Tutorial: Visualise Flow Direction with Arrows

Course: GIS OpenCourseWare for Hydrological Applications
Book: Tutorial: Visualise Flow Direction with Arrows
Printed by: Guest user
Date: Wednesday, 26 January 2022, 9:58 PM


This tutorial will guide you through the steps of visualising the output of the SAGA flow direction algorithm as a mesh with arrows using the Crayfish plugin.

1. Introduction

The flow direction layer is a raster layer indicating the direction of the water flow. The directions are encoded into discrete values. Each software does that differently.

With a directional or circular ramp we can visualise the flow direction. Creating the colour ramp and interpreting the water flow is difficult. Therefore, here another, more intuitive visualisation method will be used. In this tutorial you'll learn how to visualise the SAGA flow direction layer with arrows indication the flow direction.

After this tutorial you'll be able to:

  • Convert the SAGA flow direction Raster layer to a mesh (GRIB format)
  • Style the flow direction mesh with arrows
  • Visualis the flow direction in the 3D view

2. Style the background layers

First we're going to style the background layers on top of which we will later visualise the arrows of the flow direction.
1. Start QGIS

2. Add the DEM (dem.tif), flow direction raster (flowdir.sdat) and channels (channels.shp) to the Layers panel. You can drag and drop it from the browser panel or use from the main menu Layer | Add layer | Add raster layer...

3. Deactivate the fowdir layer by unchecking the box.

4. Click to open the Layer Styling panel. Make sure DEM is the active layer. Choose Singleband pseudocolor as renderer. Click on the arrow right of Color ramp and choose Create New Color Ramp.... Choose Catalog: cpt-city and then from Topography select elevation and click OK. Then click Classify. Under Layer rendering Change the Blending mode to Multiply.

5. Click right on DEM in the Layers panel and choose Duplicate Layer.

6. Deactivate DEM by unchecking the box and activate the DEM copy layer.

7. Click right on the DEM copy layer and choose Rename Layer. Rename the layer to Hillshade.

8. In the Layer Styling panel choose Hillshade as renderer. Keep the default settings. Only change under Resampling Zoomed in to Bilinear and Zoomed out to Average. This makes the visualisation of the hillshade smoother.