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The book

Using and Enhancing the Free Map of the World.
By Frederik Ramm,
Jochen Topf,
and Steve Chilton.


Karlsruhe Hack Weekend 18-19 February 2017

FOSSGIS 2017 in Passau, 22-25 March 2017


Polygon Poi Converter

Wheelmap and Wheelchair Tag

Wheelmap is a map for wheelchair accessible places. A "traffic light" system is used to flag places as either "suitable" (green), "limited" (orange), or "not suitable" (red). Everyone can participate and add these flags.

Wheelmap doesn't just use OSM as a base map; they also store the wheelchair suitability directly in OpenStreetMap. This created a few technical hurdles that we happily helped them solve.

The Problem: Data Duplication

More an more, points of interest are created as areas on OSM and not as points. This created problems for Wheelmap, whose simple map interface only used point information for POIs. It could happen that users were asked to add information about the wheelchair suitability of an object in OSM when indeed the underlying area object already carried that information.

The Solution: Pseudo Nodes

We helped solve that problem by building a special, self-updating OSM database mirror for Wheelmap that converts areas into "pseudo nodes" at the centre of an area. These nodes can then be processed normally in Wheelmap.

Technically, this is implemented using standard OSM tools (PostGIS database, and osm2pgsql), and a database trigger that updates the pseudo nodes table. A reduced osm2pgsql style file imports only the required data, keeping resource usage to a minimum. In regular intervals, an export job generates an OSC change file describing all the changes in pseudo nodes since the last export, to be processed by the core wheelmap database.