View source for Slope Analysis using Quantum GIS Processing Toolbox

==Purpose==
The purpose of this tutorial is to guide the [http://www.qgis.org/en/site/ Quantum GIS (QGIS)] user through the steps necessary to create a slope analysis model workflow using the [http://docs.qgis.org/2.0/en/docs/user_manual/processing/toolbox.html 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 [http://en.wikipedia.org/wiki/Python_%28programming_language%29 Python] script as well as preform geoprocessing tasks using the QGIS [http://docs.qgis.org/2.0/en/docs/user_manual/working_with_raster/raster_calculator.html 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 [https://plugins.qgis.org/ "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 [http://en.wikipedia.org/wiki/Carp,_Ontario Carp, Ontario] in order to create a [http://en.wikipedia.org/wiki/Digital_elevation_model 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.

[[File:QGIS2.18.13.png|center]]

==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 [https://www.dropbox.com/s/651jlnr63k92qxc/CarpPointData.csv?dl=0 here].
A sample of this data is shown bellow.

[[File:ExampleData.png|center]]

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 [http://maps.canada.ca/czs/index-en.html here].

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

[[File:Extent.png|700px]]

Select the [[File:Zoom.png]] 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.

[[File:DataAccess.png|center]]

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 [http://www.qgis.org/en/site/forusers/download.html 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.

[[File:DeskIcon.png|center]]

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.

[[File:ProjectionSelection.png|center]]

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''' '''(1)'''. When finished select '''OK (3)'''.

[[File:ProjectionProperties.png|center]]

===Adding Vector Shapefiles===

Adding the Carp shapefiles or your desired shapefiles requires navigating to the menu bar, select '''Layer''' and '''Add Vector File...'''.

[[File:Image004.png|center]]

A dialogue box will open asking for the destination where the shapefiles are located. Select '''Browse''' and navigate to the folder where you saved the shapefiles. Select '''Open'''.

[[File:Image005.png|center]]

==Processing Toolbox==

===Open Processing Toolbox===

If the Processing Toolbox is not already displayed on the right side of the QGIS 2.2 window, under '''Processing''' in the menu bar, select '''Toolbox'''.

[[File:Image006.png|center]]

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

[[File:Image007.png|center]]

===Creating a model using Graphical modeler===

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

[[File:Image008.png|center]]

Once the modeler window has opened, you can begin to create the desired workflow. For this particular tutorial, the Carp shapefiles 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 enter, 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.

[[File:Image009.png|center]]

===Adding Geoprocessing Algorithms===

====Inverse Distance Weighted (IDW)====

The first algorithm that will be added is an '''Inverse Distance Weighted (IDW)''' [http://en.wikipedia.org/wiki/Interpolation 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 elevation point data initially added.

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

I also used a '''GRASS region cellsize''' of '''10'''. 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'''.

[[File:Image010.png|center]]

====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 initial IDW output created (algorithm 0).

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

[[File:Image011.png|center]]

====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 initial IDW output created (algorithm 0).

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

[[File:Image012.png|center]]

====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 initial IDW output created (algorithm 0).

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

[[File:Image013.png|center]]

===Running a model===

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

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

[[File:Image014.png|center]]

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 '''carpdem''' is the '''Input elevation points''' that will be interpolated first in the model.

[[File:Image015.png|center]]

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

[[File:Image016.png|center]]

===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'''.

[[File:Image017.png|center]]

A '''Layer Properties''' window will appear. Navigate to '''Style (1)''', select the '''Render type''' to '''Singleband pseudocolour (2)'''. Under '''Generate new colour map''' settings, the properties can be changed to your personal desire. For my output I selected a '''Continuous''' mode and selected '''Classify (3)''', then '''OK (4)'''.

[[File:Image018.png|center]]

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.

[[File:Image019.png|center]]

[[File:Image029.png|center]]

===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'''.

[[File:Image020.png|center]]

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

[[File:Image021.png|center]]

====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.

[[File:Image022.png|center]]

==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...'''.

[[File:Image027.png|center]]

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)'''.

[[File:Image028.png|center]]

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.

[[File:Image026.png|center]]

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==
[http://docs.qgis.org/2.2/en/docs/index.html QGIS version 2.2 Documentation]

[http://www.gdal.org/ GDAL - Geospatial Data Abstraction Library]

[http://support.esri.com/en/knowledgebase/GISDictionary/term/aspect Aspect - GIS Dictionary]

[http://support.esri.com/en/knowledgebase/GISDictionary/term/hillshading Hillshade - GIS Dictionary]

[http://support.esri.com/en/knowledgebase/GISDictionary/term/interpolation Interpolation - GIS Dictionary]

[http://support.esri.com/en/knowledgebase/GISDictionary/term/slope Slope - GIS Dictionary]

[http://learnpythonthehardway.org/ Python - Learn Python the Hard Way]

[http://notepad-plus-plus.org/ 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.