python_expressions.md

Using Custom Python Expression Functions

Expressions in QGIS have a lot of power and are used in many core features -selection, calculating field values, styling, labelling etc. QGIS also has support for user-defined expressions. With a little bit of python programming, you can define your own functions that can be used within the expression engine.

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.

Overview of the task

We will define a custom function that finds the UTM Zone of a map feature and use this function to write an expression that displays the UTM zone as a map tip when hovered over the point.

Other skills you will learn

• How to use the Map Tips tool to display custom text when hovering over a feature.

Get the data

We will use Natural Earth's Populated Places dataset. Download the simple (less columns) dataset

Procedure

1. Open QGIS and go to Layers --> Add Vector --> Add Vector Layer. Or browse to the downloaded ne_10m_populated_places_simple.zip file and drag it into the map window.

   

2. Go to View --> Select --> Select By Expressions.... or click the icon:

   

3. Switch to the Function Editor tab. Here you can write any PyQGIS code that will be executed by the expression engine.

   

4. We will define a custom function named GetUtmZone that will calculate the UTM zone number for each feature. Since custom functions in QGIS work at the feature level. We will use the centroid of the feature's geometry and compute the UTM Zone from the latitude and longitude of the centroid geometry. We will also add a 'N' or 'S' designation to the zone to indicate whether the zone is in the northern or southern hemisphere.

   Press the plus button in the lower left of the screen and type utm_zones.py as the file name. Then type the following code into the editor window. When you are finished click Save and Load Functions (on the right).

   UTM Zones are longitudinal projection zones numbered from 1 to 60. Each UTM zone is 6 degree wide. Here we use a simple mathematical formula to find the appropriate zone for a given longitude value. Note that this formula doesn't cover some special UTM zones.

   math.floor() returns the integer value nearest 0 of a double. We add 180 to the longitude so that the zone number starts at 0 and divide it by 6 to find the relevant zone number. The % 60 is so that longitudes <-180 or >180 still give the right answer by taking the remainder after dividing by 60.

   import math
   from qgis.core import *
   from qgis.gui import *
   
   @qgsfunction(args='auto', group='Custom', usesgeometry=True)
   def GetUtmZone(value1, feature, parent):
       """Return the UTM Zone of the feature's geometry as a String"""
       centroid = feature.geometry().centroid()
       longitude = centroid.asPoint().x()
       latitude = centroid.asPoint().y()
       zone_number = math.floor(((longitude + 180) / 6) % 60) + 1
   
       if latitude >= 0:
           zone_letter = 'N'
       else:
           zone_letter = 'S'
   
       return f"{int(zone_number)}{zone_letter}"

   

5. Switch to the Expression tab in the Select by expression dialog. Find and expand the Custom group in the Functions section. You will notice a new custom function GetUtmZone in the list. We can now use this function in the expressions just like any other function. Type the following expression in the editor. This expression will select all points that fall in the UTM Zone 40N. Click Zoom to Features and the map will change, if you click Select Features you should see the points in UTM zone 40N change colour to yellow.

   GetUtmZone() = '40N'
   

   

6. Back in the main QGIS window, you should see many points highlighted in yellow. These are the points falling in the UTM Zone we specified in the expression. But you can see we have zoomed in to the UTM zone.

   

7. You saw how we defined and used a custom function to select features by expression. We will now use the same function in another context. One of the hidden gems in QGIS is the Map Tip tool. This tool shows user-defined text when you hover over a feature. Right-click the ne_10m_populated_places_simple layer and select Properties.

   

8. Switch to the Display tab. Here you can enter any text that will be displayed when you hover over the features of the layer. Even better, you can use layer field values and expressions to define a much more useful message. Click on the Ɛ button.

9. You will see the familiar expression editor again. We will use the concat function to join the value of the field name and the result of our custom function GetUtmZone. Enter the following expression and click OK.

   concat("name",'|UTM Zone: ',GetUtmZone())
   

   

10. You will see the expression entered as the value of the Display text. Click Insert to add it to the HTML box and then press OK.

    

11. Before we proceed, let us de-select the features that were selected in the previous step. Go to View --> Select --> Deselect Features from All Layers.

    

12. Activate the Map Tips tool by going to View --> Map Tips.

    

13. Zoom into any area of the map and put your mouse cursor over any feature. You will see the name of the city and corresponding UTM zone displayed as the map tip.