Tips And Tricks For Debugging Geometry Generation Issues In Qgis

Geometry errors in spatial data can cause a variety of issues when working with QGIS. Identifying and resolving these errors is an important troubleshooting skill for effective QGIS usage. This article provides tips and tricks for debugging common geometry generation issues in QGIS.

Identifying Geometry Issues in QGIS

The first step in fixing geometry errors is identifying that an issue exists. QGIS has built-in tools to help validate geometries and pinpoint problems.

Using the QGIS Validity Checker

The Vector Geometry validity checker under Vector > Geometry Tools is essential for detecting geometry errors. Running this tool will analyze the geometries of a layer and identify invalid ones. The tool adds an “validity” column indicating valid and invalid records. This makes it easy to identify and investigate specific invalid geometries.

It’s important to note that some geometry errors require enabling the appropriate checks in the Geometry validity checker options. For example, self-intersecting polygons will only be flagged if the Check for self-intersections option is enabled. Always check the settings in the Geometry validity checker options dialog before running the validity check to catch all possible errors.

Identifying Specific Geometry Error Types

In addition to generally flagging invalid geometries, the Geometry validity checker provides specific information on the error type. This can assist troubleshooting by pointing directly at the underlying geometry issue.

Common error types to look for include:

  • Self-intersection – a polygon boundary crosses over itself
  • Ring Self-intersection – the boundary of a polygon hole intersects the outer ring
  • Segment intersection – line segments of a feature intersect each other
  • Too few points – a geometry does not have the minimum number of vertices
  • Invalid coordinate – a geometry vertex is invalid (NaN or infinity)
  • Ring not closed – the first and last vertices of a polygon ring are not the same

Reviewing the specific error descriptions will guide troubleshooting efforts towards known geometry defects.

Common Causes of Geometry Errors in QGIS

Understanding the typical causes of geometry problems can speed up identification and correction. Some general sources of geometry errors include:

Invalid Geometries

A geometry is considered invalid if it does not follow standard rules. For example, polygon rings must close and lines cannot self-intersect. Invalid geometries may originate from incorrect digitizing, improper geometry construction, or faulty geometry import/export.

Geometry Precision Issues

Lack of vertex precision is a common trigger for geometry errors. For example, a curved line digitized at low precision may self-intersect at tight corners. Precision also impacts geometry validity checks, causing properly defined geometries to incorrectly flag as invalid.

Topological Errors

Topology defines spatial relationships between geometries. Faulty topology can yield geometry interaction issues like invalid self-intersections. These errors typically arise when topology rules are undefined, incomplete, or not properly applied.

Diagnosing the underlying cause of a geometry error guides the appropriate resolution tactic.

Fixing Geometry Errors in QGIS

Once an invalid geometry is identified, correcting the specific error is essential. Targeted tools in QGIS assist remedying geometry defects.

Correcting Invalid Geometries

The Check Geometries tool under Vector > Geometry Tools provides options to repair common invalid geometries. This includes fixing polygon winding order, redefining ring orientation, and removing duplicate vertices. The tool outputs a new layer with corrected geometries, preserving the original attribute data.

For more complex geometry correction, the Reshape Features tool enables manual editing of invalid feature geometries. Users can redraw lines and redraw polygon boundaries to mend overlaps, gaps, and other defects in the geometry.

Increasing Geometry Precision

The Simplify tool found under Vector > Geometry Tools is used to increase or decrease geometry precision. Simplifying geometries removes extraneous vertices while retaining essential shape. This avoids precision-related self-intersections and other geometry errors.

Increasing geometry precision is especially important when experiencing precision-related validity check errors. Set an appropriately small Simplify tolerance when correcting these issues.

Resolving Topological Errors

Topology errors occur when spatial relationships between geometries are inaccurate. Defining topology rules and resolving violations is key to fixing these problems.

The Topology Checker tool under Vector > Topology Checker validates topology relationships according to defined rules. Any violations are highlighted as geometry errors for correction.

Using the snapped geometries output from the Integrate tool is an alternative approach. This tool conforms geometries to layer topology, correcting any interaction issues.

Establishing complete topology rules suited to the data prevents invalid topological geometries.

Preventing Geometry Errors in QGIS

While resolving existing geometry issues is important, prevention is ideal. Various QGIS best practices help avoid generating geometry errors when working with spatial data.

Setting Appropriate Geometry Precision

Using proper geometry precision settings when digitizing or converting data avoids downstream precision-related defects. This includes sufficiently small snapping tolerances and simplify tolerances.

An appropriate validate tolerance is also crucial when validity checking geometries. Too strict a check may erroneously flag valid features based solely on precision differences.

Ensuring Valid Input Data

Verifying the validity of source data before import into QGIS is key. This avoids importing existing invalid geometries. Check source data with the same validity tools available within QGIS to catch any geometry issues.

When digitizing new features, continuously use the validity checker to immediately catch digitizing errors. Identify and fix issues early before their impacts cascade.

Building Comprehensive Topology Rules

Topology rules validate spatial relationships between geometries in a layer. Defining a complete set of topology rules suited to the data is necessary. Standard rules to always implement include:

  • Must not have invalid geometries
  • Must not have multi-part geometries
  • Must not overlap
  • Must not intersect
  • Must not have gaps

Building these rules through the Topology Checker will guide geometry generation according to essential topological constraints.

Following QGIS best practices, especially regarding precision and topology, leads to clean geometry generation and avoidance of troublesome errors.

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