Every two weeks or-so, the London OpenStreetMap group comes together for a mapping party. The main idea is to go to an area of London to improve the map there. Sometimes that's adding streets, but as most of those are now completed, time is spend on buildings, address information and points-of-interest.

As an experiment I recorded a snapshot of the map for the area (http://mapcraft.nanodesu.ru/pie/73) every hour to follow the progress of people adding new and improved data to the map. I then rendered an image of each map snapshot and put those together to create a video:

You can see the video in HD on Vimeo.

How Did He Do That?

Well, first of all, I already have a script to keep a local version of the London XML map data (as london.osm) up to date. Each hour, this script:

• Read the old london.osm file and merge in changes through the OpenStreetMap replication service and write it as london-new.osm.

• Compare the old file and the new file and write out a changes file to changes/diff-<timestamp>.osc. This is not needed for this to work, but I am interested in this information as well.

• Move the new file to the old file.

• Import it into my local PostGreSQL/PostGIS instance.

In script form, that looks like:

#!/bin/bash

NOW=`date +%Y%m%d-%H%M`

export JAVACMD_OPTIONS="$JAVACMD_OPTIONS -Djava.net.preferIPv4Stack=true"
../osmosis-0.39/bin/osmosis --rri --simc --rx london.osm --ac --bb left=-0.563 right=0.28 top=51.68 bottom=51.26 --wx london-new.osm
../osmosis-0.39/bin/osmosis --rx london-new.osm --rx london.osm --derive-change --write-xml-change file="changes/diff-$NOW.osc"
mv london-new.osm london.osm
../try4/bin/osm2pgsql/osm2pgsql -S ../try4/bin/osm2pgsql/default.style --slim -d gis -C 2400 london.osm #-e 15 -o expiry.log

And from that I then can render maps of London quite easily:

cd /backup/osm/try4/bin/mapnik
./generate_tiles_london.py | grep -v " exists"

In order to make a snapshot of South Kensington, or at least, the area that we were going to map in, I added another task. I placed it just after the moving of the old file to the new file. This task cuts out the South Kensington area and puts it in a file south-ken/south-kensington-<timestamp>.osm:

../osmosis-0.39/bin/osmosis --rx london.osm --bb left=-0.18256 right=-0.16853 top=51.49426 bottom=51.48733 --wx south-ken/south-kensington-$NOW.osm

This generated about 100 files in the south-ken directory, where each XML file contained a snapshot of the map on each specific hour.

I then ran the following script to create a image of each of those map files:

#!/bin/bash

export MAPNIK_MAP_FILE=/backup/osm/try4/bin/mapnik/osm.xml
mkdir -p images
LAT1=-0.18256
LON1=51.48833
LAT2=-0.16853
LON2=51.49326

for i in south*osm; do
  /backup/osm/try4/bin/osm2pgsql/osm2pgsql -S /backup/osm/try4/bin/osm2pgsql/default.style --slim -d gis -C 2400 $i
  /backup/osm/try4/bin/mapnik/generate_image.py ${LAT1} ${LON1} ${LAT2} ${LON2}

  mv image.png images/${i}-0.png
  LAT1=`echo "${LAT1} + 0.000005" | bc -q`
  LON1=`echo "${LON1} + 0.00001" | bc -q`
  LAT2=`echo "${LAT2} - 0.00001" | bc -q`
  LON2=`echo "${LON2} - 0" | bc -q`

  /backup/osm/try4/bin/mapnik/generate_image.py ${LAT1} ${LON1} ${LAT2} ${LON2}
  mv image.png images/${i}-1.png
  LAT1=`echo "${LAT1} + 0.000005" | bc -q`
  LON1=`echo "${LON1} + 0.00001" | bc -q`
  LAT2=`echo "${LAT2} - 0.00001" | bc -q`
  LON2=`echo "${LON2} - 0" | bc -q`
done

I had to modify the generate_image.py script from the Mapnik distribution slightly to allow the bounding box to be passed in through the command line. I also set the image width and height to 1280 by 720:

...
ll = [];
for arg in sys.argv:
    ll.append(arg)
ll.pop(0);
for index, arg in enumerate(ll):
    ll[index] = float(arg)

z = 15
imgx = 1280
imgy = 720

m = mapnik.Map(imgx,imgy)
...

Once all the PNG files were created, I converted them to the JPEG format and ran mencoder to generate the AVI film:

for i in *png; do convert -quality 100 $i $i.jpg; done
mencoder "mf://*.jpg" -mf fps=10 -o test.avi -ovc lavc -lavcopts vcodec=msmpeg4v2:vbitrate=7200

And this video I then uploaded to Vimeo. Voilà! One OpenStreetMap mapping party progress animation!