QGIS 3 Plugin Tutorial – Geocoding with Nominatim Part 2

QGIS Tutorials

This tutorial is part of our QGIS tutorial series:


QGIS 3 Plugins - Geocoding with Nominatim Part 2

This tutorial follows up on the first QGIS plugin development tutorial, which built a plugin using Nominatim's reverse geocode endpoint to query user generated coordinates for street addresses. If you have no idea what I'm talking about, go through the tutorial or get the previously prepared plugin from our repository.

In this walk-through you'll extend the functionality so that a user doesn't have to manually copy/paste coordinates from a third-party service/document. A user will want to be able to click inside the map and use this point for reverse geocoding. And it's explicitly an extension, a user would still be able to paste coordinates manually.

The final extended plugin can be found in our tutorial repository.

Goals:

  • capture interactive map input from users
  • create your own Map Tool (like Pan or Zoom Tool in QGIS)
  • learn applied examples for signals/slots to create interactivity

Plugin functionality:

  • User either copies Lat/Long or X/Y to the plugin dialog or uses an interactive Map Tool to capture a coordinate in the map canvas
  • Upon OK button click, the Nominatim API is queried on its Reverse Geocoding endpoint
  • A Point layer is generated in-memory, displayed on the map and zoomed to

Disclaimer

Validity only confirmed for Ubuntu 18.04 and QGIS <= v3.6.3 Occassionally, the author might choose to give hints on Windows-specific setups. Ctrl+F for WINDOWS flags. Mac OS users should find the instructions reasonably familiar.

Prerequisites

Hard prerequisites

Recommendations

  • Basic Python knowledge
  • Familiarity with

1 Preparation

First, copy the previous plugin files to a new structure, so you don't mess with a working solution. You can leave all file names the way they are though.

mkdir quick_api_interactive
cp -r quick_api/* quick_api_interactive

Now all the definitions will be the same for the "new" plugin as it was for the old one, which will conflict with each other in QGIS. So you need to remove the old plugin and add the new one:

rm -r ~/.local/share/QGIS/QGIS3/profiles/default/python/plugins/quick_api
ln -s ~/.local/share/QGIS/QGIS3/profiles/default/python/plugins

You can already create the new files and folders which you'll need throughout the tutorial:

cd quick_api_interactive
mkdir icons  # We'll provide some icons later on
touch maptool.py  # This will hold our interactive map tool later

Now your file structure should look like this:

quick_api_interactive
├── icon.png
├── icons
├── __init__.py
├── maptool.py
├── metadata.txt
├── quick_api_dialog_base.ui
├── quick_api_dialog.py
├── quick_api.py
├── resources.py
└── resources.qrc

2 Qt Designer - Add Map button

If you're not too familiar with Qt Designer, we have a reference guide for you.

Open the quick_api_dialog_base.ui in Qt Designer and perform the following steps in sequence:

  1. Change the layout of the main QDialog to Grid by pressing Lay Out in a Grid button in the main toolbar
  2. Drag a QToolButton to the right side of the QgsFilterLineEdit widget and name it map_button
  3. Resize the rest of the widgets to fill the entire width of the dialog
  4. Save the UI file

Now, your UI should look approximately like this:

new UI

3 PyQGIS - Add icons

Typically icons are created with Qt's QIcon class and there are multiple ways how to create one:

  • file-based classic Python approach: simply supply the path of an existing icon file
  • access QGIS resources: QGIS has a resource store holding all its file-based resources such as icons. Tapping into that you can simply use what's already in QGIS without having to worry to come up with your own icons.

We'll show you both approaches here.

File-based

Let's replace the plugin's main icon. Our proposal is the icon of our Pelias geocoder plugin: https://github.com/nilsnolde/pelias-qgis-plugin/raw/master/PeliasGeocoding/gui/img/icon_reverse.png

Add the icon to the ./icons and adapt the path in initGui to reflect the new path:

icon_path = os.path.join(current_dir, 'icons', 'icon.png')

Note, whatever you choose the image should be SVG or PNG with background transparency, otherwise it will look awful.

QGIS resources

Another way is to re-use what QGIS offers out-of-the-box in its own resource store. The real challenge is to find out how to reference the path to the icon/resource to supply it to QIcon.

One solution is this fantastic collection by Geotribu. Finding a suitable icon requires some guesswork w.r.t. its name. For our purposes we choose one icon for both the map tool button and the cursor when the map tool is active:

test: ":images/themes/default/cursors/mCapturePoint.svg"

To use that icon for the map tool button, you can add the following to the run() method:

if self.first_start == True:
    # ...
    self.dlg.map_button.setIcon(QIcon(":images/themes/default/cursors/mCapturePoint.svg"))

Test in QGIS

You already made a lot of changes to the initial plugin, so it's time to make sure everything worked smoothly. Restart QGIS, enable the "new" plugin (called quick_api_interactive now) and choose it from the dropdown of Plugin Reloader before reloading it.

4 PyQGIS - Set up Map Tool and signal

Finally you can do a bit more fun work! You'll start with maptool.py

This will be the actual tool which will let users choose a point on the map canvas for reverse geocoding. It's surprisingly little code actually.

What you will create is a real Map Tool, like e.g. the Zoom Tool in QGIS or the Pan Tool. Using the new tool will only emit the point clicked in the map canvas to a listening function which will transform the point to WGS84 and subsequently send it to Nominatim. So, let's get started.

First, import the relevant modules:

from PyQt5.QtCore import pyqtSignal
from PyQt5.QtGui import QCursor, QPixmap
from PyQt5.QtWidgets import QApplication

from qgis.gui import QgsMapToolEmitPoint, QgsVertexMarker
from qgis.core import (QgsCoordinateReferenceSystem,
                       QgsCoordinateTransform,
                       QgsProject
                       )

Then build a class PointTool based on QgsMapToolEmitPoint and also create a module-wide CRS based on WGS84:

WGS = QgsCoordinateReferenceSystem("EPSG:4326")


class PointTool(QgsMapToolEmitPoint):
    pass

The point tool also needs to know when it needs to capture a click from the user, i.e. it needs to know which event is has to listen to. QgsMapToolEmitPoint has a few pre-defined event methods available which you can listen to. In this case, you'll use canvasReleaseEvent, which is the event triggered when the mouse button was pressed and released while hovering over the map canvas. Other events include canvasMoveEvent and canvasPressEvent. These events are managed by QGIS and need to be implemented before they're doing anything. The event it accepts is a QgsMapMouseEvent, which, among other properties, knows the point coordinates of the click event in the current CRS of the map canvas:

# class PointTool(QgsMapToolEmitPoint):
canvasClicked = pyqtSignal('QgsPointXY')

def canvasReleaseEvent(self, event: QgsMapMouseEvent):
    # Get the click and emit a transformed point

    crs_canvas = self.canvas().mapSettings().destinationCrs()
    xformer = QgsCoordinateTransform(crs_canvas, WGS, QgsProject.instance())

    point_clicked = event.mapPoint()
    point_wgs = xformer.transform(point_clicked)

    self.canvasClicked.emit(point_wgs)

This might be the first time you come across pyqtSignal, so we recommend to go through our short primer on PyQt Signal/Slots concept.

Above you define a custom signal canvasClicked, which can emit a QgsPointXY variable type. It will only emit in the above case when a canvasReleaseEvent was triggered and after the point was transformed to WGS84. This emitted signal however needs a slot to connect to, i.e. a function which does something useful with the emitted point. As you will see shortly, that slot will be the function adding the emitted WGS84 coordinates the user clicked to the QgsFilterLineEdit widget in your GUI, which a user had to fill manually before.

5 PyQGIS - Implement Map Tool and slot

Now that you set up the signal that will emit the clicked point, you need to tell the plugin what should happen with that QgsPointXY.

This is a 2 stage process:

  1. connect the map_button QToolButton's clicked signal to a function initializing your map tool
  2. in this initialization function, connect the canvasClicked signal of your map tool to the function which will populate the lineedit_yx QgsFilterLineEdit with the emitted QgsPointXY.

First, import the Map Tool to quick_api.py:

from .maptool import PointTool

Initialize the Map Tool

you will introduce a new function to quick_api.py:QuickApi, which will be called whenever the map_button is clicked:

#class QuickApi:
def _on_map_click(self):
    self.dlg.hide()
    self.point_tool = PointTool(self.iface.mapCanvas())
    self.iface.mapCanvas().setMapTool(self.point_tool)
    self.point_tool.canvasClicked.connect(self._write_line_widget)

This will:

  • hide the plugin's dialog
  • set the current Map Tool to your PointTool, i.e. now the signal canvasClicked will be emitted if the user clicks inside the map canvas
  • connect the canvasClicked to a new function self._write_line_widget, which doesn't exist yet but will come soon

_on_map_click() will need to be called when map_button is clicked, who doesn't know of its existence yet. So let's change that. Connect the button's signal to the new function in run() (note it should be after self.dlg is defined obviously):

#class QuickApi:
#def run():
if self.first_start == True:
		...
    self.dlg.map_button.clicked.connect(self._on_map_click)  # added line

Quick re-write of plugin startup

For the plugin to handle this correctly, you need to re-arrange a bit the start-up logic. Without going into too much detail, the QDialog.exec_() is not ideal in this case, as that line is blocking, i.e. the code stops there until the user presses the OK or Cancel button. It's usually alright, but it stops working when you introduce new asynchronous functions like event loops, as you did above. The whole logic to show the plugin to the user is a little flawed, so let's rewrite some of it:

In quick_api.py:QuickApi.run(), first delete self.dlg.show(), which is superfluous anyways. Then replace self.dlg.exec_() with self.dlg.open(). QDialog.open() will just open a modal dialog and immediately return control to the code. Its asynchronous nature means you have to know when the user presses OK/Cancel, so you have to listen for the appropriate signal and connect that to a function which does your work in case OK was clicked. The right signal is the dialog's built-in finished() signal, which emits the result code (0 for Cancel button, 1 for OK), similar to the return value of the previous QDialog.exec_() method.

Now the plugin needs to know what to do when the finished() signal is emitted: of course it needs to run your actual code. So, take the whole if result: statement and put it into a new method called result(), which has to accept the result code the finished() signal emits. Your code should finally look approximately like this:

#class QuickApi:
def run(self):
    if self.first_start == True:
        self.first_start = False
        self.dlg = QuickApiDialog()
        self.dlg.map_button.setIcon(QIcon(":images/themes/default/cursors/mCapturePoint.svg"))
        self.dlg.finished.connect(self.result)
        self.dlg.crs_input.setCrs(QgsCoordinateReferenceSystem(4326))
        self.dlg.map_button.clicked.connect(self._on_map_click)

    self.dlg.open()

def result(self, result: int):
    if result:
        project = QgsProject.instance()
				...

Write the clicked coordinates to the GUI

At last you have to set up the _write_line_widget() function, which will take the emitted QgsPointXY from the canvasClicked signal and write it to our lineedit_xy QgsFilterLineEdit widget:

#class QuickApi:
def _write_line_widget(self, point: QgsPointXY):
    self.dlg.lineedit_xy.setText(f"{point.y():.6f}, {point.x():.6f}")
    self.iface.mapCanvas().unsetMapTool(self.point_tool)
    self.dlg.show()

This will:

  • set the text to the formatted latitude and longitude of the emitted point
  • unset the current map tool, which will take care of some cleaning up (you could also save a reference to the previous map tool in _on_map_click() and restore it here)
  • finally show the plugin dialog again which will now have the new clicked coordinate in its text field

6 Final plugin functionality

Finally the plugin is ready to be fully tested. Reload the plugin in QGIS and see the result. The plugin didn't change much visually, just a tiny new button. But the user experience just went through the roof!

The expected behavior at this point:

  • When you click the Map button the plugin window disappears
  • When you click the map canvas, the plugin window re-appears and contains the clicked coordinates in WGS84

For all practical purposes you could stop here. However, if you want to learn a few more tricks how to improve this plugin to be more user-friendly, keep on reading.

7 Bonus material

Give the cursor a custom icon

Another neat functionality would be if the cursor of your custom Map Tool would have its own icon, not the default cross-hairs symbol. We opt for the same icon we used for the QToolButton.

Open ./maptool.py and add the following just after declaring WGS:

CUSTOM_CURSOR = QCursor(QIcon(':images/themes/default/cursors/mCapturePoint.svg').pixmap(48, 48))

We have to do a little bit of a roundabout here: QCursor only accepts a QPixMap, so our QGIS internal icon will need to be converted first and set to a size of 48 pixels.

Next, you'll have to let QGIS know that it should use that cursor when the map_button is clicked. You can also do this in your Map Tool, using its predefined activate() method, which is called when the Map Tool is activated (e.g. from self.iface.mapCanvas().setMapTool(self.point_tool)):

#class PointTool(QgsMapToolEmitPoint):
def activate(self):
    QApplication.setOverrideCursor(CUSTOM_CURSOR)

Now the icon will change to our image, but it won't change back once the operation is completed. For that, you can hook into the deactivated signal which will be emitted when e.g. the map tool in unset. Connect that signal to a function restoring the old cursor:

from PyQt5.QtWidgets import QApplication

def _write_line_widget(self, point):
    ...
    self.point_tool.deactivated.connect(lambda: QApplication.restoreOverrideCursor())

And that's it! Best of luck!

Next steps

In the next tutorial we'll you some useful tricks around QGIS plugin development and apply some software development best practices by restructuring the whole code base a little. This will help us for the last tutorial in this series which is about an automated test infrastructure for QGIS plugins: https://gis-ops.com/qgis-3-plugin-tutorial-geocoding-with-nominatim-part-3/.


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