Introduction
When working with spatial data—especially farm plot polygons—it's not uncommon to encounter geometries with unwanted "spikes." These sharp, unnatural protrusions in polygon edges may indicate errors during data collection or processing. If left uncorrected, these spikes can introduce critical risks in geospatial analysis platforms.
In the Meridia Verify portal, farm data with spikes is detected by the GET05 (Farm plot boundary has spikes) test.
How to Fix Spikes
This tutorial walks you through identifying and resolving such issues using QGIS, a widely used open-source GIS application.
Step 1: Inspecting the Dataset
Once the data is loaded:
Right-click the layer in QGIS
Choose "Open Attribute Table" to start navigating through the data.
Using the Attribute Table:
Identify the data row of the affected polygon.
Right-clicking its row and selecting "Zoom to Feature".
At this point, you should be able to spot the spikes visually—sections of the polygon that unnaturally protrude from the boundary.
Step 2: Understand the Cause
Spikes in polygon boundaries often result from inaccurately recorded coordinate points. These anomalies can originate from:
Field data collection errors, such as:
Poor GPS signal or satellite obstruction
Device malfunction or calibration issues
Human error during manual point entry or navigation
Desktop data processing issues, including:
Incorrect digitisation or tracing of features
Projection mismatches or misalignments
Faulty data imports or format conversions
Step 3: Begin Editing
To fix the polygon:
Right-click the layer
Select "Toggle Editing". Ensure the Digitising Toolbar is enabled (right-click the toolbar area and check the option).
Use the Vertex Tool (also known as "Vertex Tool" or similar) to display the polygon's vertices.
Click on the spike vertex to start editing the polygon.
Step 4: Modify the Geometry
Click on the polygon to display its vertices. You can then:
Move misplaced vertices to their correct locations, or
Delete redundant or incorrect points if they do not belong.
Adjust each spike carefully to smooth out the polygon’s shape.
Step 5: Save Your Work
Once corrections are done:
Save your edits via the toolbar.
Toggle editing off by right-clicking the layer and selecting "Toggle Editing" again.
Your polygon should now appear clean, without any unnatural spikes.
Removing the spikes restores the polygon’s integrity, ensuring accurate spatial representation. This not only prevents analytical errors but also avoids downstream issues when uploading data to platforms like Meridia Verify.
Clean geometries contribute to more reliable validation processes and better decision-making.
📌Some Important Considerations
Always verify the root cause: is the error from the field or the desktop? If the problem originates from field mapping, it’s best to request a remap from the field team.
Save frequently during editing sessions to avoid data loss.
Enable snapping options in QGIS for better precision if needed.
Summary
This tutorial has shown how to identify and resolve spikes in polygon datasets using QGIS. Such geometrical errors are common but manageable with the right tools and understanding. Whether you're correcting them in the field or on the desktop, these steps will help ensure your spatial data is accurate and ready for validation.
We hope this has been a helpful guide. Thanks for reading!