Automating Vector and Raster Workflows using the Graphical Modeler in QGIS

Disclaimer

Please note that this tutorial was created as a term project for the Carleton University class GEOM4008 - Advanced Topics in Geographic Information Systems. The tools used below are just a some of many that can be found within QGIS and outputs may heavily vary depending on the inputs. The results should not be used as conclusive findings for others work.

Introduction

The objective of this tutorial is to provide an in-depth walk through of automating two workflows in the QGIS Graphical Modeler. The first is a simple workflow using sample vector data that covers geoprocessing tools such as; buffer, clip and merge. It will also demonstrate how to perform a field calculation within a model. The second workflow is more complicated and works with a sample DEM. The workflow covers the various steps required to determine the watershed basins within an area of interest. The different algorithms covered in this tutorial are; fill sinks, catchment area, channel network, watershed basins and vectorising grid classes. These algorithms are native to SAGA GIS, but through QGIS's advanced interface processing toolbox, they as well as GRASS GIS algorithms are accessible. This tutorial provides a highly explanatory and easy walk through for individuals with varying levels of GIS knowledge. The purpose is to aid those looking for a quicker, more efficient way of performing long workflows without the use of expensive GIS software.

Software

As mentioned above, the software that will be used for this tutorial is Quantum GIS, otherwise known as QGIS. It was chosen for two reasons; first, it is one of the most powerful GIS FOSS (Free and Open-Source SoftwareFree and Open-Source Software) with a very user friendly interface; second, it has a very intuitive Graphical Modeler that has access to many different algorithms from GDAL, GRASS, ORFEO Toolbox, QGIS and SAGA.