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Element

Several services, amongst them the OSMPythonTools.api and OSMPythonTools.overpass return OSM objects, which are represented by an instance of the class Element, which provides functions to access its properties.

Accessing general properties

Consider the following example of a query to the OSM Api:

from OSMPythonTools.api import Api
api = Api()
busStop = api.query('node/42467507')

The result, that is, busStop, is an element, whose properties can easily be access as follows:

busStop.id()
# 42467507
busStop.type()
# 'node'
busStop.lat()
# 40.7014417
busStop.lon()
# -73.9430797

Accessing tags

The tags can be accessed as a python dictionary:

busStop.tags()
# {'asset_ref': '2030', 'highway': 'bus_stop', 'location': 'Broadway / Thornton Street', 'name': 'Broadway / Thornton Street', 'route_ref': 'B46;B46-LTD'}

The value of a single tag can be accessed like follows:

busStop.tag('highway')
# 'bus_stop'

Accessing nodes and members

Ways incorporate nodes, and relations other elements, called members. Nodes of a way can easily be accessed:

way = api.query('way/108402486')
way.nodes()
# [<OSMPythonTools.api.ApiResult object at 0x104c06940>, <OSMPythonTools.api.ApiResult object at 0x104c2d8d0>, ...
way.nodes()[0].id()
# 1243967857

The members of a relation can also easily be accessed:

relation = api.query('relation/1539714')
relation.members()
# [<OSMPythonTools.api.ApiResult object at 0x104c20240>, <OSMPythonTools.api.ApiResult object at 0x104c2db38>, ...
relation.members()[0].id()
# 108402486

It should be noted that the nodes and members accessed this way are shallow by default. That is, the objects representing these elements (nodes and members) have only preloaded the type and the ID of the element. If more properties of the elements are accessed, the corresponding data is automatically downloaded and cached. Accordingly, the command

way.nodes()[0].id()

does not download the data for the nodes but rather just outputs the ID. If the latitude of the first node shall be accessed, this can be done as follows:

way.nodes()[0].lat()

As the data is downloaded only when needed and as the elements (in this case the nodes) are shallow by default, the data is only downloaded for the first node. In rare cases, the data should be downloaded for elements. In this case, the parameter shallow=False needs to be added:

way.nodes(shallow=False)

Members of a relation are shallow by default as well, and the same parameter shallow=False can be used.

Accessing the geometry

In order to receive geometry information, the argument includeGeometry=True needs to be provided to the overpassQueryBuilder in order to let him generate a query that downloads the geometry data. As an example, one can query for all waterbodies in Vienna:

from OSMPythonTools.nominatim import Nominatim
from OSMPythonTools.overpass import overpassQueryBuilder, Overpass

vienna = Nominatim().query('Vienna, Austria')
query = overpassQueryBuilder(area=vienna.areaId(), elementType=['way', 'relation'], selector='"natural"="water"', includeGeometry=True)
result = Overpass().query(query)

The geometry of the downloaded elements can be obtained in the GeoJSON format:

result.elements()[0].geometry()
# {"coordinates": [[[16.498671, 48.27628], [16.4991, 48.276345], ... ]], "type": "Polygon"}

Some relations contain invalid geometries. If this is the case, an exception is returned.

In order to receive the center of a way or relation, the argument includeCenter=True needs to be provided to the overpass QueryBuilder:

zandvoort = nominatim.query('Zandvoort')
query = overpassQueryBuilder(area=zandvoort, elementType='way', selector='"highway"', includeCenter=True)
result = overpass.query(query)

Then, the center can be obtained as follows:

result.elements()[0].centerLat()
# 52.3657055
result.elements()[0].centerLon()
# 4.5598827

Accessing the history and data about the creation

If explicitly queried, the resulting elements contain information about the history. As an example, one can query for the changes made by the user franz-benjamin in Heidelberg in March 2017:

from OSMPythonTools.nominatim import Nominatim
from OSMPythonTools.overpass import overpassQueryBuilder, Overpass

heidelberg = Nominatim().query('Heidelberg, Germany')
query = overpassQueryBuilder(area=heidelberg.areaId(), elementType='node', since='2017-01-01T00:00:00Z', to='2017-02-01T00:00:00Z', user='franz-benjamin', out='meta')
changedNodes = Overpass().query(query)

It is important to use out='meta' because this will fetch the meta data about the history and the creation of the corresponding elements:

changedNodes.elements()[0].version()
# 8
changedNodes.elements()[0].changeset()
# 44871772
changedNodes.elements()[0].user()
# 'franz-benjamin'
changedNodes.elements()[0].userid()
# 4913117
changedNodes.elements()[0].timestamp()
# 2017-01-03 13:47:23+00:00'