Adjusting imprecise building footprints

Digitizing vector data from raster data is a tedious process. A common task in digitizing is to outline buildings in an image to make footprints for urban analysis. While most buildings are rectangular in shape, building footprints often end up with incongruent angles due to analysts quickly clicking points. In this recipe we'll automatically transform some non-rectangular polygons into rectangles using a simple algorithm developed by José Guerrero, which can be found at the following URL:

http://gis.stackexchange.com/questions/212003/how-to-modify-a-polygon-to-be-more-rectangular/212325#212325

Getting ready

You need to download the following zipped shapefile into a directory named shapes in your qgis_data directory:

https://github.com/GeospatialPython/Learn/raw/master/irregular.zip

How to do it...

First we'll set up some reusable functions for geometry calculations. Then we'll load our footprints layer and create a memory layer...