P6 - GGY283 - 2021 - Instructions and Notes

GGY 283 Introductory Geographic Information Systems PRACTICAL 6
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PRACTICAL 6: GEOREFERENCING, DIGITISING, FIELD CALCULATIONS AND DISPLAY

Contents
LIST OF FIGURES ...................................................................................................................................... 1
LIST OF TABLES ........................................................................................................................................ 2
MAIN OUTCOMES (What must you submit?) ......................................................................................... 3
TASK 1 – main outcomes..................................................................................................................... 3
TASK 2 – main outcome ...................................................................................................................... 3
SKILLS ...................................................................................................................................................... 4
SOFTWARE .............................................................................................................................................. 4
ADDITIONAL HELP ................................................................................................................................... 4
PRACTICAL DATA ..................................................................................................................................... 4
TASK 1 ..................................................................................................................................................... 5
Task 1.1 Explore, Clip and extract project data .................................................................................. 5
OPEN A NEW PROJECT .................................................................................................................... 5
SELECTING DATA BASED ON ATTRIBUTES (Attribute query) .......................................................... 6
Task 1.2 Creating features from coordinates (E.g. GPS coordinates) ................................................. 8
Task 1.3 Joining Attributes by Field values ......................................................................................... 9
Task 1.4 Georeferencing images ....................................................................................................... 12
Task 1.5 Digitise new spatial features (Digitize dam surface area for 2014 and 2017) .................... 17
Task 1.6 Calculate the surface area of the dam for the various time periods. ................................. 20
Task 1.7 Creating, exporting and submitting a suitable map layout ................................................ 21
TASK 2: PEER REVIEW ASSIGNMENT ..................................................................................................... 22
References ............................................................................................................................................ 23
LIST OF FIGURES
Figure 1: Example - QGIS browser .......................................................................................................... 5
Figure 2: Example of loaded data layers ................................................................................................. 5
Figure 3: Clip ........................................................................................................................................... 5
Figure 4: Extract from the WC_dams attribute field .............................................................................. 6
Figure 5: Select by Expression function .................................................................................................. 6
Figure 6: Extract for attribute table with filter options below. .............................................................. 7
Figure 7: Save selected features ............................................................................................................. 7
Figure 8: Save selected features parameters and result ........................................................................ 8
Figure 9: Field with only one decimal value after refacturing .............................................................. 12
Figure 10: Georeferencer windows and toolbar................................................................................... 14
Figure 11: Example of control points table for the 2014 image ........................................................... 15
Figure 12: Georeferenced 2014 image ................................................................................................. 15
Figure 13: Example of 2017 image control points and table ................................................................ 16
GGY 283 Copyright University of Pretoria 1

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Figure 14: Example of a georeferenced 2017 image ............................................................................ 16

Figure 15: Digitizing in progress ............................................................................................................ 18
Figure 16: Example of an attribute table for the digitized 2014 Theewaterskloof water surface
polygon ................................................................................................................................................. 18
Figure 17: Example of the digitised 2014 polygon (red boundary) ...................................................... 19
Figure 18: Adjusted vertices ................................................................................................................. 19
Figure 19: Example of a 2014 polygon .................................................................................................. 20
Figure 20: Example of a 2017 digitised polygon ................................................................................... 20
Figure 21: Calculating the geometry of a field. ..................................................................................... 21
LIST OF TABLES
Table 1: Practical 6 data description ....................................................................................................... 4
Table 2: Example of surface area data table......................................................................................... 21
Table 3: Summary of important aspects that the final map layout must adhere to – these aspects will
form the basis of the peer marking criteria. ......................................................................................... 22

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MAIN OUTCOMES (What must you submit?)

For this practical exercise you must complete TWO tasks:
TASK 1 – main outcomes

A task group on water management in the Western Cape requires an A4 map layout which clearly
shows the following:
 Five main dams relevant to the water to their area of interest: Wemmershoek, Berg River,
Theewaterskloof, Steenbras (Upper) and Steenbras (Lower).
 The change in dam surface area at specific intervals for the Theewaterskloof dam in the
Western Cape. The map layout must specifically show:
o the decline in surface area between 2012 and 2017, e.g.
o specific surface area figures for 2012, 2014 and 2017 in Hectare (Ha), e.g.
Changes in dam surface
area
Year Surface area (Ha)
2012
2014
2017
o a graphic presentation of the dam levels for the last 5 years, e.g. you can use simple
diagrams like histograms from the layer properties options, e.g.
o rivers in the area of interest.
NB! Remember to use orientation layers to give context to your map layout. Whether to add an inset
map or grid is your own logical decision. Your map must have the required map elements and adhere
to established map design principles. The final map layout must be submitted as a PDF document on
clickUP under Assessments and Tasks→Practical Classwork Submissions→ P6 Practical 6 Map
submission.
TASK 2 – main outcome

You must evaluate the maps submitted by three other students (as well as your own) using the online
rubric under Practical 6 Task 2 Peer review.

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SKILLS
The aim of this practical is to ensure that you can:
 Clip a vector dataset;

 Select and extract data based on layer attributes;
 Geo-reference an image;
 Add data from a cloud-based server to a QGIS project;
 Covert and display coordinates;
 Join layer attributes;
 Create and edit features (digitizing);
 Add fields and complete field calculations;
 Communicate results by creating and exporting a suitable map layout;
 Evaluate (with integrity) maps as produced by your peers.
SOFTWARE
This practical was created using QGIS. Your computer may have a slightly different version loaded.
If/When instructions do not reflect items on your screen exactly, please use common sense and an
investigative approach to continue with the exercise.
ADDITIONAL HELP
GGY 283 clickUP page – Videos and help files – QGIS videos and the QGIS manual.
The manual is also available online at https://docs.qgis.org/3.16/en/docs/user_manual/index.html.
PRACTICAL DATA
Data data layers for this practical are available on clickUP in the GGY283_2021_Practical6_data zipped
folder. Descriptions for the data files are listed below. Remember to unzip the folder after
downloading and storing. You may also use web layers like OSM data from “Quick map services” for
orientation and when geo-referencing.
Please note that this is a dataset created for training purposes only and it should not be used for any
other purpose.
Table 1: Practical 6 data description
Data file Description

Roads_WC.shp Western Cape Roads
Mainroad.shp Western cape main roads
Cape_Town.shp Location point for Cape Town
Major_Towns_and_cities_polygon.shp Major towns and cities in the Western Cape
WC_Dams Western Cape dams
WC_Province Western Cape provincial boundary
Hydro_Riv_Pri.shp South African Primary rivers
Hydro_Riv_Sec.shp South African Secondary rivers
Theewaterskloof2012.shp Theewaterskloof dam outline from 2012
Image_Theewaterskloof_2014.jpeg Theewaterskloof image from 2014
Theewaterskloof2017_2cl.jpg Theewaterskloof image from 2017 (Source: Google earth)
Dam_Levels_WC_main.xls Excel file with dam levels (%) for main Cape Town reservoirs
2014-2021

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TASK 1
Task 1.1 Explore, Clip and extract project data
OPEN A NEW PROJECT
Purpose: To open a new dedicated project for this assignment.
1. Open a new empty QGIS project and use the Browser to explore the properties of the data
layers as you did in the previous practical exercises. Note that:
a. The shapefiles have a geographic coordinate reference system (CRS), but they are not
projected.
b. The images have no CRS.
c. The dam level data is an Excel file
Figure 1: Example - QGIS browser
2. Add the rivers, roads, dams, province and cities&towns shapefiles.
Figure 2: Example of loaded data layers
3. Reduce the size of the dataset by clipping the two river layers to the Western Cape boundary.
Select the layer – choose Vector-Geoprocessing tools-Clip. Save the clipped layer in your data
folder as a shapefile with a suitable name. (Short names without spaces or punctuation is best)
Figure 3: Clip
(Note: You can clip each layer separately or both together as a batch process)
4. Save the project in your data folder with a suitable name.

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SELECTING DATA BASED ON ATTRIBUTES (Attribute query)

Purpose: For this exercise you are only interested in the five main dams that supply Cape Town. You
will select them and create a new layer with only the dams of interest.
1. Right-click WC_Dams and select Open Attribute Table. Explore the available fields.
Figure 4: Extract from the WC_dams attribute field
2. You can use the NAME field to select only the five dams that you have a particular interest in.
3. You can make a selection from the attribute table menu or from the project main menu. To
use the attribute table menu:
4. Select the (Select by Expression) function.
5. Use the Wizard window to complete the Expression:

"NAME" = 'Berg River' OR "NAME" = 'Steenbras (Lower)' OR "NAME" = 'Steenbras (Upper)'
OR "NAME" = 'Theewaterskloof' OR "NAME" = 'Wemmershoek'
Figure 5: Select by Expression function
6. Then click on Select Features and close the window.

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7. You will see that there are now 5 features selected in the attribute table:
Figure 6: Extract for attribute table with filter options below.
8. Choose Show Selected Features at the bottom of the table to see only the five selected
records in the attribute table, e.g.
9. The selection will also show in yellow in the map window. If you right click on the dams layer
name, you can export the selected features as a new file.
Figure 7: Save selected features

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10. Save the new shapefile in your data folder with a suitable name:
Figure 8: Save selected features parameters and result
11. Remember to save your project.
Task 1.2 Creating features from coordinates (E.g. GPS coordinates)

Purpose: You want to display the dam level coordinates as points in your project to make sure that
the information is correctly added.
1. Open the Dam_Levels_WC_Main.xls in MS Excel or a similar spreadsheet application (not in

ArcGIS).
2. The Latitude and Longitude fields have the Geographic coordinates for the dams.
3. Note that there are inconsistencies in the way that the coordinates have been entered.
4. GIS software may require specific formats and QGIS may be able to read various formats
depending on the version of your software etc. For this exercise we will be using the Add
delimited text layer tool in the layers Toolbar or from Layers, Add layer in the top menu.
5. To add the dams to your map, you must first calculate the coordinates, for the six dams in
decimal degrees (Lecture 7 – Slide 25) and then save the result in your data folder in a .csv
delimited format. (Reminder: Southern latitude should have a negative sign)
NOTE: You can do this manually or in MS Excel (or a similar application) – please see the P6a
Helpfile if you need assistance.
6. Once you created the .csv file, you can add the coordinates as points to your project.
7. Select the Add delimited text layer tool.
8. Select your data folder location and give the layer a suitable name. Use the newly created Lat
& Long columns in Decimal degrees. (Note the issues that QGIS has with the format in some
of the other columns. Click Add and close the window.

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9. The points will display in the map window – you can alter the default symbol and colour if
needed.
10. Save your project!
Task 1.3 Joining Attributes by Field values

Purpose: You want to add the dam level information from the dam level layer to the layer with main
dam polygons.
1. Look at the attribute information in the Dam level ‘points’ layer and the layer with the five
main dam polygons. You will see that the dam names can be found in both attribute tables.
2. You can use the dam names to join the dam level information to the polygon layer.
3. Select the polygon layer (5 main dams) in the Layers panel.
4. Open the Processing Toolbox.

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5. Search for and open a suitable join tool: Join attributes by field value.
6. Read the tool information and think! The polygon layer is the Input layer 1 and the point layer
is Input layer 2. The field in the table that you want to use to join the information is the name
field in both layers. Save the joined layer in your data folder with a suitable name. Choose a
shapefile format. Then also choose the fields that must be added to your new polygon layer
and select RUN.

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7. Close the processing window

8. The new layer should be available to your project – Open the attribute table:
9. You may find the several decimal places were added to the dam level fields. If this is not
suitable, you can use a tool like the Refactor fields tool (note that you can save the result as
a new layer) to change the field format, e.g.
10. Adjust the field precision on the dam level fields: e.g.
11. Select “Run” and view the resultant file’s attribute table to examine the result:

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Figure 9: Field with only one decimal value after refacturing
Task 1.4 Georeferencing images

Purpose: In order to digitise the dam surface area of the Theewaterskloof dam for 2014 and 2017,
you need to geo-reference (add location on earth) the two pictures.
Note: If you struggle to understand this process – please watch the video “Georeferencing with
QGIS” as available on clickUP under Videos and help files - QGIS videos.
1. Zoom in to the Theewaterskloof dam area. (Use the identify button or labels on the dams layer
to identify the dam.)
2. Order the layers so that you can see the Roads_WC layer, the selected main dams and a
basemap, e.g. Open Street Map.
3. Change the symbology of the main dams layer to “No brush” so that you it does not cover the
layers underneath but you can see the outline.
4. Use the Freehand raster georeferencer tool to add control points to the images and geo-
reference them. One image at a time.
5. With the 2014 raster layer selected in the browser window – choose Raster – Georeferencer,
e.g.

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Note: If your Georeferencer tool is not available you may need to add the plug-in:
Choose Plugins- Manage and Install Plugins
Then search for, and install, the georeferencer plugin.
6. Choose the transformation settings and file name and location that you want to apply. Make
sure to tick “Load in QGIS when done”, e.g.

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7. The georeferencer window has two windows and several tools additional to the normal pan and
zoom tools.
Figure 10: Georeferencer windows and toolbar
8. Zoom in to a feature (like a road crossing) that you can identify on the image in the tool window
AND on the map.
9. Choose the Add point tool to add control points. Click the location on the image and then
choose “From map canvas” and click on the corresponding point on the map. Use the roads layer
and the basemap and try to get the location as good as you can.
E.g.

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10. Add as many points as you can identify and make sure that they are distributed over the entire
area. Try to keep the Residual values and Mean error value as low as possible. Move or delete
points that have high residual values (e.g. higher than 4), e.g.
Figure 11: Example of control points table for the 2014 image
11. When you are happy with your points, hit the run button and close the toll when
completed.
12. The image should display mostly correctly in your project map window.
Figure 12: Georeferenced 2014 image

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13. Then follow the same process (steps 5-11) to geo-reference and display the 2017 image. (Note:
It is more challenging to find good control points for the 2017 image because the raster
resolution is courser. Just do your best and try to get at least 6 points with a reasonable
distribution.)
Figure 13: Example of 2017 image control points and table
Result
Figure 14: Example of a georeferenced 2017 image

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Task 1.5 Digitise new spatial features (Digitize dam surface area for 2014 and 2017)
Purpose: You must create polygons for the surface area covered by the Theewaterskloof dam in 2014
and 2017 respectively so that you can calculate the surface area and illustrate the difference between
the two years on a map.
1. Start by creating a new shapefile for the 2014 dam surface area – using the main menu.
2. Choose your file location and give the file a suitable name. Indicate the geometry type. Will it be
a Point, Line or Polygon layer? Leave the Selected Coordinate Reference System (CRS) as the
default WGS 84. Add a name field (or a year field) if you want to – you can add the required area
field later. Click OK to add the layer.
3. You will see that the new shapefile was added to the ‘Layers Panel’ but nothing displays in the
map window. That is because you have not digitized the polygon yet.
4. Select the newly created file name in your project – right click and choose Toggle Editing.

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5. You should now see new options activated in the top ribbon. You can explore all of them by
hovering with your mouse over them, but in this simple exercise you will mostly need three
options:
 Add Polygon Feature – to start digitizing the dam water surface.
 Vertex tool – to adjust vertices if you need to.
 Toggle editing – to save your edits and exit editing mode.
6. Select and start digitising by clicking (adding vertices) all around the area
covered by the dam water surface in the image. (Notes: Try to be do this faily well, but it does
not have to be perfect because you can update the vertices later. Alternate the “Add polygon”
tool with the Zoom and Pan tools as you go along)
Figure 15: Digitizing in progress
7. When you are happy that you have captured the water surface area, right click on the mouse to
stop adding nodes and complete the polygon. A window will pop asking for an ‘id’ for the
polygon you created. Give it an ‘id’ like 1 or 101. Also add information to the name field if you
added one, e.g. type in the name of the dam.
Figure 16: Example of an attribute table for the digitized 2014 Theewaterskloof water surface polygon

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Figure 17: Example of the digitised 2014 polygon (red boundary)
8. If you want to adjust some of the vertices, you can use the vertex tool for the current layer:
E.g. Click on a vertex and then click on where you want it to be. Click on the little x between two
vertices to add a vertex. Change the layer symbol to “No brush” if you need to see the image
underneath, e.g.
Figure 18: Adjusted vertices
9. When you are happy with your digitised polygon, click on Toggle editing again and save your
updates.

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Figure 19: Example of a 2014 polygon
10. NOW follow steps 1-9 again to digitize the surface area as on the 2017 image. (Note: As with the
georeferencing, the resolution on the 2017 image is coarser and it may be more difficult to
identify the areas with water – just capture the areas that you are sure of – do not spend
excessive amounts of time on it.)
Figure 20: Example of a 2017 digitised polygon
Task 1.6 Calculate the surface area of the dam for the various time periods.
Purpose: To determine the differences in the dam surface areas for the three years, you want to
calculate the surface for 2012, 2014 and 2017. (Note: Due to the relative small size of the dams, and
the fact that you are working with approximations and trends in this case, it is fine to continue using
the default geographic ellipsoid. If you were working with precise measurements, an equal area
projected coordinate system is preferred. Some software applications may restrict area and distance
calculations if your data is not projected)

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Figure 21: Calculating the geometry of a field.
NOTE: you must calculate the surface areas for:
 2012 (using the Theewaterskloof2012 shapefile),

 2014 Theewaterskloof (using the 2014 outline that you digitised from the georeferenced image),
and
 2017 Theewaterskloof (using the 2017 outline that you digitised from the georeferenced image).
For use in your map layout, add the dam surface areas to a table (in MS Word / Excel) with the dam
area results converted from sq.m to Ha from 2012 to the last one (2017), e.g.
Table 2: Example of surface area data table
Changes in dam surface area

Year Area in Hectare (Ha)
2012
2014
2017
Task 1.7 Creating, exporting and submitting a suitable map layout

1. Create a suitable A4 map layout for your project.
2. Your map must clearly represent components as listed in the TASK 1 – main outcomes. Also see
the criteria in Table 3 on the next page.
E.g.
i. Zoom in to the area of interest
ii. Illustrate the change in dam levels for the Theewaterskloof dam (2012 and 2017).
iii. Add a table or text box with specific surface area figures for 2012, 2014 and 2017 in Hectare.

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iv. Add a graphic presentation of the dam levels for the last 5 years
v. Change the symbology of all layers to symbols that are suitable to the purpose of the map.
vi. Use orientation layers and/or a grid and/or inset map to give context to your map layout.
vii. Add map elements (a legend, scale bar, north arrow and title/s) to your map.
viii. Use language and fonts that are suitable for a printed map.
ix. Add metadata (Note: To facilitate anonymity, please do not add your real name or st. no. this
time - you may use a fictitious name)
x. Ensure that the layout is well balanced and the components are well aligned.
3. Export the final map layout to a PDF document.
4. Open the exported document to ensure that the file format and map layout is suitable for
submission.
5. Submit the file on clickUP under Assessments and Tasks →Practical Classwork Submissions →P6
Practical 6 Map submission.
Click on View/Complete – and then on Question 1 – browse to your local files to attach and submit
your map layout. Make sure that you attach the correct file!!! Please see the additional document
on Self and Peer Assessment if you are not sure how to submit.
 Due date for the map submission is 3 May 2021 at 23:30.

 NOTE: LATE submissions CANNOT BE ACCEPTED as all the maps must be submitted in
time for the peer review process to work correctly.
Please save your project, data and map layout for possible future use.
Table 3: Summary of important aspects that the final map layout must adhere to – these aspects will form the basis of the
peer marking criteria.
Criteria % Description
Overall map 10 All required parts of the map layout e.g main map, map elements, additional information,
layout etc. must be present, well balanced regarding hierarchy, priority and alignment.
Map elements: 10 Map title/s must be descriptive (what and where), suitably positioned and formatted
Title(s) and (font/size/language). The map legend should clearly (legibly) describe map features in
legend suitable language, font and size.
Map content, 10 Main map should clearly show the required detail using suitable symbols and methods, e.g.
Styles and the differences in surface area of the Theewaterskloof dam (2014 and 2017), the table/text
symbols with the 2012, 2014 and 2017 surface areas in Ha and the relevant dam levels over the past
five years. Symbols, colours and display options for all map layers must be well chosen and
used in an optimal way.
Labels, 10 - Important features on the map (for content and orientation purposes) must be clearly
Orientation and labelled using suitable fonts and locations
context - Orientation data (other data layers/grid/inset) should clearly provide context to show the
location and extend of the main map.
Map elements: 10 The scale bar should be correct, legible and well positioned. Units should be in km.
Scale, north arrow Additional text must be added in a suitable manner e.g. metadata like a name for the map
and metadata creator (you may use a fictitious name), the date and coordinate system used. You may also
add text about the map. e.g. to explain how the 2014 and 2017 dame areas were created.
A compass rose (or north arrow) of suitable type, size and position should be added.
TASK 2: PEER REVIEW ASSIGNMENT

Between 4 May 06:00 and 7 May 23:00 you must evaluate your own map and three maps as
submitted by your peers. To complete the evaluation return to the assessment and click on the
“View/Complete Assessment” link once again.

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 Click on the user name (e.g. User 1)

 Download the submitted file and open it, evaluate using the criteria.
 Provide a mark out of 10 and constructive feedback - click on Save and Next Criteria (Repeat
for all Criteria before you submit)
 Click on Submit
Note: The submissions will be assigned to you by the system. The descriptions in Table 3 above will
align to the criteria that you must use for the marking. Please use this opportunity to critically
assess the submissions and add constructive comments.
Please note! Students must complete both P6 tasks to obtain a final mark for this exercise!
References
1. City of cape Town Weekly Water Dashboard
https://resource.capetown.gov.za/documentcentre/Documents/City%20research%20reports%20an
d%20review/damlevels.pdf
2. GroundUp: What’s causing Cape Town’s water crisis?
http://www.groundup.org.za/article/whats-causing-cape-towns-water-crisis/
END

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GGY 283 Introductory Geographic Information Systems PRACTICAL 6

PRACTICAL 6: GEOREFERENCING, DIGITISING, FIELD CALCULATIONS AND DISPLAY

GGY 283 Copyright University of Pretoria 1

Figure 14: Example of a georeferenced 2017 image ............................................................................ 16

GGY 283 Copyright University of Pretoria 2

MAIN OUTCOMES (What must you submit?)

TASK 1 – main outcomes

diagrams like histograms from the layer properties options, e.g.

o rivers in the area of interest.

TASK 2 – main outcome

GGY 283 Copyright University of Pretoria 3

 Clip a vector dataset;

The manual is also available online at https://docs.qgis.org/3.16/en/docs/user_manual/index.html.

Data file Description

GGY 283 Copyright University of Pretoria 4

Figure 1: Example - QGIS browser

2. Add the rivers, roads, dams, province and cities&towns shapefiles.

Figure 2: Example of loaded data layers

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SELECTING DATA BASED ON ATTRIBUTES (Attribute query)

Figure 4: Extract from the WC_dams attribute field

4. Select the (Select by Expression) function.

5. Use the Wizard window to complete the Expression:

Figure 5: Select by Expression function

6. Then click on Select Features and close the window.

GGY 283 Copyright University of Pretoria 6

Figure 6: Extract for attribute table with filter options below.

Figure 7: Save selected features

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Figure 8: Save selected features parameters and result

11. Remember to save your project.

Task 1.2 Creating features from coordinates (E.g. GPS coordinates)

1. Open the Dam_Levels_WC_Main.xls in MS Excel or a similar spreadsheet application (not in

7. Select the Add delimited text layer tool.

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10. Save your project!

Task 1.3 Joining Attributes by Field values

4. Open the Processing Toolbox.

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7. Close the processing window

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Figure 9: Field with only one decimal value after refacturing

Task 1.4 Georeferencing images

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Choose Plugins- Manage and Install Plugins

Then search for, and install, the georeferencer plugin.

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Figure 10: Georeferencer windows and toolbar

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Figure 12: Georeferenced 2014 image

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Figure 13: Example of 2017 image control points and table

Figure 14: Example of a georeferenced 2017 image

14. Save your project!

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Figure 15: Digitizing in progress

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