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Overpass API

Performing queries

OSM data can be accessed using the [http://wiki.openstreetmap.org/wiki/Overpass_API](Overpass API). While the Overpass API is powerful, it cannot automatically reverse geocode place names. We thus use Nominatim again to query the area id of, for example, NYC:

from OSMPythonTools.nominatim import Nominatim
nominatim = Nominatim()
nyc = nominatim.query('NYC')

Overpass queries are simple enough to be written by hand, but we will demonstrate how to use the overpassQueryBuilder:

from OSMPythonTools.overpass import overpassQueryBuilder
query = overpassQueryBuilder(area=nyc.getAreaId(), elementType='node', selector='"highway"="bus_stop"', out='body')

The variable query is just a string containing the query:

'area(3600175905)->.searchArea;node(area.searchArea);node._["highway"="bus_stop"]; out body;'

We can now query an Overpass endpoint:

from OSMPythonTools.overpass import Overpass
overpass = Overpass()
busStops = overpass.query(query)

Please observe that the constructor of the class Overpass again accepts the parameters endpoint, cacheDir, and waitBetweenQueries. To query historical data, we can easily add a date:

overpass.query(query, date='2014-01-01T00:00:00Z')

Also a timeout can be set:

overpass.query(query, timeout=25)

Accessing the result of a query

The result of the overpass query is an object containing several functions to easily access the data. All elements returned by the query can be accessed:

busStops.elements()
# [<OSMPythonTools.element.Element object at 0x10963c9b0>, <OSMPythonTools.element.Element object at 0x10963c8d0>, ...

Each element is of the type OSMPythonTools.element.Element, and easy methods to access its properties exist.

Also only elements of a certain type can be accessed:

busStops.nodes()
busStops.ways()
busStops.relations()
busStops.areas()

In our case, the query only contains nodes because we explicitly queried only for nodes.

In many cases, the number of elements shall be counted. This can easily be achieved using the corresponding functions:

busStops.countElements()
# 542
busStops.countNodes()
# 542
busStops.countWays()
# 0
busStops.countRelations()
# 0
busStops.countAreas()
# 0

Overpass queries contain information about the verbosity of the result. If the verbosity is body, like in our example, all important information about the bus stops is returned. If count is used instead, only the number of elements is returned. The function busStops.elements() will return a list of elements in the first case, and None in the second case. The function busStops.countElements() will return the number of elements in both cases.

Not only the data itself but also meta data about the query can be accessed. We can, for example, test whether the query was valid:

busStops.isValid()
# True

Also the raw data can be accessed:

busStops.toJSON()

Also other meta data provided by the overpass endpoint can be accessed:

busStops.version()
busStops.generator()
busStops.timestamp_osm_base()
busStops.timestamp_area_base()
busStops.copyright()
busStops.remark()