For ease of following the examples in this book, it is recommended that you create a directory called qgis_data in your root or user directory,which will provide a short pathname. This setup will help prevent the occurrence of any frustrating errors resulting from path-related issues on a given system. In this recipe, and others, we'll use a point shapefile of the New York City museums, which you can download from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip this file and place the shapefile's contents in a directory named nyc within your qgis_data directory.

Now, we'll walk through the steps of loading a shapefile and adding it to the map...

Installing and configuring PostGIS is beyond the scope of this book, so we'll use a sample geospatial database interface from the excellent service http://www.qgiscloud.com/. It has its own Python plugin called QGIS Cloud. You can sign up for free and create your own geodatabase online by following the site's instructions, or you can use the example used in this recipe.

Perform the following steps to load a PostGIS layer into a QGIS map:

We'll use the same New York City Museums layer from the Loading a vector layer from a file recipe of this chapter. You can download the layer from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip that file and place the shapefile's contents in a directory named nyc within your qgis_data directory, within your root or home directory.

In this recipe, we will load the layer, get the features, grab the first feature, obtain its geometry, and take a look at the values for the first point:

Once again, we'll use the same New York City Museums layer from the Loading a vector layer from a file recipe in this chapter. You can download the layer from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip that file and place the shapefile's contents in a directory named nyc within your qgis_data directory, within your root or home directory.

In the following steps, we'll load the layer, access the features iterator, grab the first feature, and then view the attributes as a Python list:

We will need two new shapefiles that have not been used in the previous recipes. You can download the point layer from https://github.com/GeospatialPython/Learn/raw/master/MSCities_Geo_Pts.zip.

Similarly, you can download the geometry layer from https://github.com/GeospatialPython/Learn/raw/master/GIS_CensusTract.zip.

Unzip these shapefiles and place them in a directory named ms within your qgis_data directory, within your root or home directory.

In this recipe, we will perform several steps to select features in the point layer that fall within the polygon layer, as follows:

We'll use the same New York City Museums layer used in the previous recipes in this chapter. You can download the layer from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip that file and place the shapefile's contents in a directory named nyc within your qgis_data directory, within your root or home directory.

In this recipe, we'll filter the layer by an attribute, select the filtered features, and zoom to them, as follows:

Once again, we'll use the same New York City Museums layer. You can download the layer from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip that file and place the shapefile's contents in a directory named nyc within your qgis_data directory, within your root or home directory.

This recipe involves both a spatial operation and multiple visualizations. To do this, perform the following steps:

If you don't already have the New York City Museums layer used in the previous recipes in this chapter, download the layer from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip that file and place the shapefile's contents in a directory named nyc within your qgis_data directory, within your root or home directory.

In the following steps, we'll extract the first and last points in the layer's point order and measure the distance between them:

For this recipe, we'll use a line shapefile with two features. You can download the shapefile as a .zip file from https://github.com/GeospatialPython/Learn/raw/master/paths.zip.

Unzip the shapefile into a directory named qgis_data/shapes within your root or home directory.

The steps for this recipe are fairly straightforward. We'll extract the geometry from the first line feature and pass it to the measurement object, as shown here: