Lab 6: Introduction to QGIS

Becoming Familiar with QGIS

This part of the exercise runs through an example problem written by Brock University. It involves the identification trees on their campus which are in critical condition and must be removed from roadsides.

1.1 Introducing the QGIS Interface

The data for the lab was downloaded and extracted and the project was saved.

1.2 Adding Data

Under Project > Properties > CRS, the coordinates are set to NAD83 / UTM zone 17N. The data was found in the Browser section of the interface and dragged into the Layers section. The properties / the colour of the buildings were changed depending on the building type. This is done by right clicking on the layer and going into Properties > drop down menu: Categorize >Value: Type.

Figure 1. Inputted data of streets, water, contours and buildings for Brock University campus.

1.3 Adding Point Data from a Table

Layer > Add Layer > Add Vector Layer was used to input the .csv file containing the tree data. The coordinate system was set to the same as the other data before it was added to the map. Then, the data was right clicked on to save it to a shapefile.

Figure 2. Map of campus from figure 1 with trees added.

1.4 Buffering and Selecting by Location Query

In order to remove any trees in poor condition that are within 20 meters of roads, buffers are used.

Vector > Geoprocessing Tools > Buffer

In order to select the trees within this buffer zone, select by location is used.

Vector > Research Tools > Select by Location

The resulting trees that satisfy the condition are now yellow. To remove them, right click on the tree shapefile and click the following:

Export > Save Selected Features As > ESRI Shapefile, browse and select data folder > ok.

Then the previous tree file was removed to keep only the new tree_buffer file with the trees that need to be cut down.

Figure 3. Map of trees that intersect with the road buffer of 20m.

1.5 Select by Attribute

Now the trees in poor condition must be selected.

Right click on tree_buffer layer > Open Attribute Table > Select Features by using an Expression > Expand "Fields and Values" (in Function List" > CanopyCond = > Under values click "All Unique" > Critical > OR > CanopyCond = > Poor >> Select Features > Close

These selected attributes were then saved as a new shapefile.

Figure 4. Conditional statement for the tree conditions to be cut down.

1.6 Printing the map

Project > New Print Layout > rename it, check off "Include report header" > Edit > Add Item > Add Map.

From here you can customize the report in whichever way you prefer in the editor as seen below.

Figure 5. Example of report layout editor.

2. Interpolate Point Data from Lake Depth Soundings

The task of this section of the lab is to interpolate data across a landscape. This must be done for datasets in order to create continuous data that can be analyzed or used in modeling. This specific lab uses field depth measurements, taken by a Depth Sounder at locations recorded by DGPS equipment, for Lake Arlington in Texas to create an elevation relief map with elevation contours.

2.1 Entering the data into QGIS

Open QGIS > Layer > Add Layer > Add Vector Layer > add Shapefiles zip data into the program.
The identifier tool (depicted by the blue circled i) can be used to select points and view their individual data.

Figure 6. Lake boundary and elevation data entered into QGIS.

2.2 Interpolating the data

The Tin interpolation tool is found under Processing > Toolbox > search interpolation > select Tin Interpolation.

Figure 7. Locating the TIN Interpolation tool.

Figure 8. Using the TIN Interpolation tool.

2.3 Cleaning up the interpolated data

The results were then clipped to the boundary.
Raster > Extraction > Clip Raster by Mask Layer
The layer properties were opened and the Wingleband Pseudocolor render and Spectral colour ramp were selected for the data with inverted colours so that blues represent deeper regions of the lake and reds represent shallower regions.
To remove the black pixels from the output, 0 was entered as the Additional no data value on the Transparency tab.

2.4 Adding in contour lines

To create contours: Raster > Extraction > Contour.
The interval between contour lines was set to 5 and the resulting map was as follows. The contour layer properties were opened and the elevation labels were added. The final map is seen below.

Figure 9. Interpolated elevation data with contour lines for Lake Arlington.

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