[GIS] How to Extract data from OSM within a polygon

openstreetmaposmosisoverpass-api

Although I am a Openstreetmap's regular user for quite some time, I am pretty new to GIS, so I assume this is an easy question: I would like to know how can I extract data (eg: all schools) from OSM within a certain area defined by a polygon (eg: a city boundary or a city's neighbourhood) using QGIS.

Extracting data

I have searched a bit about it and I found out (correct me if I am wrong) that there are different ways to perform queries on OSM, although I don't understand how/when do I have to use each one:

Using polygons

The problem is that as far (or as little) as I know all these methods rely on a square area, and I would like to know how can I do that within two types of polygons:

A city's boundary (this polygon exists in OSM, since is the one you see when searching for an specific city)
An arbitrary area, which would require to draw a new polygon using an external software (possibly QGIS? JOSM?).

Best Answer

Overpass API allows you to extract by polygon:

http://wiki.openstreetmap.org/wiki/Overpass_API/Language_Guide#Select_Region_by_Polygon

which should fill your needs.

You can use QGIS to draw the desired polygon, then extract the coordinates with the MMQGIS plugin.

Related Solutions

OpenStreetMap Data – Processing Raw OSM Data for OpenStreetMap.org

There are a few different angles to this, and since it's unclear how you're processing data initially, I guess I'll just give an overview.

There are two main ways to consume OSM data - by using osm2pgsql, an older utility that supports 'stylesheets' and differential updates, and Imposm, a newer, Python-based system that supports Python-based stylesheet transforms. When people do processing, a lot of it is in that kind of script. For instance, here's an imposm mapping for osm-bright, the stylesheet upon which MapBox Streets (disclosure/employee) is based.

To be more specific to what you're encountering, it's likely that you aren't properly processing osm relations properly, which, in the data model are what allow multiple linestrings to form polygons. Tools like Imposm and osm2pgsql generally handle this kind of data transformation for you.

As far as how OSM.org itself does things: edits are in a 'semantic' Postgres database, and continuously imported into a PostGIS database with osmosis, and rendered with Mapnik. There's no manual cleanup step between the database and map rendering, since the two are highly coupled and the map aims to be up-to-date.

Overpass API – Get Coordinates of Postal Boundary

I would recommend the following approach: it returns coordinates for ways' nodes 'in place', like you described in your question.

The only thing, which is missing in the output is the technical OSM node-id (ref). I assume that for use case of drawing some boundary area on a map, this is most likely not needed. If you take a look at the screenshot below, overpass turbo is drawing all postal code areas on the map without any issue.

area(3600062649);
rel(area)["boundary"="postal_code"];
out geom;

http://overpass-turbo.eu/s/7In

enter image description here

Example output:

  <relation id="1301656">
    <bounds minlat="51.3145882" minlon="12.2366519" maxlat="51.3391521" maxlon="12.3034696"/>
    <member type="way" ref="87537618" role="outer">
      <nd lat="51.3179538" lon="12.2426125"/>
      <nd lat="51.3178723" lon="12.2414464"/>
      <nd lat="51.3176008" lon="12.2399198"/>
    [...]
    </member>
    [...]
    <tag k="boundary" v="postal_code"/>
    <tag k="note" v="04205 Leipzig"/>
    <tag k="postal_code" v="04205"/>
    <tag k="postal_code_level" v="8"/>
    <tag k="type" v="boundary"/>
  </relation>