Dealing With Errors And Incomplete Results From Polygon Centreline Tools
Troubleshooting Polygon Centreline Tool Errors
Polygon centreline tools, designed to extract the central spine line from area features in vector GIS datasets, are invaluable for many analysis tasks. However, these tools can sometimes produce errors or incomplete results that require troubleshooting.
Locating Gaps in Polygon Borders
Gaps along the borders of area polygons prevent the centreline algorithm from generating continuous central lines, instead creating multiple disjointed segments. Visually inspecting polygon boundaries to identify and resolve gaps is therefore an important first step.
Use snapping and topology rules to mend disconnects between end vertices of polygon borders. Examine areas where multiple polygons intersect and verify shared borders align precisely, closing small defects. Enabling edge overlay visualization flags boundary crossings needing realignment. Correcting gaps ensures unbroken polygon shells essential for quality centrelines.
Handling Self-Intersecting Polygons
Self-intersections within area polygon geometry generates invalid structures that obstruct centreline tools. Detection strategies include angle filtering to highlight sharp reversals potentially denoting interleaved borders. Spatial indexing clustered close point pairs checks for feature crossing. Validate borders with winding order rules that disqualify backwards sequences.
Eliminate self-intersections through reshaping interventions that sever and reconnect improperly crossing ring edges. Restrict editing to minimal partitions that preserve overall form. Double-check repaired polygons satisfy topological prerequisites before attempting centreline extraction to avoid residual defects short-circuiting process.
Fixing Invalid Geometry Errors
Malformed polygon structure triggers invalid geometry messages blocking centreline generation. Causes include mismatched start and end points of rings, unclosed perimeters, discontinuities, and misconfigurations. Confirm number, order and orientation of vertices comply with polygon specifications.
Rewrite rules standardizing connectivity, directionality and completeness correct many corruption issues. Supplementary utility scripts target specialized defects. Handle exceptions and warn of unresolved errors that require manual rebuilding. Validating geometry integrity eliminates bulk obstruction to centreline workflows.
Cleaning Small Sliver Polygons
Sliver polygons with very narrow form and acute angles resist centreline computation which struggles traversing tightly constrained paths. Detection tools help screen for size and shape outliers using dimensional filters and symbolic approximation. Aggregate minor fragments through merging and elimination functions.
Apply dimensional tolerances setting minimum area and width thresholds to govern sliver removal while avoiding overzealous processing. Visually inspect flagged output balancing automated generalization against preservation of essential detail and variation.
Setting Appropriate Tolerances
The parsing sensitivity of centreline algorithms relies on configuring suitable coordinate tolerances measuring allowable deviation. Excessively strict thresholds cause failure from insignificant variance while overly relaxed parameters miss substantive defects.
Set key metrics like maximum distance, Fuzzy tolerance and dangle length based on dataset accuracy and intended use scale plus allowable generalisation. Assess results at different settings to determine optimal values. Document special cases needing distinct parameters.
Best Practices for Vertex Density
Centreline quality strongly depends on appropriate border vertex density with sparse under-sampling producing disjointed approximate lines while over-densed boundaries have closely stacked vertices straining computation.
Adaptive smoothing algorithms automatically optimize borders by adding and removing vertices guided by angle, distance and shape criteria. User-directed decimation tools with density-based thinning help manual resampling towards moderate target thresholds while minimizing loss of significant inflections.
Example Script for Automated Cleaning
This python script standardizes a directory of area polygon layers to conform to centreline tool prerequisites through a sequence of correction functions targeting common defects.
Import libraries enable input/output, analysis and processing. Defect diagnosis methods embed validation rules and issue indicators. Correction functions reshape boundaries, restructure rings and rewrite vertices to resolve identified problems. Optional buffers add protective tolerances. Output report & metadata document changes.
Strategies for Incomplete Polygon Coverage
Gaps in source polygon data propagate downstream as coverage voids in centreline outputs. Absence of input area geometry blocks creation of corresponding central axes resulting in patchy incomplete results.
Inspector tools help quantify and characterize missing elements by summarizing grid statistics and spatial distributions. Clustering and machine learning methods predict likely gap locations to guide manual digitizing. Use line simplification, smoothing and interpolation techniques to bridge minor data voids.
Leveraging Supplementary Dataset Layers
Add secondary grid, contour and image sources as references to help extrapolate polygons into poorly mapped areas and reconstruct fragmentary baseline data. Integrate partially overlapping niche datasets covering distinct subregions.
Mosaic together available geospatial assets with optimal edge matching and featherting to provide a unified supplementary foundation. Derive enhanced base outlines by tracing reliable indicators in source imagery and elevation models. Align reconstructed polygons with original nucleus via constraints and regularizations.
Manual Editing Tips and Tricks
Despite algorithmic corrections, some imperfections persist requiring selective user improvements for quality centreline derivation. Target incremental interventions to maximize downstream benefits relative to editing effort.
Focus changes along critical blue lines with most significant network impact. Smoothing targeted border segments avoids causing distortions elsewhere. Small targeted tweaks to close minor gaps or align named features can greatly boost global utility if well placed.
