Tutorial: Automate Stream & Catchment Delineation using the Graphical Modeller

Site: OpenCourseWare for GIS
Course: QGIS Advanced Tutorials
Book: Tutorial: Automate Stream & Catchment Delineation using the Graphical Modeller
Printed by: Guest user
Date: Wednesday, 17 April 2024, 12:23 PM

1. Introduction

QGIS has some great ways to automate your processes. You can use PyQGIS to build scripts. But if you're looking for a more graphical way, the Graphical Modeller is a great way to automate your processes using the tools from the Processing Toolbox.

After this tutorial you're able to:

  • Define input layers and variables for your graphical model
  • Add processing tools to your graphical model
  • Save results of your graphical model
  • Save your graphical model to the Processsing Toolbox
  • Create a graphical model for stream delineation
  • Create a graphical model for catchment delineation
In this tutorial we'll make two models that you can apply to any area for which you have downloaded SRTM data and created a bounding box polygon of the study area. For this tutorial the data is provided.

Here we use QGIS 3.14 Pi, but the procedure is similar in 3.10 LTR.

2. Data preparation

We'll make two graphical models:

  • Stream delineation
  • Catchment delineation
The Stream Delineation Model should be applicable to SRTM 1-Arc Second tiles that you can download for many areas in the world. For this tutorial four tiles are provided, which cover the area of the Rur catchment.

1. Start QGIS
2. Add the DEM tiles
Four SRTM tiles

Because the four tiles cover much more than the Rur catchment area, we also need to digitize a bounding box. You can digitize a polygon or zoom in to the study area and use the Create layer from extent tool from the Processing Toolbox. Make sure that the on-the-fly projection of your project is changed to the projection that you want to use, which should not be a Geographic Coordinate System with lat/lon coordinates in degrees!

For this tutorial we have provided the bounding box.

3. Add the bounding box to the map canvas.

4. Click right on the boundingbox layer in the Layers panel and choose Set CRS | Set Project CRS from Layer.

This changes the project CRS from GCS (EPSG:4326) to UTM Zone 32N/WGS-84 (EPSG: 32632) that we're going to use.