QGIS has a powerful programming interface that allows you to extend the core functionality of the software as well as write scripts to automate your tasks. QGIS supports the popular Python scripting language. Even if you are a beginner, learning a little bit of Python and QGIS programming interface will allow you to be much more productive in your work. This tutorial assumes no prior programming knowledge and is intended to give an introduction to python scripting in QGIS (PyQGIS).
This tutorial is based on “QGIS Tutorials and Tips v1.0” by Ujaval Gandhi, updated to use Python 3 and QGis 3 by Ian Turton.
We will load a vector point layer representing all major airports and use python scripting to create a text file with the airport name, airport code, latitude and longitude for each of the airport in the layer.
We will use the Airports dataset from Natural Earth.
Download the Airports shapefile.
ne_10m_airports.zip
file and click Open. Select the
ne_10m_airports.shp
layer and click OK. Or drag and drop the
.zip
or .shp
file into the map window.You will see the ne_10m_airports
layer loaded in QGIS.
Select the Identify tool and click on any of the points to examine
the available attributes. You will see that the name of the airport
and it’s 3 digit code are contained in the attributes name
and
iata_code
respectively.
QGIS provides a built-in console where you can type python commands and get the result. This console is a great way to learn scripting and also to do quick data processing. Open the Python Console by going to Plugins –> Python Console.
You will see a new panel open at the bottom of QGIS canvas. You will
see a prompt like >>>
at the bottom where you can type commands.
For interacting with the QGIS environment, we must use the iface
variable. To access the currently active layer in QGIS, you can type
the following and press Enter. This command fetches the reference to
the currently loaded layer and stores it in the layer
variable.
layer = iface.activeLayer()
There is a handy function called dir()
in python that shows you
all available methods for any object. This is useful when you are
not sure what functions are available for the object. Run the
following command to see what operations we can do on the layer
variable.
dir(layer)
You will see a long list of available functions. For now, we will
use a function called getFeatures()
which will gets you the
reference to all features of a layer. In our case, each feature will
be a point representing an airport. You can type the following
command to iterate through each of the features in the current
layer. Make sure to add 2 spaces before typing the second line.
for f in layer.getFeatures():
print(f)
As you will see in the output, each line contains a reference to a
feature within the layer. The reference to the feature is stored in
the f
variable. We can use the f
variable to access the
attributes of each feature. Type the following to print the name
and iata_code
for each airport feature.
for f in layer.getFeatures():
print(f"{f['name']}, {f['iata_code']}")
So now you know how to programatically access the attribute of each
feature in a layer. Now, let’s see how we can access the coordinates
of the feature. The coordinates of a vector feature can be accessed
by calling the geometry()
function. This function returns a
geometry object that we can store in the variable geom
. You can
run asPoint()
function on the geometry object to get the x and y
coordinates of the point. If your feature is a line or a polygon,
you can use asPolyline()
or asPolygon()
functions. Type the
following code at the prompt and press Enter to see the x and y
coordinates of each feature.
for f in layer.getFeatures():
geom = f.geometry()
print(geom.asPoint())
What if we wanted to get only the x
cordinate of the feature? You
can call the x()
function on the point object and get its x
coordinate.
for f in layer.getFeatures():
geom = f.geometry()
print(geom.asPoint().x())
Now we have all the pieces that we can stitch together to generate
our desired output. Type the following code to print the name,
iata_code
, latitude
and longitude
of each of the airport features.
The %s
and %f
notations are ways to format a string and number
variables.
for f in layer.getFeatures():
geom = f.geometry()
print(f"{f['name']:40s}\t{f['iata_code']},\t({geom.asPoint().y():.2f},{geom.asPoint().x():.2f})")
You can see the output printed on the console. A more useful way to
store the output would be in a file. You can type the following code
to create a file and write the output there. Replace the file path
with a path on your own system. Note that we add \n
at the end of
our line formatting. This is to add a newline after we add the data
for each feature.
with open("/home/ian/airports.csv","w") as output_file:
for f in layer.getFeatures():
geom = f.geometry()
line = f"{f['name']:50s}\t{f['iata_code']},\t({geom.asPoint().y():.2f},{geom.asPoint().x():.2f})\n"
o=output_file.write(line)
You can go to the output file location you specified and open the text file. You will see the data from the airports shapefile that we extracted using python scripting.