Geographic Information System Lesson 6 IT

Geographic Information System Lesson 6

1. QGIS (GEOGRAPHIC INFORMATION SYSTEM)

2. Working with Attributes GIS data has two parts - features and attributes. Attributes are structured data about each feature.

3. Overview of the task The dataset for this lesson contains information about populated places of the world. The task is to query and find all the capital cities in the World that have a population greater than 1 million and save the resulting subset as a GeoJSON file.

4. Other skills you will learn Select features from a layer using expressions. Using the  Attributes  toolbar. Exporting selected features in a layer.

5. Get the data Natural Earth provides a  Populated Places  dataset. Download the  simple (less columns) dataset . For convenience, you may directly download a copy of the dataset from the link below: ne_10m_populated_places_simple.zip Data Source  [NATURALEARTH]

6. Procedure

7. Locate the  ne_10m_populated_places_simple.zip  file in the QGIS Browser and expand it. Select the  ne_10m_populated_places_simple.shp  file and drag it to the canvas.

8. A new layer  ne_10m_populated_places_simple  will now be loaded in QGIS and you will see many points representing the populated places of the world. The default view in the QGIS canvas shows the geometry of the GIS layer. Each point also has associated attributes. Let’s view them. Locate the  Attributes Toolbar . This toolbar contains many useful tools to inspect, view, select, and modify attributes of a layer.

9. Note If you do not see the toolbar, you can enable it from  View ‣ Toolbars ‣ Attributes Toolbar .

10. Click the  Identify  button on the  Attributes Toolbar . Once the tool is selected, click on any point on the canvas. The associated attributes of that point will be displayed in a new  Identify Results  panel. Once you are done exploring attributes of different points, you can click the  Close  button.

11. Rather than viewing the attribute one feature at a time, we can view them all together as a table. Click the  Open Attribute Table  button on the  Attributes Toolbar . You can also right-click the  ne_10m_populated_places_simple  layer and select  Open Attribute Table .

12. You can scroll horizontally and locate the  pop_max  column. This field contains the population of the associated place. You can click twice on the field header to sort the column in descending order.

13. Now we are ready to perform our query on these attributes. QGIS uses SQL-like expressions to perform queries. Click  Select features using an expression  button.

14. In the  Select By Expression  window, expand the  Fields and Values  section and double-click the  pop_max  label. You will notice that it is added to the expression section at the bottom. If you aren’t sure about the field values, you can click the  All Unique  button to see what the attribute values are present in the dataset. For this exercise, we are looking to find all features that have a population greater than 1 million. So complete the expression as below and click  Select Features  and then  Close . "pop_max" > 1000000

15. Note In the QGIS Expression engine, text with double-quotes refers to a field and text with single-quotes refer to a string value.

16. You will notice that some rows in the attribute table are now selected. The label window also changes and shows the count of selected features.

17. Close the attribute table window and return to the main QGIS window. You will notice that a subset of points is now rendered in yellow. This is the result of our query and the selected points are the ones having  pop_max  attribute value greater than  1000000 .

18. Let’s update our query to include a condition that the place should also be a capital in addition to having a population greater than 1 million. To quickly get to the expression editor, you can use the Select Features by Expression button in the Attributes Toolbar.

19. The field containing data about capitals is  adm0cap . The value  1  indicates that the place is a capital. We can add this criteria to our previous expression using the  and  operator. Enter the expression as below and click  Select Features  and then  Close . "pop_max" > 1000000 and "adm0cap" = 1

20. Return to the main QGIS window. Now you will see a smaller subset of the points selected. This is the result of the second query and shows all places from the dataset that are country capitals as well as have population greater than 1 million.

21. Now we will export the selected features as a new layer. Right-click the  ne_10m_populated_places_simple  layer and go to Export ‣ Save Selected Features As…

22. You may choose any format of your liking as the Format. For this exercise, we will choose  GeoJSON . GeoJSON is a text-based format that is used widely in web mapping. Click the … button next to File name and enter  populated_capitals.geojson  as the output file.

23. The input data has many columns. You are able to choose only a subset of the original columns for export. Expand the Select fields to export and their export options section. Click Deselect All and check the  name  and  pop_max  columns. Click OK.

24. A new layer  populated_capitals  will be loaded in QGIS. You can un-check the  ne_10m_populated_places_simple  layer to hide it and view the points from the newly exported layer.

25. THANK YOU!