31 August 2017

Map matching 101 - part 1

Map matching is an interesting problem which is used in car navigation, self-driving cars, traffic analysis, map correction and more. Let’s write a simple map matching algorithm in Julia.

Map matching

Map matching is the process of estimating actual trajectory traveled by a vehicle/bicycle on a road network (map) from certain observations. These observations could be:

  • track points of Global Navigation Satellite System (e.g. GPS, GLONASS, Galileo)
  • sensor data (e.g. LiDAR, gyroscope, accelerometer)
  • known locations of mobile network cell sites
  • even Wi-Fi access points

All these observations contain measurement/hardware errors. A digital road network (map) does not match real world either. Map matching algorithms suppose to account for such inaccuracies and estimate actual locations on the road network.

Julia

It can be done in almost any programming language. But I decided to go with Julia as it is a fairly new programming language made specifically for high-level and high-performance numerical computing. Julia’s backend is LLVM and it supports Lisp-like macros. I think it is worth trying Julia as an alternative to expensive and buggy (especially on Linux) MATLAB.

Map data

To map match input observations a digital map data must be available. OpenStreetMap can expose all its map data in a single file. There are also extracts which contain OpenStreetMap data for individual countries, cities and areas of interest. See Planet.osm for more information. To extract your own region you can use bbbike.org.

OpenStreetMap mainly uses two of the following formats:

  • OSM XML is a human readable format
  • PBF is a binary format design for faster access and smaller size

I’m going to use Amsterdam’s extract in OSM XML format. The extract contains data primitives: nodes, ways and relations.

Node

It is represented by identifier, latitude and longitude. It can represent some standalone features or shape of continuous ways.

Way

A way is an ordered list of nodes to represent linear features like rivers, roads, boundaries (“closed way”).

Relation

A relation represents a relationship between two or more elements.

Example:

        <node id="16568166" lat="52.3776574" lon="4.8988379" version="1">
                <tag k="ref" v="29-V"/>
                <tag k="railway" v="switch"/>
        </node>
        <node id="16568167" lat="52.3777968" lon="4.899034" version="1"/>
        <node id="25596477" lat="52.367" lon="4.9060967" version="1"/>

        <way id="7045991" version="1">
                <nd ref="46385578"/>
                <nd ref="252129511"/>
                <nd ref="330036893"/>
                <nd ref="46384590"/>
                <tag k="ref" v="s116"/>
                <tag k="name" v="Kattenburgerplein"/>
                <tag k="lanes" v="1"/>
                <tag k="oneway" v="yes"/>
                <tag k="bicycle" v="use_sidepath"/>
                <tag k="highway" v="secondary"/>
                <tag k="surface" v="asphalt"/>
                <tag k="maxspeed" v="50"/>
        </way>

        <relation id="2816" version="1">
                <member type="way" ref="382853413" role=""/>
                <member type="way" ref="504938884" role=""/>
                <tag k="ref" v="LF2"/>
                <tag k="name" v="Stedenroute"/>
                <tag k="type" v="route"/>
                <tag k="route" v="bicycle"/>
                <tag k="remark" v="relation contains two alternative routes"/>
                <tag k="network" v="ncn"/>
        </relation>

Observations (GNSS)

OpenStreetMap also provides access to huge database with GNSS traces. These traces can be used to test map matching algorithms.

The format used is GPX or GPS Exchange Format.

Example:

<?xml version='1.0' encoding='utf-8'?>
<gpx xmlns="http://www.topografix.com/GPX/1/0" version="1.0" creator="OSM gpx_dump.py">
  <trk>
    <name>Track 0</name>
    <number>0</number>
    <trkseg>
      <trkpt lat="52.0709250" lon="5.1255720">
        <ele>11.19</ele>
      </trkpt>
      <trkpt lat="52.0729630" lon="5.1228470">
        <ele>-1.79</ele>
      </trkpt>

To be continued

In the next posts I’m going to write a program which loads map data, plots a GPS trace and start with map matching.