Creating a Polyline with User Input Points

Oct 13, 2025 by pynkocean.com 67 views

Introduction

QGIS is a powerful Geographic Information System (GIS) that provides a wide range of tools for data analysis, visualization, and editing. One of the key features of QGIS is its ability to create and edit vector layers, including polylines. In this article, we will explore how to create a polyline in QGIS using Python by selecting points on a layer.

Prerequisites

Before we begin, make sure you have the following:

QGIS installed on your system
Python 3.x installed on your system
PyQGIS plugin installed in QGIS
A basic understanding of Python programming

Creating a Polyline with User Input Points

To create a polyline with user input points in QGIS using Python, we will use the PyQGIS plugin. Here is a step-by-step guide to achieve this:

Step 1: Create a New Layer

First, we need to create a new layer in QGIS where we will draw the polyline. We can do this using the following Python code:

import qgis
from qgis.core import QgsProject, QgsVectorLayer, QgsField, QgsFields
layer = QgsVectorLayer("Point?crs=EPSG:4326", "Polyline Layer", "memory")
QgsProject.instance().addMapLayer(layer)

This code creates a new layer with a CRS of EPSG:4326 (WGS 84) and adds it to the QGIS project.

Step 2: Create a Dialog for User Input

Next, we need to create a dialog where the user can input the points for the polyline. We can use the QgsGui module to create a dialog with a text field for the point coordinates.

from qgis.gui import QgsGui, QgsMapCanvas, QgsMapCanvasLayer, QgsMapToolEmitPoint
canvas = QgsMapCanvas()
canvas.setCanvasColor(Qt.white)
tool = QgsMapToolEmitPoint(canvas)
dialog = QgsGui.Dialog()
dialog.setWindowTitle("Polyline Creator")
dialog.setLayout(QGridLayout())
point_field = QLineEdit()
point_field.setPlaceholderText("Enter point coordinates (x, y)")
dialog.layout().addWidget(point_field, 0, 0)
add_button = QPushButton("Add Point")
add_button.clicked.connect(lambda: add_point(point_field.text()))
dialog.layout().addWidget(add_button, 0, 1)

This code creates a dialog with a text field for the point coordinates and a button to add the point to the polyline.

Step 3: Add Points to the Polyline

When the user clicks the "Add Point" button, we need to add the point to the polyline. We can do this by creating a new feature in the layer and setting its geometry to the point.

def add_point(point_str):
    # Parse the point coordinates from the text field
    x, y = map(float, point_str.split(","))
    point = QgsPoint(x, y)
# Create a new feature in the layer
feature = QgsFeature()
feature.setGeometry(point)

# Add the feature to the layer
layer.addFeature(feature)

This code adds a new feature to the layer with the specified point coordinates.

Step 4: Draw the Polyline

Finally, we need to draw the polyline on the map canvas. We can do this by creating a new map layer and adding the features from the polyline layer to it.

# Create a new map layer for the polyline
polyline_layer = QgsVectorLayer("LineString?crs=EPSG:4326", "Polyline Layer", "memory")
QgsProject.instance().addMapLayer(polyline_layer)
for feature in layer.getFeatures():
polyline_layer.addFeature(feature)

This code creates a new map layer for the polyline and adds the features from the polyline layer to it.

Putting it all Together

Here is the complete code for creating a polyline with user input points in QGIS using Python:

import qgis
from qgis.core import QgsProject, QgsVectorLayer, QgsField, QgsFields
from qgis.gui import QgsGui, QgsMapCanvas, QgsMapCanvasLayer, QgsMapToolEmitPoint
from PyQt5.QtWidgets import QApplication, QWidget, QVBoxLayout, QGridLayout, QLineEdit, QPushButton
from PyQt5.QtCore import Qt
layer = QgsVectorLayer("Point?crs=EPSG:4326", "Polyline Layer", "memory")
QgsProject.instance().addMapLayer(layer)
canvas = QgsMapCanvas()
canvas.setCanvasColor(Qt.white)
tool = QgsMapToolEmitPoint(canvas)
dialog = QWidget()
dialog.setWindowTitle("Polyline Creator")
dialog.setLayout(QGridLayout())
point_field = QLineEdit()
point_field.setPlaceholderText("Enter point coordinates (x, y)")
dialog.layout().addWidget(point_field, 0, 0)
add_button = QPushButton("Add Point")
add_button.clicked.connect(lambda: add_point(point_field.text()))
dialog.layout().addWidget(add_button, 0, 1)
draw_button = QPushButton("Draw Polyline")
draw_button.clicked.connect(lambda: draw_polyline())
dialog.layout().addWidget(draw_button, 1, 0)
dialog.show()
def add_point(point_str):
# Parse the point coordinates from the text field
x, y = map(float, point_str.split(","))
point = QgsPoint(x, y)
# Create a new feature in the layer
feature = QgsFeature()
feature.setGeometry(point)

# Add the feature to the layer
layer.addFeature(feature)

def draw_polyline():
# Create a new map layer for the polyline
polyline_layer = QgsVectorLayer("LineString?crs=EPSG:4326", "Polyline Layer", "memory")
QgsProject.instance().addMapLayer(polyline_layer)
# Add the features from the polyline layer to the polyline layer
for feature in layer.getFeatures():
    polyline_layer.addFeature(feature)

This code creates a dialog with a text field for the point coordinates and a button to add the point to the polyline. When the user clicks the "Draw Polyline" button, it creates a new map layer for the polyline and adds the features from the polyline layer to it.

Conclusion

Q: What is the purpose of this article?

A: This article provides a step-by-step guide on how to create a polyline with user input points in QGIS using Python. It covers the basics of creating a new layer, creating a dialog for user input, adding points to the polyline, and drawing the polyline on the map canvas.

Q: What are the prerequisites for this article?

A: To follow this article, you need to have QGIS installed on your system, Python 3.x installed on your system, and the PyQGIS plugin installed in QGIS. You should also have a basic understanding of Python programming.

Q: How do I create a new layer in QGIS using Python?

A: To create a new layer in QGIS using Python, you can use the following code:

import qgis
from qgis.core import QgsProject, QgsVectorLayer, QgsField, QgsFields
layer = QgsVectorLayer("Point?crs=EPSG:4326", "Polyline Layer", "memory")
QgsProject.instance().addMapLayer(layer)

This code creates a new layer with a CRS of EPSG:4326 (WGS 84) and adds it to the QGIS project.

Q: How do I create a dialog for user input in QGIS using Python?

A: To create a dialog for user input in QGIS using Python, you can use the following code:

from qgis.gui import QgsGui, QgsMapCanvas, QgsMapCanvasLayer, QgsMapToolEmitPoint
from PyQt5.QtWidgets import QApplication, QWidget, QVBoxLayout, QGridLayout, QLineEdit, QPushButton
from PyQt5.QtCore import Qt
canvas = QgsMapCanvas()
canvas.setCanvasColor(Qt.white)
tool = QgsMapToolEmitPoint(canvas)
dialog = QWidget()
dialog.setWindowTitle("Polyline Creator")
dialog.setLayout(QGridLayout())
point_field = QLineEdit()
point_field.setPlaceholderText("Enter point coordinates (x, y)")
dialog.layout().addWidget(point_field, 0, 0)
add_button = QPushButton("Add Point")
add_button.clicked.connect(lambda: add_point(point_field.text()))
dialog.layout().addWidget(add_button, 0, 1)

This code creates a dialog with a text field for the point coordinates and a button to add the point to the polyline.

Q: How do I add points to the polyline in QGIS using Python?

A: To add points to the polyline in QGIS using Python, you can use the following code:

def add_point(point_str):
    # Parse the point coordinates from the text field
    x, y = map(float, point_str.split(","))
    point = QgsPoint(x, y)
# Create a new feature in the layer
feature = QgsFeature()
feature.setGeometry(point)

# Add the feature to the layer
layer.addFeature(feature)

This code adds a new feature to the layer with the specified point coordinates.

Q: How do I draw the polyline on the map canvas in QGIS using Python?

A: To draw the polyline on the map canvas in QGIS using Python, you can use the following code:

# Create a new map layer for the polyline
polyline_layer = QgsVectorLayer("LineString?crs=EPSG:4326", "Polyline Layer", "memory")
QgsProject.instance().addMapLayer(polyline_layer)
for feature in layer.getFeatures():
polyline_layer.addFeature(feature)

This code creates a new map layer for the polyline and adds the features from the polyline layer to it.

Q: What are some common issues that may arise when using this code?

A: Some common issues that may arise when using this code include:

The dialog not appearing on the screen
The text field not accepting input
The button not working as expected
The polyline not being drawn on the map canvas

To troubleshoot these issues, you can try the following:

Check that the QGIS project is open and that the PyQGIS plugin is installed
Check that the dialog is being created correctly and that the text field and button are being added to it
Check that the point coordinates are being parsed correctly and that the feature is being added to the layer correctly
Check that the polyline layer is being created correctly and that the features are being added to it correctly

Q: How can I customize this code to suit my needs?

A: You can customize this code to suit your needs by modifying the following:

The CRS of the layer
The type of layer (e.g. point, line, polygon)
The dialog layout and design
The button functionality
The polyline layer creation and feature addition

To customize this code, you can modify the code directly or use a code editor to make changes. You can also use the QGIS API documentation to learn more about the available functions and classes.