# Feature Subtypes When we talk about **types** of features, there are actually three distinct concepts: - The **object type**: The GeoDesk API type (derived from [`Feature`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Feature.html)) that corresponds to the three basic OSM datatypes --- [`Node`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Node.html), [`Way`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Way.html) and [`Relation`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Relation.html) - The **query type**: The letter code used in [GOQL](goql) queries, which also includes areas - The **geometry type**: The six JTS [`Geometry`](https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/Geometry.html) subtypes created by the [`toGeometry()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Feature.html#toGeometry()) method Note that neither OSM nor the GeoDesk API have an "area" type --- an area is represented by a `Way` (for simple polygons without holes) or a `Relation` (which can define more complex polygonal geometries). This chart illustrates the three type concepts:In this chapter, we'll look at the characteristics and additional methods of the `Node`, `Way` and `Relation` subtypes. ## `Node` {#node} A feature represented by a single coordinate pair. A [`Node`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Node.html) can be stand-alone, or form part of a `Way`. If the latter, [`belongsToWay()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Node.html#belongsToWay()) returns `true`. [`parentWays()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Node.html#parentWays()) returns all ways to which this node belongs (or an empty collection for a stand-alone node). An optional query string can be passed: ```java node.parentWays("w[waterway=river,stream]") // only returns rivers and streams ``` Sometimes it is more convenient to inverse a query using [`with()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Features.html#with(com.geodesk.feature.Feature)): ~~0.2~~ ```java library.ways("w[waterway=river,stream]").with(node) // same as above ``` ### Anonymous nodes {#anonymous-nodes} An **anonymous node** has no tags and does not belong to any relations --- it merely serves to define the geometry of a `Way`. By default, feature libraries omit the IDs of such nodes to save space, in which case [`id()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Feature.html#id()) returns `0`. ## `Way` {#way} A linear or simple polygonal geometry, represented by two or more nodes. Linear rings and polygons have a minimum of four nodes (their first and last node are the same). [`nodes()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Way.html#nodes()) returns an ordered collection of a way's nodes. An optional query string can be passed: ```java way.nodes("[traffic_calming]") // only speed bumps etc. ``` Sometimes it is more convenient to inverse a query using [`of()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Features.html#of(com.geodesk.feature.Feature)): ~~0.2~~ ```java library.nodes("[traffic_calming]").of(way) // same as above ``` A `Way` is iterable: ```java for(Node node: way) ... ``` - Iteration only retrieves nodes that have tags or are part of a relation; to query *all* nodes, use `nodes()`. [`toXY()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Way.html#toXY()) returns an `int` array with the way's coordinates (X coordinates are stored at even index positions, Y at odd), which is the most space-efficient representation of the Way's geometry. ## `Relation` {#relation} A `Relation` is used to tie together multiple features to build a larger singular feature (such as a [long river](https://wiki.openstreetmap.org/wiki/Relation:waterway) or a [complex polygon](https://wiki.openstreetmap.org/wiki/Relation:multipolygon)) or to create a new conceptual feature, such as a [bus route](https://wiki.openstreetmap.org/wiki/Relation:route) or [turn restriction](https://wiki.openstreetmap.org/wiki/Relation:restriction). [`members()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Relation.html#members()) returns an ordered collection of a relation's members. There are several related methods that filter members based on their object type, also with an optional query argument: ```java rel.members() // all members rel.memberNodes() // only nodes rel.memberWays("[highway]") // only roads, paths, etc. rel.memberRelations() // only members that themselves are relations rel.members("a") // only areas rel.members("a[leisure=park]") // only park areas ``` Note that there is no "memberAreas" method, since areas can be `Way` or `Relation` objects. If you want only polygonal ways or area relations, use `memberWays("a")` / `memberRelations("a")`. Member queries can also be phrased using [`of()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Features.html#of(com.geodesk.feature.Feature)): ~~0.2~~ ```java library.ways().of(rel) // ways that are members of the given relation ``` To restrict members to specific roles, use `role` in the query as if it were a tag: ~~0.2~~ ```java rel.memberWays("w[waterway=canal][role='*_stream']") // canals whose role ends with "_stream" ("main_stream", "side_stream") ``` [`first()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Features.html#first()) is useful if you expect at most one member with a given role: ```java Node capital = rel.memberNodes("[role=admin_centre]").first(); ``` A `Relation` is iterable: ```java for(Feature member: rel) ... // is equivalent to for(Feature member: rel.members()) ... ``` - A feature may appear among a relation's members more than once (for example, a bus route may pass a road segment multiple times). Usually, each occurrence will have a different role (e.g. `forward` and `backward`), but there is no rule requiring this. [`toGeometry()`](https://apidocs.geodesk.com/0.2/com/geodesk/feature/Feature.html#toGeometry()) creates a [`Polygon`](https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/Polygon.html) or [`MultiPolygon`](https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/MultiPolygon.html) for area relations, and a [`GeometryCollection`](https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/GeometryCollection.html) (comprised of the geometries of its members) for all others. The bounding box of a `Relation` is the union of the bounding boxes of its members.

---