Chapter 1: An Introduction to QGIS

Part III A: Opening and Symbolizing Vector Data

Vectors are points, lines, and polygons representing data. Vectors can represent borders, roads, and towns along with many other types of data. The key characteristic of GIS is that it assigns geographic coordinates to pieces of vector data. By contrast, a graphics software program can also create points, lines, and polygons, but it will not assign them a geographic location.

All of the data that we just downloaded is vector data. See Chapter 4: Digitization for instructions on how to create your own points, lines, and polygons.

The PEI Coastline Layer

Adding the Layer

Let’s start by adding PEI’s coastline to our base map.

  • On the left-hand side of your screen, in the Browser, navigate to the Chapter1 folder that we created earlier.
Tip: once you have navigated to this folder, right-click it and click Add as Favourite. From now on, you can access it from the Favourites menu.
Figure 1.35. In the browser screen, right clicking on the chapter and add as a favourite
Figure 1.35
Figure 1.36. In browser the favourites tab will show all the favourited sections
Figure 1.36
  • Within the Chapter1 folder, navigate to the Data folder.
  • Expand the folder called coastline.SHP.
    • The .shp file extension indicates that it is a shapefile.
  • Right-click the layer called coastline_polygon.shp and click Add Layer to Project.
Figure 1.37. Right clicking on section data, coastline.SHP, coastline_polygon opens the option to “add layer to project”(the third option down).
Figure 1.37
  • You will now see the coastline_polygon layer in your Layers table of contents.
Figure 1.38. After adding the layer to the project it can be seen in the layers panel.
Figure 1.38

Your map will look similar to this now. The coastline file has a fill layer by default, and we can remove this in order to see our base map with the highlighted coastline.

Figure 1.39. This shows a highlighted coastline.
Figure 1.39

Symbolizing the Layer

  • Right-click on the coastline_polygon layer and click Properties.
  • Click Symbology.
Figure 1.40. After right clicking on the coastline_polygon layer properties is selected.
Figure 1.40
  • Under Fill, click to select Simple Fill.
Figure 1.41. The screen to allow the fill colour to be changed.
Figure 1.41
  • Click Fill style, and select No Brush option. Then click OK.
Figure 1.42. After choosing the fill colour the fill style can be dictated through a drop down menu.
Figure 1.42

Your map will look something like this now.

Figure 1.43. The finalized re-evaluated map based off selections.
Figure 1.43

The PEI Hydronetwork Layer

Repeat the above steps to add and symbolize the “PEI HYDRONETWORK.shp” layer.

Once added, your layers will look like this.

Figure 1.44. The layer panel with an added PEI HYDRONETWORK added.
Figure 1.44

And your map will now look something like this.

Figure 1.45. The map with the PEI HYDRONETWORK.
Figure 1.45

Further Symbolizing the Layer

We want the hydro layer to stand out, so we will change its line colour to blue.

  • Right-click the PEI HYDRONETWORK layer, and click Properties.
  • Click Symbology.
Figure 1.46. In the layers panel right clicking and select properties to change the line colour (to blue) to make the layer more prominent.
Figure 1.46
  • Click to select Simple Line.
  • Click Colour and set it to blue. You can use the sliders to change the colour or set an HTML notation for colour in Hexadecimal.
  • Click OK.
Figure 1.47. The colour changing options for the PEI HYDRONETWORK lines.
Figure 1.47

Your map will now look something like this.

Figure 1.48. The map with the blue PEI HYDRONETWORK lines.
Figure 1.48

The 1935 Forestry Layer

Adding the Layer

  • Use the Browser to add “1935 inventory_region.shp” to your layers.
Figure 1.49. Through the browser added “1935 inventory_region.shp” onto the map.
Figure 1.49

Viewing an Attribute Table

Your map will now reflect all four layers. However, the 1935 inventory_region layer is dense and covers the entire island. As mentioned at the start of this section, vectors are made of data. In our case, the data that we are concerned with is the way that the land was used. Canadian censuses recorded this data for many decades. But the important thing about GIS is that it can take this quantitative data and map it as vectors. If we want either to see or edit the data behind the vectors, we can do so by clicking Open Attribute Table.

  • Right-click on the 1935 inventory region layer and click Open Attribute Table.
Figure 1.50. After adding the 1935 inventory region, and right click on it to then select the open attribute table.
Figure 1.50

Here is the attribute data attached to the 1935 inventory_region. We want to reference the different categories in the LANDUSE column. This is the column that tells us how the land was used in 1935 and whether it was still forested or not.

Figure 1.51. Attribute table, from right to left the table reads; key, map, stand, land type, species, origin, area, and land use.
Figure 1.51

Symbolizing the Layer

We do not want to edit this data; we want to analyze this data visually. So, we can close the Attribute Table window. We want to categorize the layer to see the forestation more clearly, so we will open the properties of the layer.

  • Right-click the layer and click Properties.
  • Click Symbology.
Figure 1.52. The symbology page with the drop down menu at the top that will allow it to be changed to categorize.
Figure 1.52
  • At the very top of the Symbology window, click the dropdown menu where it currently says Single symbol.
  • Click Categorized.
Figure 1.53. The options in the symbology drop down, the third option on the list is “categorized”.
Figure 1.53

Once Categorized has been selected, the Symbology window will change.

  • Set the Value to LANDUSE by using the dropdown menu.
  • Click the Classify button towards the bottom of the window.
  • Click the dropdown arrow next to Color ramp to set it to Greens.
  • Click OK.
Figure 1.54. The menus for “categorized” in symbology. The first options to change are Value, Symbol, and Colour Ramp.
Figure 1.54

This will generate the Symbols, Value and Legend.

The layer will update with sub-layers matching each of the Categories of the LANDUSE column.

If you zoom in on the map now, you will see the various categories of Forests.

Figure 1.55. The forested view is now able to be seen on the map.
Figure 1.55
  • To return to an overall view of Prince Edward Island, right-click the coastline_polygon layer in the Layers pane and click Zoom to Layer.

The PEI Highway Layer

Adding the Layer

  • Use the browser to add the PEI_highway.shp file.

During the process of adding PEI_highway.shp file, we encounter the following error:

Figure 1.56. Error page for the highway layer.
Figure 1.56

We can ignore this by clicking Cancel. QGIS will still render the layer onto our map in accordance with our project’s CRS.

Figure 1.57. Map with the highway layer added.
Figure 1.57

Symbolizing the Layer

The roads are a little hard to see among the rest of the data. To remedy this, we will make the roads layer the uppermost layer on the map by moving it above the 1935 inventory in the Layers pane.

  • Click and drag the roads layer to the top of the list in the Layers pane.
Note: the order in which layers appear in the Layers pane determines the order in which they are displayed on the map. So, a layer at the top of the Layers pane list will appear above all other layers on the map.

 

  • Right-click on the PEI_highway layer, and click Properties.
    • Note: if the aforementioned error reappears, click Cancel once more.
  • Click Symbology.
Figure 1.58. Symbology page for the highway layer. The order for changeable options are Unit, Opacity, Color, Width.
Figure 1.58
  • Change the Width from 0.26000 to 0.75, and click OK.
Figure 1.59. The symbology for the highway layer with the changed width value for the lines.
Figure 1.59

The PEI Placenames Layer

Adding the PEI Placenames Layer

  • Add the PEI_placenames.shp to our Project and drag this layer to the top of the list in the layers pane.

 

Note: a similar error to the one that we encountered for the roads layer will appear. Click Cancel to ignore the error.

 

Figure 1.60. PEI placenames layer added to the layers panel at the top of the list.
Figure 1.60

Turning on Labels

  • Open the properties of the PEI_placenames layer and click Labels.
Figure 1.61. The labels option under symbology for the PEI placenames layer is selected.
Figure 1.61
  • From the dropdown that initially says No Labels, select Single Labels.
Figure 1.62. The labels drop down options are No Labels, Single Labels, Rule-based Labeling, and Blocking.
Figure 1.62

You can adjust the visual aspect of the labels using the various options under the Labels menu, including Text, Formatting, Buffer (Outline), Mask, Background, Shadows, and various other features.

  • Under the Text tab, change the Colour to white.
  • Under the Buffer tab, click the checkbox beside Draw text buffer and then select black as the buffer colour.
Figure 1.63. The text sample section allows for the text color to be changed and to write in text.
Figure 1.63

Here is the result:

Figure 1.64. The map with the updated label layer.
Figure 1.64

Applications of This Vector Data

Historian Douglas Sobey calculated that the number of trees in PEI decreased by two-thirds between the start of the settlement era and 1935, and he used the forest inventory and other historical GIS data to show where exactly this deforestation happened. If you are interested, you can check out his work in chapter four of Time and a Place: An Environmental History of Prince Edward Island.[1]

A GIS such as the one that we set up can also help test whether routinely generated sources such as the census are accurate records of historical land use. Historian Joshua MacFadyen and forester William Glen used a GIS to demonstrate that the census was an incomplete record of land clearing. They compared the census of agriculture with inventories created from historical aerial photographs to show the extent of this discrepancy and test the rate at which farmers underreported the amount of cleared land. In the mid- and late-twentieth century, the amount of improved land reported in the census of agriculture was much lower than the amount shown in forest inventories—over 30 percent lower in Kings County in the 1960s and 1970s. This was of course because agriculture represented a decreasing share of land use. However, even in earlier decades, when most rural land use was agricultural, farmers in Kings County reported between 10 and 16 percent less cleared land than the amounts shown by the inventories. MacFadyen and Glen used a historical GIS to determine more accurate estimates of land clearance rates across the twentieth century. [2]

 

 

  1. Douglas Sobey, “The Forests of Prince Edward Island, 1720–1900,” in Time and a Place: An Environmental History of Prince Edward Island, ed. Edward MacDonald, Joshua MacFadyen, and Irené Novaczek (Montreal: McGill-Queen's University Press, 2016). ↵
  2. Joshua MacFadyen and William Glen, “Top-down history: Delimiting forests, farms, and the census of agriculture on Prince Edward Island using aerial photography, ca.1900-2000,” in Historical GIS Research in Canada, ed. J. Bonnell & M. Fortin (Calgary: University of Calgary Press, 2014). ↵

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The Geospatial Humanities Copyright © by Joshua MacFadyen; Benjamin Hoy; and Jim Clifford. All Rights Reserved.

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