By David Crowther
I have been a big advocate of Open Source technology for a long time, regularly using and providing training course in Open Source GIS applications such as QGIS, GeoServer and PostGIS.
When I first created GIS training courses (16 years ago!), the most difficult aspect was always – where do I get data from?..... Ok now I’ve got some data, but am I licensed to use it in my training courses?
Those days, thankfully, are now long gone thanks to websites such as Data.gov.uk and the Environment Agencies GeoStore, which provide you with access to data via WMS web services at the click of a few buttons.
That just leaves us with accessing background mapping datasets which can be used both as a backdrop to provide context to these datasets, and as vector based objects which we could use to copy and snap to.
The Ordnance Survey have been leaders in the field for providing users access to Open Data, and have an easy to use download service providing access to a range of products that are getting increasingly more accurate. A simple google of the key words ‘OS Open Data’ takes you straight tothe data download pages for the free datasets that the Ordnance Survey provide.
Including Vector datasets such as,
- OS Open Local
- OS Open Names
- OS Open Rivers
- OS Open Roads
As well as Raster background maps such as;
- OS VectorMap District
- 1:250K Raster Mapping
Once you have downloaded the Open Source data for your area, the Ordnance Survey also make it easy to style your data within your chosen GIS.
Continuing the Open Source theme, I have been using QGIS to display OS Open Local to view building and other topographic features as vector objects. The Ordnance Survey not only provide access to the vector datasets, but they also provide links to a series of QGIS StyleSheets (QML) files that you can use to style your data easily within QGIS.
In the image below, we are showing OS Open Local data for two OS grid tiles. On the left is grid tile SU, where using the QML Style files QGIS is styling the data without you having to spend hours choosing line, fill and text styles. On the right is the grid tile TQ, where the OS Open Local data is displayed as unstyled objects.
Within minutes we now have access to street level vector objects, styled ready to provide to users within a desktop GIS!
On comparison of the OS Open Data Products with Ordnance Surveys’ Premium MasterMap datasets, yes, there is obviously a lower degree of detail. For example, when comparing a row of houses, OS Open Data (shown on the left) will display this as one continuous polygon, whereas MasterMap (on the right) will break these up into their component objects, with their footprint more accurately drawn.
You also have the added benefit of ‘Building Height Attributes’ available within MasterMap, which allows you to model the features within a 3D GIS application, such as Infraworks.
The next and ultimately most exciting part was that using GeoServer and specifically, the QGIS plugin ‘GeoServer Explorer’, you can now easily create Data Layers in GeoServer by simply dragging the vector layer into the GeoServer Plugin, creating a WMS Layer instantly. Using the Edit SLD option you can also directly access the SLD properties within QGIS and either make a change to the style of features by directly editing the XML,….
Or more simply, by using the Style Editor within the QGIS Layer Properties window, you can make simple changes to styles and write those directly back to GeoServer by ether updating existing or creating new Style Files.
Most recently, while having a play with QGIS, I noticed the Open Street Map > Download Data tool. Using this tool, you can easily download Open Street Map (OSM) Buildings, Roads and other features into QGIS for your area of extents. Downloading OSM data for the centre of Leicester took me less than one minute!
Using the Open Layers plugin within QGIS, you can then add context to your vector data, and immediately see the details that have been captured within OSM.
By browsing the OSM vector geometry, you have access to highly intelligent attribute information. For example, the polygon dataset included values for the ‘Name’ of features as well as their ‘Type’.
The Polyline layer allowed me to theme/style each road based on whether they were ‘Local’, ‘Primary’, ‘Secondary’, etc.
Having interrogated the OSM data, I was then able to write a simple SQL query to select only those polygon features, whose type was ‘Building’ and then save that out as a new Buildings Shapefile!
It would now be interesting to compare the accuracy of the OSM dataset to that provided in the OS Open Data.,.. but I will leave that with you to explore!