometry)

For example:

// All railway bridges across the Mississippi River
Features railwayBridges =
    features.select("w[railway][bridge]");    
return railwayBridges.crossing(mississippi);

disjointFrom

Selects features whose geometry is disjoint from A:

  • The geometries of the candidate feature and A have no common points at all.
Features disjointFrom(Feature)
Features disjointFrom(Geometry)
Features disjointFrom(PreparedGeometry)

intersecting

Selects features whose geometry intersects A:

  • The geometries of A and the candidate feature have at least one point in common.
Features intersecting(Feature)
Features intersecting(Geometry)
Features intersecting(PreparedGeometry)

maxMetersFrom

Selects features whose distance to A is less or equal to m meters (measured between the closest points of the candidate feature and A).

Features maxMetersFrom(double, Feature)
Features maxMetersFrom(double, Geometry)
Features maxMetersFrom(double, PreparedGeometry)
Features maxMetersFromXY(double, int, int)
Features maxMetersFromLonLat(double, double, double)

For example:

// All bus stops within 500 meters of the given restaurant
Features nearbyBusStops = features.select("n[highway=bus_stop]")
    .maxMetersFrom(500, restaurant);
 
// All features within 3 km of the given point 
return features.maxMetersFromLonLat(3000, 76.41, 40.12);

overlapping

Selects features whose geometry overlaps A:

  • The geometries of A and the candidate feature have the same dimension.

  • A and candidate feature each have at least one point not shared by the other.

  • The intersection of their interiors has the same dimension.

Features overlapping(Feature)
Features overlapping(Geometry)
Features overlapping(PreparedGeometry)

touching

Selects features that touch A:

  • The geometries of A and the candidate feature have at least one point in common, but their interiors do not intersect.
Features touching(Feature)
Features touching(Geometry)
Features touching(PreparedGeometry)

For example:

Features counties = features.select(
    "a[boundary=administrative][admin_level=6]");
for (Feature county: counties)
{
    System.out.printf("%s has %d neighbors\n",
        county.stringValue("name"), 
        counties.touching(county).count());
}

within

Selects features that lie entirely within A:

  • Every point of the candidate feature is a point in A, and their interiors have at least one point in common.
Features within(Feature)
Features within(Geometry)
Features within(PreparedGeometry)

Topological filters

These methods return a subset of those features that have a specific topological relationship with another Feature.

connectedTo

Selects all features that have at least one node (vertex) in common with the given Feature or Geometry.

Features connectedTo(Feature)
Features connectedTo(Geometry)

nodesOf

The nodes of the given way. Returns an empty set if the feature is a node or relation.

Features nodesOf(Feature)

membersOf

Features that are members of the given relation, or nodes of the given way. Returns an empty set if the feature is a node.

Features membersOf(Feature)

parentsOf

Relations that have the given feature as a member, as well as ways to which the given node belongs.

Features parentsOf(Feature)