I made a bus map.

Not just an image, but a dynamic “slippy map” rendered at several zoom levels.

This was my “hack” for the RewiredState, National Hack the Government Day”, a gathering of hackers who build something in a day, with the aim of tackling government/society problems, working with government data. Projects are presented at the end of the day.

I created the bus map by “rendering” OpenStreetMap data. By this I mean starting from raw map data, the underlying vector data, the coordinates and connections of every road etc, and creating raster images arranged in tiles at several zoom levels, for the map display linked above.

Clearly within the graphics routines this bright red routes data is drawn in a particular order, and in a sense it is laid on top of the map, but it’s important to realise the routes are baked into the raster images. This isn’t a javascript overlay. Rendering data in this way has advantages and disadvantages over javascript overlays. An advantage is that we can show all the bus routes of London when zoomed out, without crashing the browser. Other advantages are in the subtleties of how you can make the map look. I haven’t really demonstrated this very well here yet, but I hope to make some improvements. If you look closely some tube station labels are drawn over the routes, which at least shows it’s not an overlay. There are other (better) OpenStreetMap rendering examples elsewhere

Getting the data

You can download raw OpenStreetMap vector data for the entire planet. In fact I took an england extract in PBF format offered up by geofabrik. From this I just chopped out the London area (this bounding box) using the tool “osmosis” with this command:

wget http://download.geofabrik.de/osm/europe/great_britain/england.osm.pbf

$OSMOSIS_HOME/bin/osmosis \
   --read-pbf ./england.osm.pbf \
   --bounding-box left=-0.543 right=0.337 top=51.719 bottom=51.253 idTrackerType=BitSet \
   --write-xml ./london.osm

Once I had london.osm, it was time to put this through the Mapnik rendering tool. Very broadly the steps were:

• Install Mapnik. In fact this is several steps, but happily I already had it installed. The ‘Build your own OpenStreetMap Server’ tutorial by Richard Weait was useful for this
• Load the data into postGIS with a command such as ./osm2pgsql -S default.style --slim -d gis ./london.osm
• Edited ‘generate_image.py’ to render a single image in central london as a test
• Edit ‘generate_tiles.py’ to specify the london bounding box and zoom levels. Initially I’ve rendered zoom levels 7 to 15.
• Run ‘generate_tiles.py’ to create the tileset (png files in a directory structure)
• Made a HTML file with the javascript to launch OpenLayers fullscreen pointing at these tiles (do ‘view source’ on the site to see the javascript for this)
• Copied the stylesheet file ‘inc/layer-ferry-routes.xml.inc’ to make a new style layer for bus route relations (thanks to SK53 for suggesting this)
• Reference the new layer in osm.xml, and ‘inc/layers.xml.inc’
• Test using ‘generate_image.py’ before re-running ‘generate_tiles.py’ to re-render the tiles.
• Added more rules for varying thicknesses and a TextSymbolizer for showing route labels

My busmap stylesheet so far: style.tar.gz

öpnvkarte

Among the list of rendering examples linked above, is the one called “öpnvkarte.de” which shows bus routes throughout europe. In fact this is one of the best examples of customised OpenStreetMap rendering. Really nice colour and style choices with subdued colours for the background features enhancing the bold transport lines very clearly. So bus route rendering has certainly already been done. What’s the point in my map then?

öpnvkarte is very german. It’s named after a german transport network. The map is quite literally centred on Germany, and you’ll have noticed it has an un-typeable german character in it’s URL (although Shaun has provided openbusmap.org as a more sensibly named proxy). Petty irritations perhaps. So what else?

As far as I’m aware öpnvkarte hasn’t been updated in a while, so by re-rendering today’s openstreetmap data, we can see how the community has progressed with adding more routes. I was slightly disappointed to find that there wasn’t much difference in London actually. Just one or two details. Compare here with here. But OpenStreetMap people tend to be motivated a lot by renderings and other uses of the data they’re gathering. If the bus map stops being updated, then people stop adding bus routes.

There are other things which are less than perfect about öpnvkarte. It’s not a worldwide rendered tileset. It went worldwide for a few weeks but the server didn’t manage to cope. (I know my map is obviously not competing very well. I just rendered tiles for London!)

Wordwide tile rendering and re-rendering of updates, are tricky technical challenges. Obviously the developer of öpnvkarte is under no obligation to do these things, it’s just a shame that he doesn’t. More of a shame is that he doesn’t (as far as I’m aware) share his stylesheet config files. Again he’s not obliged to do so, and given the amount of work he must have put into it, perhaps it’s understandable. But sharing stylesheets would allow people with hardware and tuning know-how to have a go at tackling these other hosting challenges.

So those are a few weak justifications for starting all over again and attempting to build a different bus map stylesheet. My mapnik config files are hopelessly rudimentary so far, but it’s a start. Mostly though, this is just another baby step in my slow climb up the Mapnik learning curve. I wrote a diary entries back in January about this, but until this weekend I hadn’t tried anything further.

RewiredState
It was good to get up and present this rendering idea at rewiredstate, to a bunch of hackers and data mashers who I think would find this sort of thing interesting to play with themselves. I know a lot of them will reach for the boring old google maps toolsets for their map mashing work, but hopefully I gave a hint at some of the raw power and rendering fun offered by OpenStreetMap. Perhaps I should have made this important point again though: You can just use the familiar google maps javascript API on top of OpenStreetMap basemaps!.

The important point I did remember to make in my 2 minute slot: Next weekend we’re having a London OpenStreetMap hack weekend. I must try to prepare this time!

Thanks to the hard working RewiredState organisers and their sponsors (wired uk, tso, dxw) for a great day, and for all the free penguin biscuits and beers in the pub afterwards.

UPDATE: New transport map from Andy Allan. My friend Andy is creator of OpenCycleMap and all round Mapnikmeister. Unveiled today (14th April), his transport map is auto-updated and rendered worldwide (how are your local buses looking?) with beautiful pale shaded background cartographic choices. I think we can safely say I’ve been outdone on this one 🙂

(photo Copyright All rights reserved by SINFOGEO)A few weeks ago we had a big OpenStreetMap conference, the annual “State Of The Map”. This year it was in sun-baked Girona. “State Of The Map 2010 felt like a coming together of all my OpenStreetMap friends from around the world” Read my more detailed write up over on my user diary. There was a diverse variety of talk topics, from OpenStreetMap technical nitty-gritty, to big business GIS uses, to worldwide travels and humanitarian mapping. The full list of sessions is here. My talk this year was part of the “community track”.

Party Time! Good and bad ways to run OpenStreetMap mapping parties

UPDATE: Video of the talk is now available

I seem to have become the chief event organiser for OpenStreetMap in London, not because people massively admire the way I do it, but because it’s actually fair amount of effort, and I’m the one who got suckered into doing it. So this talk was mostly about my experiences of running these regular “mapping party” events, which we do every two weeks as a “mapping party marathon” (Sounds interesting? Check out when the next one is!)As I stressed in my talk, my approach is not ideal. There’s lots of things I don’t get around to doing. In particular I haven’t really tried to drum up press coverage of events. For one reason or another we never have very much luck at attracting new poeple. Although we try to be welcoming to new faces, it mostly ends up being a fun social meet-up where people know eachother. In the questions I was asked “what are my ideas for attracting new people?” to which I gave a hand-waivey response, but the truth is I don’t really know.

So I thought it counterpoised my talk quite nicely when Thea Clay got up and talked about outreach to non-technical communities and spoke about some of the successes she has had reaching out to local community groups and attracting new people to mapping parties in the U.S. This seems like a direction we need to go in, and I hope to find time to try this more in London. But then again…

Ultimately I think the most important message she had, was that OpenStreetMap is too difficult for non-techy people. They get the idea of creating open maps. They enjoy the outdoor aspect of data gathering, and they will have a lot of fun when you first take them out to do this, but the minute they sit down and take a look at our map editing tools “you can see the look on their faces change”.

Perhaps then the most important role of OpenStreetMap events, is to foster the existing OpenStreetMap community in an area. I would even go as far as saying that most outreach work at the moment should target boring old techy people and developers.

This is not the end game. We absolutely want to attract young people and old people, mothers and grandmothers, but getting techy people excited about OpenStreetMap is easier and actually far more effective than anything else at this stage. They’ll learn the tools quicker. But more importantly, these are the people who will go out and embed OpenStreetMap in their websites, taking it to new audiences, or if we’re really lucky they’ll go on to help solve the technical problems of OpenStreetMap and make the next useabilitiy break-throughs.

Obviously any publicity is good publicity. Any mapping party is a good mapping party, but I’m trying to consider where effort is best spent. Are we getting ahead of ourselves to try to engage with local community groups? Maybe I’m just working the geek angle, because that’s what I know best. As I said in my talk, press is important, and press (or PR?) is something OpenStreetMap needs to get better at.

Ruby code for converting to UK Ordnance Survey coordinate systems from WGS84? It’s one of those things I’d assumed would be a five minute search -> cut-n-paste job. But no. Well now you can cut & paste from here.

Using Proj4 and the proj4rb bindings

You can do all manner of coordinate projecting using Proj, and this is available within ruby code via proj4rb. To get set-up, install proj. On ubuntu you can do:

sudo apt-get install proj

download the proj4rb gem file (the ruby bindings), and install the gem:

sudo gem install proj4rb-0.3.1.gem

To use in a plain ruby script:

require 'rubygems'

require 'proj4'

To use in a rails app, add a line to your environment.rb file

config.gem "proj4rb", :lib => "proj4"

Probably all obvious to any pro-rubyist, but I got stuck at various stages of that. Anyway once you’re set up…

This code will do a conversion from WGS84 lat/lon to eastings and northings:

lon = -0.10322
lat = 51.52237

srcPoint = Proj4::Point.new(Math::PI * lon.to_f / 180,
                            Math::PI * lat.to_f / 180)

srcPrj  = Proj4::Projection.new("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")
destPrj = Proj4::Projection.new("+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 "+
                                      "+ellps=airy +datum=OSGB36 +units=m +no_defs")

point = srcPrj.transform(destPrj, srcPoint)

puts "http://www.openstreetmap.org/?mlat=" + lat.to_s + "&mlon=" + lon.to_s + "&zoom=16"
puts "Converts to:";
puts "http://streetmap.co.uk/grid/" + point.x.round.to_s + "_" + point.y.round.to_s + "_106"

Originally I was trying just the ‘destPrj’ string, and calling the ‘forward’ method, but this seemed to be skipping the datum conversion, resulting in everything being 100m out. It seemed to be necessary to use the ‘srcPrj’ string and the ‘transform’ method, to get the datum conversion happening.

You can look up the mystical proj strings on spatialreference.org

(UPDATE)  If you’re interested in converting in the other direction, using the proj4 gem, see Peter Hicks blog: Converting OSGB36 (Eastings/Northings) to WGS84 (Longitude/Latitude) in Ruby

A pure ruby implementation

Like many naive developers who have gone before me, I started out thinking that these coordinate transformations should be acheivable with a few simple formulea, and was expecting to be able to copy & paste a block of ruby code from somewhere to do it without installing any gems.

I didn’t find any such code, but frustration with proj problems (before I got that working) led me to create a pure ruby solution, by porting this pure javascript code by Chris Veness into ruby. So there we go:

osgbconvert.rb – Conversion to Ordnance Survey coordinates in pure ruby

You can run it to see an example conversion with the output:

wgs84 lat:51.52237, wgs84 lon:-0.10322

http://www.openstreetmap.org/?mlat=51.52237&mlon=-0.10322&zoom=16

Converts to:osgb36 lat:51.5218609279112, osgb36 lon:-0.101610123646581

Converts to:easting:531691, northing:182090  As a grid ref:TQ31698209

http://streetmap.co.uk/grid/531691_182090_106

This includes ported code for Convert co-ordinates between WGS-84 and OSGB36 and ported code in one direction for OS Latitude/Longitude to OS National Grid (coordianate systems explained below) Also I haven’t faithfully stuck to what Chris’ functions return.

There is a bunch of maths, but it was quite a straightforward syntax conversion, and it comes in at about 150 lines of code. Somebody should do this for other languages. I’m sure other languages offer more beefy libraries equivalent to proj4. The main benefit of this is for quick copy & paste coding.

Mind you, if you are copying and pasting this, be sure to worry about the fact that it is LGPL licensed by Chris Veness [Update: It’s now shown under the more permissive CC-BY license on his website. Take your pick!]  For my part, you can credit me too if you like, but I don’t care.

I also came across another implementation, license unknown.

About Ordnance Survery coordinate systems

Chris Veness’s page is well linked because the code comes with a comprehensive description of what’s going on. However I misunderstood a few things even after reading it several times. Here’s my noddy guide to Ordnance Survey coordinate systems:

WGS84 is what I would call the “normal” latitude and longitude coordinate system. These days web developers passing around latitude and longitude values are normally using this. It’s what OpenStreetMap works with if you want to point to a location with a URL such as : http://www.openstreetmap.org/?mlat=51.52237&mlon=-0.10322&zoom=16 (the placr.co.uk office).

Ordnance Survey OSGB36 – Involves coordinates which are also called “latitude and longitude”. The numbers look very similar, and in fact if you get them confused and use one in place of the other, you’re generally only ~100 metres off. At this point I could mention “datums” and “ellipsoids”, but who cares? How do we fix it? With the above ruby code, use the function convertWGS84toOSGB36 (or convertWGS84toOSGB36 in the other direction)

Couple of formatting things to note: Both WGS84 and OSGB36 latitude & longitude values can be expressed in degrees and minutes rather than decimal, and with N S E W letters on the end. For example 51°31′20.53″N 000°06′11.60″W If longitude has a W on the end it would more “normally” be a negative number. The letters on the end do not make this an “Eastings/Northings” coordinate. That’s a different system…

Ordnance Survey National Grid Reference Eastings and Northings look totally different. For the placr office the easting is 531691. The northing is 182089. On these numbers the first digit is special. It’s identifying which “grid” square we are in, counting from zero. Everything in London is in grid square 5,1

National Grid References with letters are the older more human friendly version of that. So “TQ 3169 8208” is the equivalent. The 5 and 1 are removed, and instead we represent the grid square separately at the front. Everything in London is in grid square “TQ”. Just to munge things together a bit more, we might drop one digit of precision and drop the spaces to give “TQ316820”.

Don’t ask me how Landranger sheet numbers fit in. I’m going back to my WGS84 thankyou very much.