Category Archives: openstreetmap

Finding Ley-Lines with PostGIS

I’ve recently been playing with PostGIS. This post will summarise a simple attempt to manipulate data and draw it on top of Open Streetmap. I wanted to produce a Brighton version of Steven Kay’s Pub Ley Lines. As you will see, the outcome was less interesting than the process.

What follows is, basically, a How-to-Draw-An-Owl tutorial. It summarises the steps I usually look up and is intended to share with a few specific people. If you find yourself here via Google and want more information, leave a comment and I will add more detail.

1 – Download data from OSM

The first thing I needed was the data from Open Streetmap, which contains pub locations among the points-of-interest. There are a number of options. Downloading direct from OSM failed when I last tried it, but I had an older version of the data available.

2 – Set up Postgis and osm2pgsql

I’d previously installed Postgresql and Postgis on my laptop but somehow the installation has become broken and won’t be easily repaired or uninstalled. I should fix this, but I wanted to get on with this experiment. I’ve been meaning to set up AWS for some time, and using a micro instance on Amazon allowed me to get a version of PostGIS running very quickly.

I created a new EC2 micro instance based on the basic ubuntu instance, ran ‘sudo apt-get update’ followed by ‘sudo apt-get install osm2pgsql’ and I had everything I needed.

AWS is awesome, and I love being able to run up an instance for a small task and throw it away once I’m done.

3 – Create postGIS user and table

Setting up a new user on postgres is certainly less of a hassle than doing it on MySQL, but there are a few gotchas – such as needing to add a line to the /etc/postgresql/9.3/main/pg_hba.conf file. Also, when creating a new database, remember to enable PostGIS with the command “CREATE EXTENSION POSTGIS”

4 – Load the OSM data in Postgres

Having transferred my PBF file to the AWS instance using scp, I could then load the data. The command here is a little different to the one I used previously because of this server’s limitations:

osm2pgsql -U ley -d ley  --slim --cache-strategy sparse --number-processes 4 brighton.pbf

5 – Create a table containing all of the ley lines

I pretty-much followed the recipe given by Stephen Kay here.

6 -Extract the data

This time, rather than a tab-separated format, I selected the WKT (well-known text) format for all the ley-lines with more than 8 pubs. I’m sure there are better ways to extract this. The query used was:

psql -U ley -w ley -c "COPY (select st_astext(st_transform(geom,4674)) from leys where ct> 8) TO STDOUT WITH CSV" > lines.txt

This may not be the most portable format for the data but I can bully it into something Openlayers can use.

7 – Create a page to display the data

Since the data is constant and will be low-traffic, I am using OSM via static files to display the results.  The linestring WKT representations of the leys have been copied into the javascript file rather than being loaded from a file. All quick-and-dirty, but it has worked. The source is on github and the results are online.

Getting Open Streetmap working on Ubuntu

It’s taken me quite a bit of time but I’ve finally got an open streetmap tileserver working on my laptop. The main issue appears to have been that I was following the manual Ubuntu set-up instructions rather than building a tile server from packages. Which isn’t to say that using packages was plain sailing. This post will outline some of the problems I ran into and how I solved them.

(Bear in mind that I’d already tried to install OSM using the manual method on the laptop and fragments of these installs were still on the system. Installation might be a lot easier on a clean OS. However, it’s worth documenting the issues in case anyone runs into the same issues and is googling for them)

The main point of this post is to say that I have managed the open streetmap set-up and now have a map of Brighton working on my laptop:

Screenshot from 2015-03-03 08:11:32
I can see my house from here!

The installed slippymap.html page is great as the radio buttons on the top right allow you to compare a remote instance against the local one – useful for finding out where you are when things aren’t working, or you only have a small area set up. Now I’ve fixed this, I can get on with actually using the server.

The main problems I had to solve with the packaging method were:

Mod-tile dpkg installtion hangs

The command to install mod-tile hung. Googling around, I found a patch for this  – a line was missing from the libapache2-mod-tile.postinst script. Editing the script and re-running the installation fixed this issue.

osm2psql didn’t work

When I ran the command osm2pgsql I received an error:

Osm2pgsql failed due to ERROR: SELECT AddGeometryColumn('planet_osm_point', 'way', 900913, 'POINT', 2 ) failed: ERROR:  AddGeometryColumns() - invalid SRID

This seemed to have been caused by not having set up data in the spatial_ref_sys table. To check this see if there are any lines from the SQL query

SELECT * FROM spatial_ref_sys;

If not, then this is easily fixed:

psql --username=postgres --dbname=gis --file=/usr/share/postgresql/9.1/contrib/postgis-1.5/spatial_ref_sys.sql

After re-running this script, the osm2psql script worked. Note that this may have been a problem due to me incorrectly removing previous installation attempts.

Tiles not rendering

This is more of a generic issue. Basically, when the slippymap page is working, but the images are not being generated, a lot of missing images will be displayed:

Pink tiles

You can open the blank tiles in a new page and see there where they are being generated from. Some of the help online suggested that it could be down to the slippymap.html pointing to the wrong OSM server (it defaults to localhost). In my case it was a permissions issue:

Database permissions not set correctly

There seems to be a minor issue with the installation scripts. This was fixed by explicitly setting up the postgis user.

I was able to diagnose this issue by running renderd as a foreground process (ie renderd -f) and seeing permissions errors such as the following:

renderd[26466]: An error occurred while loading the map layer 'default': :
FATAL: role "www-data" does not exist (encountered during parsing of layer 'landcover' in map '/etc/mapnik-osm-data/osm.xml')
renderd[26466]: An error occurred while loading the map layer 'default': :
FATAL: role "www-data" does not exist (encountered during parsing of layer 'landcover' in map '/etc/mapnik-osm-data/osm.xml')

First steps with OpenStreetMap

(3/3/15 EDIT – it turns out that installing OSM is simpler than I thought. I’ve published an update to this post which details the steps taken to a successful installation)

Over the past few weeks I’ve spent a lot of time wrestling with installing an OpenStreetMap server. In theory this should be simple. The tutorial for the version of Ubuntu that I use makes it look easy. Instead this is the most difficult install I worked with since the days when Oracle released their shrinkwrap products with incorrect documentation.

The OSM install involves five different products, some of which need to be compiled from source, and one which doesn’t work in the documented version. Fortunately, a lot of the problems I faced could be resolved via Google. Although using make only increases my love for maven. I don’t miss old-school build systems.

I’ve had multiple stabs at installing OSM, which seems about par for the course. I’ve not succeeded yet, but I seem to be making progress. I thought I would document what I’ve done here – partly for my own benefit and partly for anyone who comes by on Google and is as lost as I am. I’m determined to succeed at this – it’s a challenge and a puzzle and will probably seem quite simple once I’ve cracked it.

The Stack

The OpenStreetMap stack consists of four elements:

  1. mod_tile, an apache module that serves and caches map tiles
  2. renderd – a “priority queueing system” to manage the load from mod_tile’s rendering requests.
  3. mapnik – the software library that does the actual rendering of the map tiles.
  4. postgresql/postgis the datastore containing the mapping elements

OSM also needs osm2pgsql to load the OSM data into postgis. So far, I have mapnik working and mod_tile passing requests to renderd, but I cannot link renderd to mapnik. The only thing I’ve served from mod_tile is this world map, which is produced outside the main mapnik process:

0

Installation

The OSM installation is fiddly, which means Googling for the issues. There are also lots of fileswhere a small mistake can need debugging, for example replacing variables in DTD entities. Things like these would be easier if the installation files were opinionated – this works well in Spring Boot and Maven.

The main issue I’ve had is with versions of software (which is where I think I’m currently stuck). One bug had to be fixed by using a version of mod_tile from github.com:springmeyer/mod_tile.git. However, I am not convinced this is playing nicely with some versions of mapnik.

I had decided to use a limited area of the world, rather than the full-on 29GB download. To work out what tile co-ordinates are needed by mod_tile requires a mapping from long/lat to the OSM cordinates (I used the python scripts on that page). These co-ordinates can be checked against the OSM public servers. For example, Brighton is http://a.tile.openstreetmap.org/12/2046/1374.png.

As I said above, I still can’t get renderd to talk to mapnik, although mapnik is working, shown by this attempt to render Brighton:

out2

Apparently, the East Sussex data I had didn’t include Brighton. But a query in East Sussex produced a decent map:

out

What Next?

Despite my issues, I’m still excited about OSM and have a couple of ideas for using it. And I’m convinced that I can figure out what’s going wrong, although I will need to sit down and actually read the mod_tile documentation rather than trusting to installation guides and online discussions.

I regret not attempting the original installation on a Vagrant VM, as this would have enabled me to start from scratch. I may have to do a little work to remove the different versions of libraries on my laptop before I can continue the installation.

Hopefully, another few hour’s work should see me with a working version of OSM. Once I’ve done that, I will write up a step-by-step list of the steps required.