OpenStreetMap represents a lot of data. If you want to import the entire planet into a PostGIS database using osmosis, you need at least 300GB of hard disk space and, depending on how much you spent on fast processors and (more importantly) memory, a lot of patience. Chances are that you are interested in only a tiny part of the world, either to generate a map or do some data analysis. There’s several ways to get bite-sized chunks of the planet – take a look at the various planet mirrors or the cool new Extract-o-tron tool – but sometimes you may want something custom. For the data temperature analysis I did for State of the Map, I wanted city-sized extracts using a small buffer around the city border. If you want to do something similar – or are just interested in how to do basic geoprocessing on a vector file – this tutorial may be of interest to you. Instead of city borders, which I created myself from the excellent Zillow neighborhood boundary dataset, I will show you how to create a suitably generalized OSM POLY file (the de facto standard for describing polygon extracts used by various OSM tools) that is appropriate for extracting a country from the OSM planet with a nice buffer around it.
Let’s get to work.
Preparing Quantum GIS
We will need to add a plugin that allows us to export any polygon from your QGIS desktop as an OSM POLY file. We can get that OSM POLY export plugin for Quantum GIS here.
Unzip the downloaded file and copy the resulting folder into the Python plugins folder. On Windows, if you used the OSGeo installer, that might be
See here for hints where it may be for you.
Getting Country Borders
Easy. Download world borders from http://thematicmapping.org/downloads/world_borders.php
Geoprocessing step 1: Query
Open the Layer Query dialog by either right-clicking on the layer name or selecting Query… from the Layer menu with the TM_WORLD_BORDERS-0.3 layer selected (active).
Type “ISO2″ = “US” in the SQL where clause field and run the query by clicking OK.
Geoprocessing step 2: Buffering
The next step is to create a new polygon representing a buffer around an existing polygon. Because we already queried for the polygon(s) we want to buffer, there’s no need to select anything in the map view. Just make sure the TM_WORLD_BORDERS-0.3 layer is active and select Vector > Geoprocessing Tools > Buffer(s):
Make sure the input vector layer is TM_WORLD_BORDERS-0.3. Only the query will be affected, so we’re operating on a single country and not the entire world.
For Buffer distance, type 1. This is in map units. Because our source borders file is in EPSG:4326, this corresponds to 1 degree which is 69 miles (for the longitudinal axis, that measurement is only valid at the equator and decreases towards the poles). This is a nice size buffer for a country, you may want something larger or smaller depending on the size of the country and what you want to accomplish, so play around with the figure and compare results. Of course, if your map projection is not EPSG:4326, your map units may not be degrees and you should probably be entering much bigger values.
Select a path and filename for the output shapefile. Do not select ‘Dissolve buffer results’. The rest can be left at the default values. Push OK to run the buffer calculation. This can take a little while and the progress bar won’t move. Then you see:
Click Yes. Now we have a buffer polygon based on the US national border:
Geoprocessing step 3: Generalizing
We’re almost done, but the buffer we generated contains a lot of points, which will make the process of cutting a planet file slow. So we’re going to simplify the polygon some. This is also a QGIS built-in function.
Select Vector > Geometry tools > Simplify geometries:
Make sure your buffer layer is selected as the input. Set 0.1 (again, this is in map units) as the Simplify tolerance. This defines by how much the input features will be simplified, the higher this number, the more simplification.
Select a destination for the simplified buffer to be saved. Also select Add result to canvas. Click OK:
This dialog may not seem very promising, but it has worked. Also, I have sometimes gotten an error message after this process completes. Ignore these if you get them.
Geoprocessing step 4: resolving multipolygons
Now, if your simplified country border consists of multiple polygons (as is the case with the US) we have a slight problem. The POLY export plugin does not support multipolygons, so we need to break the multipolygon into single polygons. And even then, we will need to do some manual work if we want OSM .poly files for all the polygons. This is because the plugin relies on unique string attribute values to create different POLY files, and we do not have those because the polygons we are using are all split from the same multipolygon. So we need to either create a new attribute field and manually enter unique string values in it, or select and export the parts to POLY files one by one and rename the files before they get overwritten.
Finale: Export as POLY
I am going to be lazy here and assume I will only need the contiguous US, so I select the corresponding polygon. After that I invoke the plugin by selecting Plugins > Export OSM Poly > Export to OSM Poly(s):
The plugin will show a list of all the fields that have string values. Select ISO2 and click Yes. Next you will need to select a destination folder for your exported POLY files. Pick or create one and push OK.
This is it! Your POLY files are finished and ready to be used in Osmosis, osmchange and other tools that use it for data processing.