Using Custom Python Expression Functions (QGIS3)¶

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.

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 Ɛ 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).

   Note

   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.

   import math
   from qgis.core import *
   from qgis.gui import *
   
   @qgsfunction(args=0, group='Custom', usesgeometry=True)
   def GetUtmZone(value1, feature, parent):
       """Return the UTM Zone of the feature's geometry as a String"""
       centroid = feature.geometry()
       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 '%d%s' % (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 33N. Click Zoom to Features and the map will change, if you click Select Features you should see the points in UTM zone 33N change colour to yellow.

   Warning

   Due to an issue in QGIS3.4.4 this will not work! However it does work in releases from QGIS3.4.5 and QGIS3.6.

   GetUtmZone() = '33'
   

   

6. Back in the main QGIS window, you should see some points highlighted in yellow. These are the points falling in the UTM Zone we specified in the expression. You could also use the Zoom to Features button to zoom into the selection.

   

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.