Slope Analysis using Quantum GIS Processing Toolbox

Purpose

The purpose of this tutorial is to guide the Quantum GIS (QGIS) user through the steps necessary to create a slope analysis model workflow using the Processing Toolbox. The user will also learn the basics surrounding the creation of slope, hillshade and aspect maps. The user will also then learn how to export the model as a Python script as well as preform geoprocessing tasks using the QGIS Raster Calculator. This tutorial is designed for a beginner GIS user who has experience with previous GIS software packages however no experience with QGIS.

Introduction

QGIS is a free open source geographical information system application that has geographical viewing, editing and analyzing capabilities. Within QGIS are tools called "Plug-ins". Plug-ins are essentially developed geoprocessing features made by the QGIS developers as well as independent users who would like to expand and contribute to the functionality of the software. One particular plug-in extension found in QGIS is the Processing toolbox extension. The Processing toolbox extension is similar to the features that are found in ArcMap’s ModelBuilder. Processing is an efficient application that is capable of creating, editing and managing geographical models. The user is able to connect together workflow diagrams that involve a series of geoprocessing algorithms. For the purposes of this tutorial, a workflow will be created using geographical data of Carp, Ontario in order to create a digital elevation model (DEM). This will then be used to create the slope, hillshade and aspect maps.

This tutorial was also done as a partial requirement of Carleton University's Advanced Topics in Geographic Information Systems 4008 course.

Data

In this tutorial the only necessary layer is a Comma Separated Value (CSV) point elevation file of your area of interest. In this tutorial we are focusing on Carp, ON. It is recommended that you use the Carp, Ontario data for following along. It can be acquired and downloaded here. A sample of this data is shown bellow.

For the event that the dropbox link has become inactive the process of obtaining this data is described below to make this tutorial relevant for longer. Access the Government of Canada Geospatial Data Extraction Application here.

In the “Find a location” bar enter “Carp” and select the following entry from those that appear:

Select the button to center the map on this location.

Next access the bar called “Select clipping area” and select “Current Map Extent”. Under “Select Data to be extracted” Select “Elevation – Point data”. Under “Select options and submit job” fill all of the required boxes so that it matches the figure below. Make sure to enter the email address that you would link to have the file sent to in the “Email Address” field.

An email will be sent to the entered email address shortly which contains a URL for the direct download of the data.

Acquiring QGIS 2.2

QGIS version: 2.18.13 (The current long-term release version of QGIS as of October20th 2017 is QGIS 2.18.13 and this was the version used for this tutorial. A list of the download links for all of the versions of QGIS can be found here.

Getting Started

Launching QGIS 2.2

When the installation process has finished, an icon should appear on your desktop similar to the one shown below.

Double-click on that icon and QGIS 2.18.13 will display on your screen.

Setting Project Coordinate Reference System (CRS)

Following the launch of QGIS 2.18.13, navigate to the top menu bar, select Project, Project Properties, then CRS on the left column of the popup window.

Make sure that Enable 'on the fly' CRS transformation is checked. Once enabled, filter search NAD83 (1) and select NAD83 under the Geographic Coordinate Systems (2). When finished select OK (3).

Adding Point Data

Adding the Carp elevation data or your desired .csv data requires navigating to the menu bar, select Layer and Add Layer, Add Delimited Text Layer....

In the popup window browse for the .csv file that holds the elevation point data. Select CSV (comma separated values) for the file type. The rest of the options should default to the correct settings, but if the popup does not match the figure below make sure that you correct these differences.

Next you will be asked to choose the projection that the data has. As before filter search NAD83 (1) and select NAD83 under the Geographic Coordinate Systems (2). When finished select OK (3).

You should now be able to see your point data in the layers panel but your map view will not be focused on this data. Right click on the point data entry in the layers panel and select Zoom to layer.

Converting Point Data to a Shapefile

Next we will save this CSV file as a Shapfile to allow for more analysis to be done with it. Right click on the point data entry in the layers panel once again and select Save As…

In the popup that appears set the format to ESRI Shapefile, set the file name and directory and ensure that the checkbox beside Add saved file to map is filled. Select OK once this is done.

You should now be ready for analysis.

Processing Toolbox

Open Processing Toolbox

If the Processing Toolbox is not already displayed on the right side of the QGIS 2.18.13 window, under Processing in the menu bar, select Toolbox.

This will display the Process Toolbox on the right side of the screen.

Creating a model using Graphical modeler

To create a model workflow navigate to the menu bar, under Processing, select Graphical modeler.

Once the modeler window has opened, you can begin to create the desired workflow. For this particular tutorial, the Carp elevation shapefile will be used to create a slope analysis workflow.

Before creating the model, you must enter a desired model name and group name (1). I named my model Slope Analysis Turotial under a group name called GEOM 4008. Once a model and group name have been entered, select Save (2), and save the model to your desired folder. I would recommend creating a folder just for QGIS models under this project.

To begin your model, you'll first have to add an input parameter. On the left side of the modeler window, select Vector Layer (3). That will then display a Parameter definition (4) window. Enter the Parameter name as Input elevation points (or something similar), Shapefile to Point, and Required to Yes. Select Ok.

Now navigate to the Algorithm (5) tab, to being adding geoprocessing algorithms.

Adding Geoprocessing Algorithms

3D transformation

The first algorithm that will be added will transform our 2-dimensional point data into a 3-dimensional dataset. This is required because by default point data in QGIS is interpreted as 2-dimensional meaning that only the X and Y values from the attribute table are considered for analysis, not the Z which is the elevation data. The algorithm used can be found by doing a search filter for 3D (1), and select v.to.3d (2) under GRASS Commands. A window will open asking for the specifications (3). Enter the specifications that you desire. Make sure that the Input layer is the elevation point data initially added, and that the Name of attribute column used for height is Z. Also add an Output Vector name of your choice. Select OK.

Inverse Distance Weighted (IDW)

The second algorithm that will be added is an Inverse Distance Weighted (IDW) interpolation. This particular algorithm can be found by doing a search filter for IDW (1), and select v.surf.idw (2) under GRASS Commands.

A window will open asking for interpolation specifications (3). Enter the specifications that you desire.

Make sure that the Input layer is the output from the 2D to 3D transformation.

For my slope analysis, I had to enter Z into the Attribute table column with values to interpolate field because the column under the PointDataShapefile Attribute Table used for elevation points is named Z.

I also used a GRASS region cellsize of 0.0005. This can be changed if you'd like, although it may take longer to process with a smaller cell size value. Also add an Output raster name of your choice. Select OK.

Slope

Following the IDW interpolation, a slope map expressed in degrees will be created using the IDW output.

Execute a search filter for Slope (1), and select Slope (2) under GDAL/OGR.

This will display a window for the Slope geoalgorithm (3). Leave the field parameters as default except for Slope expressed as percent to No. Make sure that the Input layer is the Interpolated IDW output.

Also add an Output raster name of your choice. Select OK.

Aspect

Execute a search filter for Aspect (1), and select Aspect (2) under GDAL/OGR.

This will display a window for the Aspect geoalgorithm (3). Leave the specifications as default. Make sure that the Input layer is the Interpolated IDW output.

Also add an Output raster name of your choice. Select OK.

Hillshade

Execute a search filter for Hillshade (1), and select Hillshade (2) under GDAL/OGR.

This will display a window for the Hillshade geoalgorithm (3). Leave the specifications as default. Make sure that the Input layer is the Interpolated IDW output.

Also add an Output raster name of your choice. Select OK.

Running a model

Once all of the geoprocessing algorithms have been added, the model should look similar to the capture below.

Make sure that you Save (1) the model before Running the model (2).

When the Run model button is select, a window the same as the capture below will appear.

In this window make sure you save the output files to an appropriate destination folder. Select OK to run the model.

Also check to make sure that PointDataShapefile is the Input elevation points that will be Transformed first in the model.

Once the model is finished running, the output layers will be displayed in the Layer toolbox on the left side.

Displaying output rasters from model

Once the model is run and the output rasters appear in the Layer toolbox. They will only appear as single band image styles.

One way to change this is to change this right click on Output IDW, select Properties.

A Layer Properties window will appear. Navigate to Style (1), select the Render type to Singleband pseudocolour (2). The rest of the properties can be changed to your personal desire. For my output I selected Spectral (3) for the color and Quantile mode (4) and then selected OK (4).

This process can be done for any of the output raster layers. The IDW output was done as an example. The output should look similar to the capture below, depending on the cell size used in the modeling process.

It is also possible to change the transparencies of the layers and overlay them on top of one another for more visually pleasing final maps. The following map was created by setting the transparency of the IDW layer to 10% and overlaying it onto of the Hillshade layer.

Exporting your model

As a Python script

When your model is finished you can export the model as a Python script.

To do so, right click on the model in the Processing toolbox and select Save as Python script.

This Python script can be edited in a program such as Notepad++.

As an image

If you'd like to print out the model or include it in a report. It is possible to save the model as an image for future reference.

Just re-open your model again from the Processing toolbox and select the Export as image button in the modeler window.

Raster Calculations

In addition to creating a model using the Processing toolbox, the QGIS raster calculator can be used to input certain expressions to preform multiple geoprocessing tasks. For the purposes of this tutorial I will be creating an output Slope map with slopes between 5 and 10 degrees.

First navigate to the menu bar, under Raster, select Raster Calculator....

A Raster calculator window will open. Inside the expression field insert the following expression (1): ( "Output Slope@1" >= 5 ) AND ( "Output Slope@1" <= 10 ). I would recommend manually clicking the operators to avoid typing error. When done entering the expression, at the bottom of the window it should say "expression valid".

Remember to save the output as something that is easily recognizable (2), example, SlopeCalc.tiff. Select OK (3).

When finished you should have an output map like the one indicated below. Slopes between 5 to 10 degrees are represented in white and all other slopes in black.

The following process can be done for any of the output terrain maps. Slopes between 5 to 10 degrees was done as an example.

Conclusion

In conclusion, this tutorial was aimed to give the user the skills to create workflows using the QGIS Processing Toolbox while also creating a workslope for slope and terrain analysis. This tutorial also gives the user the very basic introduction into the GIS world of Python scripting as well. The Python scripts that were exported can be slightly edited to preform the same geoprocessing tasks but with different parameters if necessary.

Resource Links

QGIS version 2.2 Documentation

GDAL - Geospatial Data Abstraction Library

Aspect - GIS Dictionary

Hillshade - GIS Dictionary

Interpolation - GIS Dictionary

Slope - GIS Dictionary

Python - Learn Python the Hard Way

Notepad++ Homepage

References

Crosetto, M., Tarantola, S., & Saltelli, A. (2000). Sensitivity and uncertainty analysis in spatial modelling based on GIS. Agriculture, Ecosystems and Environment, 81(1), 71-79. doi:10.1016/S0167-8809(00)00169-9

Hutchinson, M.F., 1998. Interpolation of Rainfall Data with Thin Plate Smoothing Splines – Part II: Analysis of Topographic Dependence. Journal of Geographic Information and Decision Analysis, vol.2, no. 2, pp. 152-167, 1998.

R.E. Crochiere and L.R. Rabiner. (1983). Multirate Digital Signal Processing. Englewood Cliffs, NJ: Prentice–Hall.