10. Osm4j

Es una librería java para acceder y manipular la infromación procedente de OpenStreetMap. El código fuente de la librería se puede consultar en :

https://github.com/topobyte/osm4j-core

La documentación del API se encuentra en:

http://www.topobyte.de/projects/osm4j/

10.1. Data model

10.2. OsmEntity

• getId()
• getMetadata()
• getNumberOfTags()
• getTag()

10.3. OsmNode

• getLatitude()
• getLongitude()

10.4. OsmWay

• getNumberOfNodes()
• getNodeId()

10.5. OsmRelation

• getNumberOfMembers()
• getMember()

10.6. Data sets

An OSM dataset is basically a long list of nodes, ways and relations that is encoded using one of the basic storage file formats. osm4j provides access to data encoded in the most important of these data formats by providing OsmReader and OsmIterator implementations for them.

All basic data formats have in common, that they store their data in a specific order:

They contain a sequence of nodes, followed by a sequence of ways, followed by a sequence of relations. Each sequence contains its elements ordered by the objects’ ids in ascending order. Thus, when processing an OpenStreetMap dataset, you will encounter the contained data in exactly this order. Also, when writing a dataset to some output using an OsmOutputStream, it is important to feed the elements to the stream in the correct order.

It is important to understand that ways and relations reference other objects using their ids. They do not contain the data of referenced objects themselves. Hence, to work with a way or relation it is usually necessary to resolve those references and find the actual objects they reference.

For example, a way is just a sequence of node ids. To interpret the geometry of the way, you have to assemble a sequence of coordinates from the references by finding the referenced nodes by their id.

10.7. Hello world

El ejemplo que sigue descarga un dataset de ejemplo que contiene los datos correspondientes a la Relation 1005914que corresponde con la Facultad de Letras de la Universidad de Girona. Los datos están almacenados en un fichero en formato .osm.

public static void main(String[] args)
    throws MalformedURLException, IOException,
        ParserConfigurationException, SAXException {
  // Define a query to retrieve some data
  String query = "http://mercatorlab.com/geoinquietos/girona1.osm";

  // Open a stream
  InputStream input = new URL(query).openStream();

  // Create an iterator for XML data
  OsmIterator iterator = new OsmXmlIterator(input, false);

  // Iterate all elements
  for (EntityContainer container : iterator) {

    // Check if the element is a Relation
    if (container.getType() == EntityType.Relation) {

      // Cast the entity to OsmRelation
      OsmRelation rel = (OsmRelation) container.getEntity();

      // Check if the Relation is the 1005914
      if(rel.getId()==1005914l) {

        // Print basic information
        System.out.println("id: " + rel.getId());
        int numberOfTags = rel.getNumberOfTags();
        System.out.println("number of tags: "
            + rel.getNumberOfTags());
        System.out.println("tags:");
        for (int i = 0; i < numberOfTags; i++) {
          OsmTag tag = rel.getTag(i);
          System.out.println(tag.getKey() + " = "
              + tag.getValue());
        }
        int numberOfMembers = rel.getNumberOfMembers();
        System.out.println("number of members: " +
            rel.getNumberOfMembers());
        for(int i=0; i<numberOfMembers;i++) {
          OsmRelationMember member = rel.getMember(i);
          System.out.println(member.getId() + " " +
              member.getType() + " " + member.getRole());
        }
      }
    }
  }
}