Here's a quick howto for local map rendering, using
python,
mapnik
and data from
OpenStreetMap.
This tutorial shows how to create a stylesheet for mapnik from scratch, if
you're looking for how to tweak OpenStreetMap's mapnik rendering, I plan
to write a tutorial on
PostGIS-driven rendering at a later time.
This post is not for those who want to make a local slippy map, nor those
who want to render different areas of big dumps (think of big national dumps).
We'll render some features of a residential area of a city with buildings
mapped. We'll get a
small dump of the area for this, using the
XAPI.
First, you need to install mapnik. On Debian-like systems, it should be as
easy as:
# apt-get install python-mapnik
Then, you need some python code to use the mapnik library. Let's call it
render.py:
#!/usr/bin/python
import mapnik
###
# Configuration
###
style = 'style.xml'
output = 'output.svg'
width = 1280
height = 800
bbox = '12.58664,37.65759,12.5945,37.66325'
###
# Don't touch below!
###
bbox = bbox.split(',')
ll = mapnik.Coord(float(bbox[0]), float(bbox[1]))
tr = mapnik.Coord(float(bbox[2]), float(bbox[3]))
mymap = mapnik.Map(width, height)
mapnik.load_map(mymap, style)
map_bbox = mapnik.Envelope(ll, tr)
mymap.zoom_to_box(map_bbox)
mapnik.render_to_file(mymap, output)
For the purposes of this post, we're going to create a 1280x800 map -- adjust
that to suit your needs -- and we specified a
bounding box corresponding to
that area. That's composed as west,south,east,north coordinates. Be sure to
get them right :-).
Now we need to write a
Mapnik stylesheet, we'll call it
style.xml. That's
a XML document, you can find the documentation
at the official website.
The skeleton of a stylesheet is as follows:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map>
<Map bgcolor="lightblue" srs="+proj=latlong +datum=WGS84">
<Style name="...">
...
</Style>
<Layer name="...">
<Datasource>
...
</Datasource>
</Layer>
</Map>
The stylesheet has an arbitrary number of
Style elements
and an arbitrary number of
Layer elements. The former class of elements
define the appearance of the features, while the latter defines where to get
data from.
Each layer has a
Datasource child: here's where you say where
to get your data from. We're going to use the osm input plugin, which
lets you parse the dump directly. This has some drawbacks:
- not scalable for big dumps
- for each bbox you want to render, the full dump has to be re-parsed
But works great for small dumps.
In this tutorial, we want to pretty render a residential area, so we first
need to render the road network. This will mainly be made of
highway=residential roads.
Each
style can have one or more
Rule element. These define what
elements that particular style should apply to. Each rule can have different
rendering settings.
The given dump has different roads: residential, unclassified, tertiary and
secondary. Let's add a rule for each. Here's the first of them:
<Rule>
<Filter>[highway] = 'residential'</Filter>
<LineSymbolizer>
<CssParameter name="stroke">#ffffff</CssParameter>
<CssParameter name="stroke-width">5</CssParameter>
<CssParameter name="stroke-linejoin">round</CssParameter>
<CssParameter name="stroke-linecap">round</CssParameter>
</LineSymbolizer>
<TextSymbolizer name="name" face_name="DejaVu Sans Book" size="8" fill="#000" halo_radius="1" spacing="300" placement="line" />
</Rule>
It is quite straightforward: you define what elements the rule should apply
to, and then add some
symbolizer. For a detailed description of each
symbolizer, please read the
documentation.
The example dump also has a railway and some steps. To render them, we use
the
stroke-dasharray property:
<Rule>
<Filter>[highway] = 'steps'</Filter>
<LineSymbolizer>
<CssParameter name="stroke">#ff0000</CssParameter>
<CssParameter name="stroke-width">5.0</CssParameter>
<CssParameter name="stroke-dasharray">2,1</CssParameter>
</LineSymbolizer>
</Rule>
<Rule>
<Filter>[railway] = 'rail'</Filter>
<LineSymbolizer>
<CssParameter name="stroke">#222222</CssParameter>
<CssParameter name="stroke-width">3</CssParameter>
<CssParameter name="stroke-linejoin">round</CssParameter>
</LineSymbolizer>
<LineSymbolizer>
<CssParameter name="stroke">white</CssParameter>
<CssParameter name="stroke-width">1</CssParameter>
<CssParameter name="stroke-linejoin">round</CssParameter>
<CssParameter name="stroke-dasharray">10,20</CssParameter>
</LineSymbolizer>
</Rule>
Also here, it is quite simple. The values given to stroke-dasharray determine
the size of the dashes, and they can be alternated. So, for the steps we have
2px of #ff0000 and 1px of trasparency, while for the rail we have 10px of
white and 20px of #222222.
What if we want to add some markers to the map? Easy, just use another
symbolizer, called
PointSymbolizer:
<Rule>
<Filter>[highway] = 'stop'</Filter>
<PointSymbolizer width='10' height='22' file='stop.png' type='png' allow_overlap='true' opacity='1.0' />
</Rule>
Here we mark stop signals with a pre-rendered stop.png (only png and tiff are
currently supported, and svg support is only in the development version).
What to do next? Well, we can make this map
really nice. We'll render
buildings with a pseudo-3D effect, which, however, does not reflect real
heights -- but it's rather nice to see. How to achieve this?
BuildingSymbolizer is the answer.
<Rule>
<Filter>[building] <> ''</Filter>
<BuildingSymbolizer>
<CssParameter name="fill">#00ff00</CssParameter>
<CssParameter name="fill-opacity">0.4</CssParameter>
<CssParameter name="height">0.00007</CssParameter>
</BuildingSymbolizer>
</Rule>
Here we just draw
building=*. Regarding the height, you need to play a bit
-- it took a while before I found a good value of it.
One thing missing in this stylesheet, is the rendering of coastlines. Since
we don't have a coastline here, I'm skipping it, but the easy way is: use
another rule. This is really just a trick; OpenStreetMap renders the coastline
in a different way respect to other features, that's why I'm using this
trick.
I'm not posting the whole stylesheet, but you can
download it. You can
also get
the script,
the OSM dump and the
stop.png image.
Now, you should be able to just do:
$ ./render.py
and you will have your map in
output.svg 
Here's what you would get if you use the files provided above (click to see
the original SVG version):
I travel across the Sedgemoor/North Somerset/Bristol council borders, so I m a big fan of FixMyStreet which puts fault reporting for things like potholes, flytipping and broken lights behind a common interface. Credit to the MySociety charity project, once again.
So I m quite happy that something similar is starting for website faults. If you know disabled people who are willing to report web accessibility issues for volunteers to pick up, please let them know they can currently:
The 
A fresh minor release of
It's not very easy to find information on how to adjust 
My blog will move soon to a new URL and software. In fact, I've written my
own static HTML based blog generator, wrapping around Atom for data storage.This is why the feed will also be only available in Atom format in the
future - it works a lot better, and why support 2 formats?The most important feeds are:
Currently, the main product of the Debian project is its stable release
When talking about free culture or free software, many people suggest
that they would love to support free models, but that they don't see
how to make it all work. Until they have an alternate model in front
of them, they cannot bring themselves to argue for a more ethical
alternative. I disagree with this approach. Instead, I say, "this is
the world I want to live in and, even though I don't know exactly how
to get to there from here, I'm going to refuse to settle for anything
short of this ideal." Most people dismiss such thinking as
"impractical" and "unrealistic." I think most people are being
unimaginative.
One more personal record broken for my 6th marathon (including those I ran
alone). This time, it was a small marathon, the 
I've been trying to work out how to handle (or whether to handle) the vestigial debconf values from emdebian-tools which was replaced in Debian some months ago. I tried to transfer the postrm script to libemdebian-tools-perl which itself is currently a transitional package. Turns out that when debconf tries to purge libemdebian-tools-perl, it doesn't purge the old emdebian-tools settings (it looks for libemdebian-tools-perl.templates instead of emdebian-tools.templates) - unless those were themselves setup by libemdebian-tools-perl. i.e. users with the original emdebian-tools setup will likely have debconf settings that are not purged by removing the package to which those settings should have migrated.
I m currently working on something which will eventually amount to a
re-bootstrapping of sorts of the Debian/m68k (Linux) port patches to
the Linux kernel, gcc, etc. are prepared (some have been accepted into
upstream or the packages already). I will probably have more, once the
compile processes finish, anyway (even emulated, it s slow).
I think that, once I get past that TLS (thread-local storage, needed
by eglibc) migration, I will try to find out a list of packages needed
for debootstrap (AFAICT: all packages marked Essential, or of priority
important, and all marked Build-Essential (for the *-builder variant),
and their dependencies (although I ll substitute sysv-rc with file-rc,
which is better and needs less deps)) and pull arch:all from sid, then
build the rest myself using a consistent snapshot of sid possibly with
patches going to unreleased. Then I can use cowbuilder to make cleaner
packages, which can eventually be uploaded (once I get enough to get a
buildd running kernel, bootloader, etc) binNMUs are way to go here
I suppose. I will only upload once it s self-hosted, installable (seen
by edos-debcheck), clean, etc. (i.e. I ll rebuild all
binaries) and probably keep a bootstrap repo around (until m68k caught
up) so that unstable (possibly amended by unreleased for a while) will
not again become uninstallable, e.g. if arch:all packages change their
dependencies (Python, gcc-defaults are some I ve seen). That bootstrap
repository is needed anyway because debootstrap can t install from two
separate repositories (unstable+unreleased for example).
Progress is slow because I try to keep as close to official packages
as possible, refuse to cross-compile, and try to produce uploadable if
possible packages all the time. Getting patches into packages, so that
I can build from unstable, instead of debian-ports.org unreleased, has
proven time-consuming and occasionally frustrating as well. Although I
would like to thank the people who helped me on the way already. (I am
not naming any in fear of forgetting some, but you know who you are )
They are among the Debian (gcc, kernel, m68k) and Linux-68k crowd.
(Why does genattrtab in gcc-4.4 take 3 hours when it took less than
half an hour in gcc-4.3 anyway?)
I m also still working on 