Selecting data

Selecting data is very useful in GIS. We can create a new layer from a selection, or we could add new data specific to that selection. QGIS allows multiple ways of selecting data, and we will cover some of these here. Later, in Chapter 6, Extending QGIS with Python, we will look at spatial selections. The data attributes toolbar allows selecting by area (this means drawing on the map by hand) and selecting by value (this means passing a query to select the data). The selection tools are highlighted in the following screenshot:

Working from left to right, the tools are Select by Area, Select by Attribute, and Clear Selection. Click on the Select by Location button and click on the largest area associated with the Alaska layer. It will change to yellow to highlight the selected layer. The following screenshot shows the other Select by Area options in the drop-down list:

Click on Clear Selection to remove any selections from the layer.

The Select by Value button is the second button on the toolbar described previously. Click on this button to open a new window:

In the Select Features by Value window, we have clicked on the Exclude field button associated with AREA_MI and selected Less than or equal to (<=). As a parameter, we have typed 1000 into the box. We then clicked Select features and QGIS selected the features that match this expression. At the top of the screen, a notification tells us how many features have been selected. In this case, it is 640.

If we click on close and then press F6 or click on the button immediately to the right of the Clear Selection button, the attribute table will appear with the selected rows highlighted. This attribute table for the Alaska layer is shown as follows:

When you are finished inspecting this table, close it down by clicking the close button, on Windows this is an Χ symbol in the top-right corner. Keep this selection handy, as we will use it in the next section.

There is one more very useful tool worth mentioning, and that is the Field Calculator tool. This button looks like an abacus and is next to the Attribute Table button. We will use this tool next to begin creating our own data.