I’ve been teaching myself to use Quantum GIS, an open source geographic information system, by using it to build the maps for a practical assignment to do a feasibility study of a small windfarm.

You can get a copy of the software at the QGIS website, and there are tutorials and introduction documents around the web.    This post is to pick up a few things that took me a while to figure out.  They’re notes for me, and they might be handy for someone else.

How can I add a new point symbol?

Point symbols are the icons you use to show a point on a map, like this wind turbine on a hill near Machynlleth.  By default, the software doesn’t come with a wind turbine symbol (incredible omission!), so I had to find one.

Point symbols are scaleable vector graphics (svg) files, and they’re kept in the  OSGeo4Wappsqgissvg directory whereever you installed the software.  To find a suitable one, I googled “windmill .svg”, and Google came up with one on Wikimedia Commons, which is available in the public domain, so no licencing issues.  Then I just copied it into one of the folders in that svg directory, and the next time I restarted Quantum GIS, it was available for me to choose.

My computer is grinding to a halt.  What can I do?

I had about a dozen layers, with lots of information from the Countryside Commission for Wales, who had kindly sent me data about all the restricted areas in Wales, and I’d loaded them up.  That was not smart.

There’s a little box on the bottom right hand side of the screen, with a label ‘Render’.  Turn it off, unless you want the system to redraw everything every time you move the view or make any change at all.  And if you must load dozens of layers, be prepared to spend a lot of time waiting.

I want to show a bufferzone around my feature

Working out where you can put turbines in a wind farm is all about figuring out what restrictions apply.  The Horse Riders Association don’t want one within 3 * the turbine tip height of a bridleway.  The Ramblers Assocation want them at least tip height +10% from a walking path.  There are other restrictions on roads, cables, microwave links and the rest.  What you really want to be able to do is draw all your features – paths, bridleways, roads, microwave lengths, and then push a button to show what areas of land this rules out.

To do this, you need to add the ftools plugin, which lets you do all sorts of analysis on your vector layer, and draws buffer zones.  It has a couple of dozen functions, and I’ve not had the chance to explore them yet, but it did make it much easier to draw my bridleway buffers.  If you set up the distance you need as a field when you draw the path, it can automatically use that to draw the right size buffer, allowing me to put my footpaths and bridleways in the same layer.