Introduction
This practical guide covers resolving a common yet often overlooked issue in GIS: duplicated vertices in polygons. If you’ve spent time working with spatial data, you’ll know how vital it is to keep your geometries clean and accurate.
While some duplicates may seem harmless, others, particularly non-consecutive duplicates, can distort your data in ways that quietly sabotage analysis and mapping.
In the Meridia Verify portal, data with duplicated vertices is flagged with a risk profile for GET04 (Farm plot has duplicate boundary vertices after rounding to 6 decimals) test.
Identifying and Fixing Duplicate Vertices
In this tutorial, we’ll walk through how to spot and fix these issues using QGIS.
Understand the Types of Duplicate Vertices
While reviewing polygons, you may encounter two types of duplicate vertices. This section outlines the distinction between Consecutive Duplicates and Non-Consecutive Duplicates.
Consecutive Duplicates
These are repeated vertices right after one another. They usually don’t affect the polygon's shape and topological errors, and most GIS tools ignore them. Meridia Verify's GET04 test doesn’t flag these for the same reasons.
Non-Consecutive Duplicates
This is where things get tricky. These duplicates appear separated by other vertices and can cause the polygon to fold or intersect itself. These are the real issues flagged by GET04 and must be addressed.
⚠️ Note:
Since consecutive duplicate vertices do not pose an issue in our context, the remainder of this article focuses on addressing non-consecutive duplicate vertices in QGIS.
Step 1: Inspect the Data in QGIS for non-consecutive duplicated vertices
Open your data in QGIS to begin the inspection.
Right-click the layer to inspect.
Select Open Attribute Table. Check for the farm plot IDs flagged by GET04.
After finding the polygon that requires some handling, we can start editing it.
Step 2: Fixing the Issue with Two Reliable Methods
Method 1: Automatically Clean Geometry Using v.clean
QGIS integrates GRASS GIS tools, and one of the most effective is the v.clean function.
Open the Processing Toolbox: Go to Processing > Toolbox.
Search for
v.clean: In the search bar, typev.cleanand open the tool.
In the v.clean window, follow these steps to use the tool properly.
Set Input Layer: Your problem polygon layer.
Set the Cleaning tool: Choose
rmd_upl(Remove Duplicate Points).Run the Tool: Click Run. A new cleaned layer will be added.
The cleaned polygon may be split into multiple pieces. This isn’t an error—it reflects where the geometry was previously malformed.
Next Steps: You may need to either:
Delete irrelevant fragments, or
Merge them back together if they belong to a single feature.
This method is fast, but it assumes you're okay with the possible splitting of your polygons.
Method 2: Manually Edit Using the Vertex Tool
When you need more control—say, to maintain a single feature—manual editing is your best bet.
Right-click the layer to open the menu.
Toggle Editing Mode: Click the Toggle Editing button (pencil icon) on the toolbar.
Activate the Vertex Tool: Enable the Vertex Tool to interact with the polygon vertices directly.
Click on the vertices of the polygon that require editing.
Look for identical coordinates that are not next to each other in the sequence. You can either:
Delete the extra vertex if it’s unnecessary.
Move it if it was placed incorrectly (e.g., due to a field error). If possible, confirm with field information to maintain accuracy.
When done, click Toggle Editing again and choose Save Layer Edits.
This approach ensures you preserve the original feature's identity and shape, but it takes more time.
Final Outcome & Key Reminders
Whether you use v.clean or manual editing, the goal is the same: topologically clean polygons that reflect the farm plots in reality.
With
v.clean, you often end up with multiple, distinct polygons—helpful when your original feature was accidentally merged.With manual editing, you restore the intended geometry while keeping the feature whole.
Both approaches lead to data that’s ready for precise mapping, analysis, and decision-making—free from the hidden mess that duplicate vertices can create.
📌 Considerations to Keep in Mind
Consecutive Duplicates: These are harmless and usually don’t need fixing.
Always turn to manual validation: Keep in mind that, regardless of the method you choose, manual validation and adjustments are strongly recommended to ensure data accuracy, rather than relying solely on automated tools. Exercise careful judgment when selecting your approach:
Use
v.cleanwhen speed matters or when splitting is acceptable.Use manual editing when accuracy and context are key.
Verify with Field Data: When in doubt, compare your edits with source data or imagery.
Keep Saving: Save often to avoid losing progress, especially during manual edits
Summary
Fixing duplicate vertices might seem like a minor task, but it plays a big role in maintaining spatial data quality. By learning to distinguish between harmless and harmful duplicates—and by using QGIS tools smartly—you can keep your datasets clean, reliable, and ready for action.
We hope this guide helps make your GIS workflows smoother and your data more dependable.