The concepts for geoprocessing are very similar to those you’ve learned in ArcGIS. All of the common methods are available:

  • Intersect
  • Union
  • Buffer
  • Clip
  • Difference
  • Dissolve

These are all located under the Vector menu.

Create Polygon Centroids

Add the parcels layer to your project.

Under Vector select Geometry Tools and then Polygon Centroids.

The image below doesn’t show the last portion of menu tree.


Select the layer to create centroids from, and specify the output data set name.

_images/GeoProc_Cent_02.png _images/GeoProc_Cent_03.png


Buffering the centroids by 1000 feet (because the parcel layer is in State Plane Zone 3 (US Feet))

Find the Buffer tool under Vector, Geoprocessing Tools


The “Segments to approximate” setting allows you to “tune” the detail that corners on the buffer have. The higher the number the more and shorter segments are created.

_images/GeoProc_Buff_02.png _images/GeoProc_Buff_03.png


(note I’m going to skip doing this one in the live demo)

Intersecting the centroids with the buffers will start to let us know how many parcels have centroids within 1000 feet of the center of each parcel. This would let us calculate a parcel density, and is a technique that I use frequently when doing summaries of land use.

The Intersect tool is in Vector, Geoprocessing Tools


This process takes a bit longer to run because it’s an exponential calculation.

_images/GeoProc_Int_02.png _images/GeoProc_Int_03.png

Voronoi Polygons

Voronoi Polygons require an ArcGIS advanced license to create. It can be done within QGIS.

The Voronoi Polygons tool is in Vector

_images/GeoProc_Vor_01.png _images/GeoProc_Vor_02.png

SpatialLite and PostGIS

One of the really powerful features of QGIS is the ability to link directly to SpatialLite and PostGIS databases. This allows the use of complex spatial queries that may take many steps to complete through the linking of individual Geoprocessing steps.