Understanding Line Intersections

Oct 13, 2025 by talynt.org 55 views

Introduction

When working with spatial data in QGIS, it's not uncommon to encounter complex geometries such as line intersections. In this scenario, we have a line layer where sometimes there are intersections of four lines. To modify the structure of one of the intersecting lines, we can utilize the power of QGIS's Field Calculator and Geometry Generator tools. In this article, we will explore how to change the start point location of an intersecting line using a geometry by expression.

Understanding Line Intersections

A line intersection occurs when two or more lines cross each other. In the case of four lines intersecting, we have a complex geometry that can be challenging to work with. To modify the structure of one of the intersecting lines, we need to understand the geometry of the intersection.

Geometry by Expression

A geometry by expression is a powerful tool in QGIS that allows us to create new geometries based on existing ones using a mathematical expression. We can use this tool to change the start point location of an intersecting line by creating a new geometry that is a modified version of the original line.

Changing Start Point Location

To change the start point location of an intersecting line, we need to use the geometry_by_expression function in QGIS. This function takes two arguments: the original geometry and the expression that defines the new geometry.

Step 1: Create a New Field

The first step is to create a new field in our line layer where we will store the modified geometry. We can do this by going to the Field Calculator in QGIS and creating a new field with a suitable name, such as modified_line.

Step 2: Write the Expression

The next step is to write the expression that will define the new geometry. We can use the geometry_by_expression function to create a new geometry that is a modified version of the original line. The expression will look something like this:

geometry_by_expression(
  "original_line",
  'make_line(geom_point(geom_centroid($geometry)), $geometry)'
)

In this expression, we are using the make_line function to create a new line that starts at the centroid of the original line and ends at the original line. This will effectively change the start point location of the original line.

Step 3: Apply the Expression

Once we have written the expression, we can apply it to our line layer by clicking on the "Apply" button in the Field Calculator. This will create a new field in our line layer with the modified geometry.

Step 4: Visualize the Results

The final step is to visualize the results of our expression. We can do this by going to the Layer Styling panel in QGIS and selecting the new field that we created. This will display the modified geometry in our line layer.

Example Use Case

Let's say we have a line layer that represents a network of roads. Sometimes, there are intersections of four lines, and we want to modify the structure of one of the intersecting lines. We can use the geometry_by_expression function to change the start point location of the intersecting line. This can be useful in a variety of scenarios, such as:

Modifying the geometry of a line to avoid obstacles
Creating a new line that follows a specific route
Changing the start point location of a line to match a specific coordinate system

Conclusion

In this article, we explored how to change the start point location of an intersecting line using a geometry by expression in QGIS. We learned how to create a new field in our line layer, write an expression that defines the new geometry, apply the expression, and visualize the results. This technique can be useful in a variety of scenarios, such as modifying the geometry of a line to avoid obstacles or creating a new line that follows a specific route. With the power of QGIS's Field Calculator and Geometry Generator tools, we can create complex geometries and modify them to suit our needs.

Additional Resources

For more information on using the geometry_by_expression function in QGIS, please refer to the official QGIS documentation. Additionally, you can find more examples and tutorials on using QGIS's Field Calculator and Geometry Generator tools on the QGIS website.

QGIS Version

This article was written using QGIS 3.22.0. The geometry_by_expression function is available in QGIS 3.0 and later versions.

License

Introduction

In our previous article, we explored how to change the start point location of an intersecting line using a geometry by expression in QGIS. We learned how to create a new field in our line layer, write an expression that defines the new geometry, apply the expression, and visualize the results. In this Q&A article, we will answer some common questions related to using the geometry_by_expression function in QGIS.

Q: What is the `geometry_by_expression` function in QGIS?

A: The geometry_by_expression function in QGIS is a powerful tool that allows you to create new geometries based on existing ones using a mathematical expression. It takes two arguments: the original geometry and the expression that defines the new geometry.

Q: How do I use the `geometry_by_expression` function in QGIS?

A: To use the geometry_by_expression function in QGIS, you need to follow these steps:

Create a new field in your line layer where you will store the modified geometry.
Write an expression that defines the new geometry using the geometry_by_expression function.
Apply the expression to your line layer.
Visualize the results of the expression.

Q: What is the syntax of the `geometry_by_expression` function in QGIS?

A: The syntax of the geometry_by_expression function in QGIS is as follows:

geometry_by_expression(
  "original_geometry",
  "expression"
)

Q: What is the `make_line` function in QGIS?

A: The make_line function in QGIS is a function that creates a new line geometry from two points. It takes two arguments: the start point and the end point.

Q: How do I use the `make_line` function in QGIS?

A: To use the make_line function in QGIS, you need to follow these steps:

Create a new field in your line layer where you will store the modified geometry.
Write an expression that defines the new geometry using the make_line function.
Apply the expression to your line layer.
Visualize the results of the expression.

Q: What is the `geom_centroid` function in QGIS?

A: The geom_centroid function in QGIS is a function that returns the centroid of a geometry. It takes one argument: the geometry.

Q: How do I use the `geom_centroid` function in QGIS?

A: To use the geom_centroid function in QGIS, you need to follow these steps:

Create a new field in your line layer where you will store the modified geometry.
Write an expression that defines the new geometry using the geom_centroid function.
Apply the expression to your line layer.
Visualize the results of the expression.

Q: What are some common use cases for the `geometry_by_expression` function in QGIS?

A: Some common use cases for the geometry_by_expression function in QGIS include:

Modifying the geometry of a line to avoid obstacles
Creating a new line that follows a specific route
Changing the start point location of a line to match a specific coordinate system
Creating a new geometry that is a combination of two or more existing geometries

Q: What are some common errors that can occur when using the `geometry_by_expression` function in QGIS?

A: Some common errors that can occur when using the geometry_by_expression function in QGIS include:

Syntax errors in the expression
Invalid geometry types
Missing or invalid input geometries
Errors in the make_line function

Conclusion

In this Q&A article, we answered some common questions related to using the geometry_by_expression function in QGIS. We covered the syntax of the function, how to use it, and some common use cases and errors that can occur. We hope this article has been helpful in answering your questions and providing you with a better understanding of how to use the geometry_by_expression function in QGIS.

Additional Resources

QGIS Version

This article was written using QGIS 3.22.0. The geometry_by_expression function is available in QGIS 3.0 and later versions.

License

This article is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License. You are free to share and adapt this article, as long as you give appropriate credit and provide a link to the original article.