1 Introduction

The practical will introduce you to digital elevation models (DEMs) - a key data model for any applications which are concerned with the surface of the Earth. Obvious applications areas include physical geography and environmental sciences, more specifilcilly erosion monitoring, hydrological modelling or civil engineering. The Ordnance Survey of Great Britain (OSGB) provides a free DEM for the whole of GB with a 50m spatial resolution. In other words, each cell in the grid covers an area of 50 metres by 50 metres. The data are provided as tiles of 10km by 10km and in this practical you will work with one of these tiles. The product is called OS Terrain 50 and more information about it is provided here:

https://beta.ordnancesurvey.co.uk/products/os-terrain-50

OS OpenData (of which this DEM is part) is licensed under the Open Government Licence (OLG) which allows you to use the OS OpenData datasets in any way and for any purpose. The OS ask that you acknowledge them with "Contains OS data @ Crown Copyright and database right 2024". This text must be added to the caption for all of the maps you create using the DEM provided.

1.1 Learning Aims and Objectives

In this practical you are tasked with analysing the form of the terrain in the study area. You will be developing the following GIS skills and understanding:

  1. Working with a raster grid data set (a digital elevation model).
  2. Deriving slope, aspect and 3D maps.
  3. Visualising and classifying raster grid data.
  4. Developing a narrative from the interpretation of selected maps.

1.2 Assignment

As in the previous GIS exercises, you will utilise QGIS. You will be using the software to map and to analyse a topographic surface. The surface is represented using a grid (raster) of elevation values and this is referred to as a digital elevation model (DEM). You will submit an assignment which includes three maps and some text (maximum 250 words), which summarises your findings following guidance provided at the end of this document.

There are two key tasks which you should undertake:
1. Create a map of (i) slope, (ii) surface roughness and (iii) areas with larger slope and small roughness values
2. Write a 250 word summary that describes the key characteristics of your maps

2 Part 1 - Raster data and analysis

2.1 Reminder

Geographical Information Systems (GIS) are powerful sets of tools for collecting, sorting, retrieving, transforming and displaying spatial data. GIS systems store information using spatial reference points. They are similar to other databases consisting of records but the records are referenced to digital mapping reference points or objects. There are two main ways of expressing spatial data:

  • Vector Data - these reference points or objects come in three discrete forms: a single point (e.g. a mountain summit, pollution sources), a line or vector (e.g. a narrow river, cliffs), or an area or polygon (e.g. a county or state, lakes). Vector data represents geographic features with lines, points, and polygons.

  • Raster Data - such as images and grids. Raster data involves dividing an area into discrete squares called cells. Cells are laid out in a grid, where each cell has a location relative to the origin and a value describing the feature being observed (for instance, the cell values in an aerial photograph represent the amount of light reflecting off the earth’s surface. A raster can represent: thematic data such as land use, temperature, and elevation; spectral data such as satellite images and aerial photographs; and, pictures such as scanned maps and building photographs.

2.2 Obtaining and manipulating data

2.2.1 Getting the data (Video 1.1)

  • The data are available on Canvas under GIS > Practical 3 > GIS 3 data

  • Create a new data directory (e.g., M:) and copy the data into that directory.

  • Unzip the data by:

  • Windows: (select the file (GIS3data.zip) right click > Extract All and specify the output folder).

  • Mac: Double tap the folder to extract. The unzipped folder will have the same name as the zipped folder and be placed in the same directory folder

  • Save your QGIS project into M:: Project > Save as and specify GIS3 as the name. You should save the project regularly (Project > Save, or click Save as you progress through the exercise.

2.2.2 Adding raster data into QGIS

  • First, open QGIS Desktop.

  • To open the data, go to Layer > Add Layer > Add Raster Layer navigate to your working directory and double click on the file SH65.asc, then click the Add > Close buttons

  • NOTE: If a new window pops up asking to Select Transformation for SH65, click Cancel

AddRaster
- Alternatively you can drag and drop the file to the Layers Panel

  • To see the range of elevation values click on the arrow sign to the left of SH65 in the Layers Panel: Range

You will see that the black is used to depict the lowest elevations and white the highest. The DEM you are working with covers part of the Ordnance Survey tile SH and it contains the summit of Snowdon in North Wales. Note that the coordinates corresponding to the position of the mouse cursor change as you move the mouse. The summit of Snowdon is at approximately 260986, 354375 (the units are metres).
AddRaster ————————————————–

Ordnance Survey National Grid System
The position of any point in Great Britain can be described by its National Grid reference. National Grid co-ordinates are expressed as distances measured in metres east and north from the grid origin (which is located to the west of the Isles of Scilly). These grid references are also used to identify individual Ordnance Survey maps and DIGIMAP data files. For example, a point could be described by grid reference SJ 46732 68791. An area could be covered by data files SJ46. The National Grid is divided into 100 km x 100 km squares, each identified by two letters. If you look at the map to the right, you will see that Snowdon is in square SH (See Lecture 2 - part 1).

This Ordnance Survey grid system is integral to the QGIS package, although it is capable of interchanging between many different project and co-ordinate systems.

2.2.3 Changing the display colours (Video 1.2)

Next, you will change from the greyscale scheme to one using a range of colours.

  • Right click on SH65 in the Layers Panel and select Properties. Make sure the Symbology tab is selected.
  • Under Render type select Singleband pseudocolor. Ensure Spectral is selected under the Color ramp scroll down menu.
  • Then, click Classify and then OK.
  • Click on the arrow sign to the left of SH65 layer in the Range1.

Now you can see that reds are used for low elevations (min 53.4) and blues for large elevations (max 1080.5).

Note that, often, QGIS specifies the minimum and maximum in the colour scheme as just above the true minimum (2% of the way through the values ordered from minimum to maximum) and just below the true maximum (98%).

  • If that’s the case change to the true minimum and maximum by going back to Properties > Symbology
  • Then, under Min/Max Value settings change the settings to Min / max and then click Apply. Then click Classify and then OK
  • Experiment with the other options available: Cumulative count cut and User defined see how the values change

2.3 Computing slope and aspect (Video 1.3)

Maps of terrain slope (or gradient) and aspect (the direction faced by the slope - e.g., a slope could be north-facing) are used in many applications from route-finding to erosion and hydrological modelling. In this part of the exercise, you will compute slope and aspect maps.

QGIS provides a host of tools for the analysis of DEMs.

  • To access these tools, go to Raster > Analysis
    Add

First, you will compute a slope map. To do this:

  • Click on Slope choose SH65 as your Input layer, check the box next to Slope expressed as percent instead degrees. Note that slope can be specified in percent or degrees, but in this exercise we use percent, but if you wish to compute degrees, then you can do that too;

  • Then click on the browse button and make sure that your data folder is specified as your working directory.

  • In the File name box type SH65slope, then click Save

  • Click Run.
    Add

  • Note: If you didn’t save the newly computed Slope to your working directory, the file will be temporary. You can work with it, but it will be lost once you close your GIS project.

  • Close the Finished window and the qgis-bin window, if it appears.

  • Next, as you did for the DEM (that is, the elevation values in SH65), assign colours to the slope values (again using the Singleband pseudocolor option) and make them visible in the Layers Panel.

The gradual slopes should appear in red and the steep slopes in blue.

  • Next, create an aspect map - again open the Raster > Analysis tools but this time select Aspect, select SH65 as your Input layer and specify the output as SH65aspect
  • Check the box next to: Return trigonometric angle instead of azimuth
  • Once the aspect layer has been computed turn off the SH65 and SH65slope layers by clicking on x to the left of their names in the Layers Panel.

The aspect values are between 0 and 360 degrees: 0 indicates that the slope is facing the North, 90 faces the East, 180 faces the South and 270 faces the West (assuming that north is the top of your input raster). The aspect value -9999 is the no data value and it indicates an undefined aspect (flat areas with slope=0).

As for SH65 (elevation) and SH65slope, change the greyscale to Singleband pseudocolor and make the colours visible in the Layers Panel.

2.4 Measuring other terrain characteristics

The other Modes available in the DEM tools (in Raster > Analysis) that you will use are:

Hillshade: This provides an output grid which depicts a terrain surface with a shaded relief effect. The shade effect can be altered by specifying the direction (azimuth) and altitude of a light source.

Terrain ruggedness index (TRI): The mean difference between a central pixel and its surrounding cells (the elevation of each cell is compared to the elevations of the cells which it neighbours).

Topographic position index (TPI): The difference between a central pixel and the mean of its surrounding cells (the elevation of each cell is compared to the mean elevation of the cells which it neighbours).

Roughness: The largest inter-cell difference of a central pixel and its surrounding cell (the largest difference between a cell and any of the cells which it neighbours).

You should compute each of these in turn and experiment with changing the colour schemes used to display the outputs. The file output names should be SH65hills, SH65TRI, SH65TPI and SH65rough.

The file 'Wilson et al., (2007).pdf' (available under GIS in Canvas) is a paper that details these measures. Feel free to take a look if you'd like to know more.

2.5 3D visualisation

The DEM model of Snowdonia can be also easily visualised in 3D using QGIS.Follow the steps below:

  • Go to View menu > 3D Map Views > New 3D MapView and expand the 3D window

  • In the 3D Map 1 window click the Configure tool, then Terrain.

  • Select DEM (Raster Layer) as your Type and choose the SH65 layer as your Elevation

  • Set the Vertical scale to 3 and make any changes to the other settings if you wish, hit OK 3D settings

  • In the 3D Map 1 window hold down the Tilt Down button (located just under the compass symbol) until you have a good horizontal view of the model, you can also use zoom in/out and rotate view functions on the right hand side to appreciate more the 3D map. Your output should look similar to the one below:

3D view
3D view
  • Note: If the Compass Symbol is missing, click on the Toggle On - Screen Navigation tool

2.6 Summarising the raster cell values (Video 1.4)

Summary statistics for a raster grid can be obtained using the Properties of each layer:

  • Properties > Information and look under Properties for Band 1. Do this for the layer SH65. The figures (to one decimal place; the units are metres) are:

STATISTICS_MAXIMUM=1080.5
STATISTICS_MEAN=440.6
STATISTICS_MINIMUM=53.4
STATISTICS_STDDEV=203.6

As you are probably aware, the standard deviation is the square root of the average squared difference between the values and the mean of the values. So, a small standard deviation indicates that the values are close to the mean (the values tend to be similar) while a large standard deviation indicates that values tend to be far from the mean (the values tend to be dissimilar).

Histograms provide a graphical summary of the frequency of values in a dataset. This is best illustrated with an example.

  • Right click on SH65 in the Layers Panel. Select Properties > Histogram.

  • Repeat this for the SH65slope layer
    Add

  • To save the histogram click the Save plot button

  • Save the file to your working directory and name it SH65slope_Hist ensuring that Save as type is TIFF.

You will use this tool again later.

2.7 Combining information (Video 1.5)

You now have a range of raster grids which depict particular terrain characteristics. It is straightforward in a GIS environment to identify cells which have a combination of characteristics. In QGIS, this can be done using the raster calculator.

  • Go to Raster > Raster Calculator

Firstly, you will identify all cells which have a slope of greater than or equal to 30 percent.

  • Specify the output layer as SH65slopeGE30

  • In Raster Calculator under Raster bands double click ‘’,

  • Click on the >= sign and then type 30. The expression should then appear as shown on the picture below.
    Add

  • Click OK.

You now have a new map where 0 = cells with slopes of less than 30 percent and 1= cells with a slope of greater than or equal to 30 percent.

Runoff has an impact on soil erosion and speed of runoff depends on slope and roughness whereby areas which are rougher may reduce runoff and thus be subject to less erosion (although that may be difficult to assess using these data). So, you will combine the new slope map with a map showing cells with a relatively small roughness value (less than or equal to 80).

Open Raster calcultaor and following the instructions above, and this time using SH65rough as your input layer, create a new map which indicates cells with a roughness value of less than or equal (<=) to 80. Name the output SH65roughLE80.

Expression
Expression

Next, multiply the two new maps together to create a new raster called SH65sl30ro80. In this case, the expression should read: ‘’ * ‘’

3 Part 2 - Producing publishable maps (Video 1.6)

You should create a map of (i) slope (SH65slope), (ii) surface roughness (SH65rough) and (iii) areas with larger slope and small roughness values (SH65sl30ro80).

The GIS 1 exercise introduced map creation in QGIS. The key steps you need to follow are repeated here using SH65rough as an example:

You will need to switch between these two options to reorganise your map.

3.0.1 Add a legend:

  • Go to Add Item > Add Legend. Click on the white space around the map using the left mouse button. You can then click on and move the resulting legend.

  • To change the legend text to something more appropriate, go back to the main QGIS window, right click on SH65rough in the layer list and select Properties.

  • Select the Source tab and in the Layer name box replace SH65rough with a label which reflects what you have mapped (e.g., Roughness index). Alternatively right click the layer and click Rename Layer

  • Then go back to the Print Layout window and select the legend window; under Item Properties > Legend > Legend items, the label should then be automatically updated.

  • To remove layers from the legend that are not required, untick the Auto Update box. Click on the layer you would like to remove and click the minus sign This will remove that layer from the legend.

3.0.2 Adding a scale bar

  • Go to Add Item/Add Scale Bar and left click on the map.
  • In case the scale bar isn’t showing up, make sure that a correct CRS is used as your QGIS project coordinate system - this should be British National Grid EPSG:27700.
  • Check the bottom right-hand corner of your QGIS map and if it says Unknown CRS then,
  • click on it or go to Project > Properties > CRS and type 27700 in the Filter box. select British National Grid and then click Apply and OK
CRS
CRS
  • To change the scale units, values and the number of segments, adjust the values under Item Properties.
  • Under Units > Scalebar units open the drop down menu and choose Meters
  • Then go to Segments and use one segment on the left and two on the right.
  • In the Appearance > Line style window change Line width to 0.2mm
  • Under the Font menu change the font size to 12

3.0.3 Adding a north arrow

  • Go to Add Item > North Arrow and draw the extent of it on the map.
  • Alternatively under Item properties > Picture > SVG browser choose one of the SVG images of a north arrow you like

Once you have finished your map, go to Layout > Export as Image.

Specify the output type as TIFF and name the file in a sensible way - e.g., call it Roughness if it is a map of roughness.

The image files (TIFFs) you have saved should be inserted into a Word document (Insert > Picture.) and your written report should be provided in the same document. When you insert the map, be sure to include the following text below it: CopyR

4 Assessment

You should have created THREE maps: (i) slope (SH65slope), (ii) surface roughness (SH65rough) and (iii) areas with larger slope and small roughness values (SH65sl30ro80).

Using your knowledge of the tools outlined in this practical you should write a MAXIMUM of 250 words describing the geographical patterns you see in your three maps and how the maps of slope and roughness relate. For example, do most areas with larger slope values have large roughness values? Also, what is the range of slope and roughness values judging by their summary statistics and the histograms, and are these similar to one another?

4.1 To submit on Canvas

To submit on Canvas you need to export the document (containing three maps and your text) as a PDF file. In Word, select Save As and set Save as type to PDF, then click Save, at this point the file will be saved as a PDF which can be uploaded onto Canvas.