Spatial Analysis in QGIS

It is time to talk about spatial analysis.  Many open source GIS software have at least some analytic capability--more functionality is being added frequently.  Earlier, I showed a simple map of wifi locations in New York City using QGIS.  Let's take a look at the density or in this case area surrounding these points.  Since I have had trouble with kernel density, let's use Thiessen/Voronoi polygons.  Interestingly, these are only available with an ArcInfo license in ArcGIS, which is extremely expensive.  I am not going to compare results here, but let's see what the resulting map looks like.  The lighter/whiter the color the less area between wifi locations and the better the wifi availability.  (Of course I don't show whether the wifi locations are free or cost-based on this map).  Not bad for free data and free data analysis!  I used the nifty vector transparency plugin from QGIS so you can also see some of the land cover.

Click on the map and a larger version will appear.