Vector Analysis

Vector analysis in WbW-QGIS covers geometry validation, overlay operations, attribute enrichment, spatial selection, proximity analysis, and spatial joining. Whitebox supplements the native QGIS vector toolbox with high-performance tools built on the wbtopology spatial index.

This chapter walks through a complete parcel-attribute enrichment workflow — a common task in land management and environmental assessment.

Key Concepts

• Geometry validity: Self-intersecting rings, duplicate vertices, and unclosed polygons cause silent failures in overlay tools. Always validate and repair geometry before any overlay operation.
• Spatial join: Assigns attributes from one layer to features in another based on spatial relationship (intersects, contains, nearest). Supports aggregation modes (first, last, sum, mean, count, min, max).
• Near analysis: Finds the nearest feature (or features within a distance) from a source layer to a target layer. Returns distance and optional target attributes.
• Clip / Intersection / Difference: Standard polygon overlay operations. Clip retains the geometry of input A bounded by B. Intersection produces the geometric overlap. Difference removes the overlap.
• Add geometry attributes: Computes and appends area, perimeter, length, centroid coordinates, or bounding box dimensions as new attribute fields.
• Select by location: Spatial predicate query (intersects, within, contains, etc.) that produces a feature selection or a new filtered layer.

End-to-End Workflow: Parcel Attribute Enrichment

This workflow assigns catchment statistics and proximity-to-road measurements to a parcel layer.

Inputs

Step 1 — Validate and Repair Geometry

Processing Toolbox → Vector Geometry → Fix Geometries (QGIS native)

Run Check Validity on both catchments.shp and roads.shp and fix any errors before proceeding.

Step 2 — Add Geometry Attributes to Parcels

Processing Toolbox → Whitebox Workflows → Vector Analysis → Add Geometry Attributes

This appends AREA, PERIMETER, and centroid X/Y fields to each parcel.

Step 3 — Spatial Join: Assign Catchment Attributes to Parcels

Processing Toolbox → Whitebox Workflows → Vector Analysis → Spatial Join

Each parcel now carries the attributes of the catchment it intersects most.

Step 4 — Near: Distance from Each Parcel to Nearest Road

Processing Toolbox → Whitebox Workflows → Vector Analysis → Near

Appended fields: NEAR_DIST (metres to nearest road segment), NEAR_FID (FID of nearest road feature).

Step 5 — Select High-Priority Parcels

Processing Toolbox → Whitebox Workflows → Vector Analysis → Select By Attribute or use QGIS Select by Expression:

"AREA" > 10000 AND "NEAR_DIST" < 500 AND "mean_slope" < 10

Export the selection as priority_parcels.shp using Layer → Export → Save Selected Features As.

Step 6 — Clip Parcels to Study Area (Optional)

Processing Toolbox → Whitebox Workflows → Vector Analysis → Line Polygon Clip (for lines) or QGIS Clip for polygon-on-polygon.

TopoJSON Conversion Chain (QGIS Interop)

Use this workflow when you need to exchange shared-boundary vector data with web clients while keeping a GeoPackage working copy for analysis.

Inputs

Step 1 — Run a Whitebox vector operation and emit TopoJSON

Processing Toolbox → Whitebox Workflows → Vector Analysis → Add Geometry Attributes

This confirms the plugin accepts .topojson output targets in a normal vector processing chain.

Step 2 — Re-open TopoJSON and convert back to GeoPackage

1. Add zones_metrics.topojson to the QGIS project.
2. Right-click the layer, then choose Export → Save Features As....
3. Set format to GeoPackage and save as zones_metrics_roundtrip.gpkg.

Step 3 — Validate roundtrip integrity

Check these before publishing or reusing the roundtrip layer:

• Feature count matches source layer.
• Core attributes (e.g., ID fields and geometry metrics) are preserved.
• CRS is correctly populated on the roundtrip GeoPackage.

Recommended use pattern

• Keep .gpkg as the editable analysis master.
• Generate .topojson as interchange or web-delivery artifacts.
• Re-import to .gpkg for heavier downstream spatial analysis.

TopoJSON Boundary-Preserving Generalization Chain

Use this chain when you need smaller delivery payloads while preserving shared boundary consistency during simplification.

Inputs

Step 1 — Simplify and emit TopoJSON

Processing Toolbox → Whitebox Workflows → Vector Analysis → Simplify Features

Step 2 — Inspect topology consistency in QGIS

1. Add admin_units_simplified.topojson to the map.
2. Inspect shared boundaries at high zoom for slivers/gaps.
3. Validate feature count versus source before publication.

Step 3 — Export analysis copy

Export to admin_units_simplified.gpkg for downstream joins/overlay work.

TopoJSON Transport + Enrichment Return Chain

Use this chain when TopoJSON is used only for transport and you need to return to an analysis-grade format for attribute enrichment.

Inputs

Step 1 — Convert transport input to GeoPackage

1. Add transport_in.topojson to the project.
2. Export as transport_stage.gpkg.

Step 2 — Apply enrichment tools

Run Whitebox vector tools against transport_stage.gpkg:

• Add Geometry Attributes for geometry metrics.
• Spatial Join for contextual attribute enrichment.
• Near for proximity attributes.

Step 3 — Emit deliverables

Write two outputs:

• transport_enriched.gpkg for analytic persistence.
• transport_enriched.topojson for interchange/web handoff.

Python Console Equivalent

import processing

# Step 1: fix geometry
processing.run('native:fixgeometries', {
    'INPUT': '/data/parcels.shp',
    'OUTPUT': '/data/parcels_valid.shp',
})

# Step 2: add geometry attributes
processing.run('whitebox_workflows:add_geometry_attributes', {
    'input': '/data/parcels_valid.shp',
    'units': 'Metres',
    'output': '/data/parcels_geom.shp',
})

# Step 3: spatial join
processing.run('whitebox_workflows:spatial_join', {
    'input': '/data/parcels_geom.shp',
    'join': '/data/catchments.shp',
    'spatial_relation': 'Intersects',
    'join_method': 'First',
    'output': '/data/parcels_joined.shp',
})

# Step 4: near
processing.run('whitebox_workflows:near', {
    'input': '/data/parcels_joined.shp',
    'near': '/data/roads.shp',
    'max_dist': 5000.0,
    'output': '/data/parcels_near.shp',
})

print("Parcel enrichment complete.")

Advanced: Simplify Features for Cartographic Output

Large polygon datasets with many vertices slow down rendering and tile export. Simplify geometries while preserving topology.

Processing Toolbox → Whitebox Workflows → Vector Analysis → Simplify Features

processing.run('whitebox_workflows:simplify_features', {
    'input': '/data/priority_parcels_clipped.shp',
    'algorithm': 'DouglasPeucker',
    'tolerance': 5.0,
    'output': '/data/parcels_simplified.shp',
})

Common Pitfalls

Validation Checklist

• All input layers pass geometry validity check.
• All vector layers share the same projected CRS.
• Spatial join result preserves original feature count (check attribute table row count).
• NEAR_DIST values are plausible (inspect histogram).
• Simplified geometry does not self-intersect at the chosen tolerance.
• Attribute field names in output do not exceed shapefile 10-character limit.