Trabajando con atributos (QGIS3)

Los datos SIG tienen dos partes - características (features) y atributos. Los atributos son datos estructurados sobre cada característica. Este tutorial muestra cómo ver los atributos de cada capa vectorial SIG y hacer consultas básicas sobre los mismos en QGIS.

Resumen de la tarea

The dataset for this tutorial 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.

Other skills you will learn

  • Select features from a layer using expressions.
  • Using the Attributes toolbar.
  • Exporting selected features in a layer

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 datasets from the link below:

ne_10m_populated_places_simple.zip

Data Source [NATURALEARTH]

Procedure

  1. 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.
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  1. 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.
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Nota

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

  1. 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.
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  1. 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.
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  1. 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.
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  1. 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.
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  1. 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
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Nota

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

  1. 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.
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  1. 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.
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  1. 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.
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  1. 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
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  1. 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.
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  1. 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...
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  1. 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.
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  1. 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.
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  1. 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.
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