Introduction
Invalid geometries can lead to errors in spatial operations such as overlays, buffering, and spatial joins.
These errors can compromise the accuracy of analyses, cause software tools to fail, and produce unreliable results. Ensuring valid geometries helps maintain data integrity, enables seamless processing, and supports effective decision-making based on spatial data.
You can watch the video below for a step-by-step guide on how to identify errors in invalid geometries, or follow the written instructions provided.
Steps for checking error types in the GET09 results for determining action:
Step 1: Download the Farm Plots scoring critical for GET09
After uploading the data set to the Verify Portal, you can view the GET09 test results on the Results page. To investigate further, download these plots as a GeoJSON file.
Step 2: Open the farm polygon in QGIS
Next, open QGIS and drag the GeoJSON file into your project.
Optionally, you can also add a street map to see exactly where the farm plots are located.
Step 3: Check for Geometry Errors
To understand the errors, go to the View tab and access the Processing Toolbox (1). Locate the Check Validity (2) tool, making sure you select the layer you want to analyse.
Run the tool, and it will generate three temporary layers:
Valid features
Invalid features
Error output (3)
Open the attribute table of the Error output to view the Error messages (4).
Step 4: Understand the Errors
Open the error output layer to see the specific issues. Below are some examples of some of the possible error messages received.
Self-intersection errors occur when a polygon crosses over itself.
Ring self-intersections (holes) appear as gaps in the polygons.
You can explore these features by zooming in and toggling layer visibility. The dots indicate where the errors occur.
Step 5: Fix the Errors
Fixing these errors and corrections may require editing the polygons. These issues can be resolved using software tools such as SpikeRemover from GIS applications or Python libraries to remove such geometric anomalies.
Please refer to this article on How to Edit Farm Plot Polygons in QGIS.
Step 6: Try the Fix Geometry Tool
You can try the Fix Geometry (1) tool to resolve some of the error messages:
Select the polygon you need to fix.
Run the tool to create a temporary layer with the corrected geometry. it will appear under Fixed geometries (2).
⚠️ Note:
This tool may not resolve all errors. In many cases, manual editing will be necessary.
The tool attempts to recreate a valid polygon using all existing vertices. While it can provide a quick solution if it returns correct geometry, a manual review and potential manual edits should always be recommended, regardless of whether the 'Fix Geometry' option is used.
Summary
This tutorial explains how to identify and resolve invalid farm polygons flagged by the GET09 test. After downloading the flagged farm plots as a GeoJSON, you open them in QGIS and use the Check Validity tool to pinpoint errors such as self-intersections or ring self-intersections (holes).
While the Fixed Geometry tool can address some issues, manual editing by a GIS expert is often required. Additional tutorials are available to help you fix these errors and ensure your data is valid.