“Graphical Model is useful for automatic GIS task by providing workflows that link various processing tools and user input.”
An analysis in QGIS requires users running algorithms in a predetermined order. In some cases the sequence of steps are identical between different data sets and generating models which are generic can be useful to do repetitive tasks. In this module, we explore how to solve a real-world problem using the graphical model. We inherit the example from the previous exercise where we were running the steps manually.
Goal: To learn how to automate manual workflows using the Graphical Model
You are a city official and you have been instructed to investigate the effect of expanding OR Tambo Internation Airport by 50km. Use the geoprocessing tools (buffer, intersect, selection) to show how the expansion will affect the surrounding urban areas.
Input Vector and Input Urban Areas
Select by Attribute, Extract selected features, Buffer, Intersect , Dissolve
The graphical model in QGIS consists of mainly two parts Inputs and Algorithms.
|Inputs||These are user-defined inputs required for running the algorithms||Input vector layer or buffer distance|
|Algorithms||Actual geoprocessing algorithm responsible for spatial analysis||Buffer, intersection, Select by location etc|
When analyzing a GIS problem it is important to visualize it first and determine the steps and order that you need to execute them. This will affect the final result if they are not executed in the correct way. When creating models in QGIS it is recommended to do it in a series of steps testing them along the way. The graphical model is also useful for running analysis where you are not concerned by the intermediary products but the final one. The model allows you to skip generating them but only focus on the end product.
When creating a model is it recommended to make the model generic as the saved model can also be used as an * Algorithm* for another process.
Your final output should look like the image below:
Click here to download the sample data for the lesson.