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Glossary

Essential GIS and FilterMate terminology explained in plain language.

A​

Attribute​

A property or characteristic of a geographic feature stored as data in a table.

Example: A building feature might have attributes like height: 25, type: 'residential', year_built: 2015.

In FilterMate: Used in expression-based filtering to select features matching specific criteria.

Attribute Filter​

A filter that selects features based on their attribute values rather than spatial relationships.

Example:

population > 100000
city = 'Paris'
year >= 2020

See: Filtering Basics

B​

Backend​

The database engine or processing system FilterMate uses to execute spatial operations.

Analogy: Like choosing between a bicycle (OGR), motorcycle (Spatialite), or sports car (PostgreSQL) for your journey.

Types:

  • PostgreSQL: Fastest, best for >50k features
  • Spatialite: Good balance, <50k features
  • OGR: Universal compatibility, slowest

See: Backend Overview

Buffer​

A zone of specified distance around a geometry, like a "force field" extending outward.

Visual:

Point buffer:     O → ⭕ (circle)
Line buffer:      ─ → ▭ (capsule)
Polygon buffer:   □ → ▢ (expanded polygon)

Example: "200m buffer around a road" = all land within 200 meters of the road centerline.

Buffer Types:

  • Round (Planar): Fast, accurate for small areas
  • Round (Geodesic): Accurate for large areas (accounts for Earth's curvature)
  • Square: Computational optimization (rarely used)

See: Buffer Operations

Buffer Distance​

The radius or width of a buffer zone, measured in specified units.

Common Values:

  • Walkability: 400-800 meters (5-10 minute walk)
  • Bikeability: 1-2 kilometers
  • Service Areas: 5-10 kilometers
  • Environmental Zones: 50-500 meters

Units: meters, kilometers, feet, miles, degrees (avoid for distance)

C​

Coordinate Reference System (CRS)​

A mathematical framework that defines how coordinates relate to locations on Earth's surface.

Common CRS Types:

  • Geographic (EPSG:4326): Latitude/Longitude in degrees
  • Projected (EPSG:3857): X/Y in meters (flat map)
  • Local (various): Optimized for specific regions

FilterMate Behavior: Automatically reprojects layers when needed (you'll see 🔄 indicator).

See: CRS Handling

E​

EPSG Code​

A standardized numerical identifier for Coordinate Reference Systems.

Common Codes:

  • EPSG:4326 - WGS84 (GPS coordinates, worldwide)
  • EPSG:3857 - Web Mercator (Google Maps, web mapping)
  • EPSG:2154 - Lambert 93 (France)
  • EPSG:32633 - UTM Zone 33N (Central Europe)

Usage: Specify CRS when exporting or reprojecting layers.

Find Your CRS: epsg.io search engine

Expression​

A formula or query written in QGIS expression language to filter or calculate values.

Types:

  • Boolean: Returns true/false (for filtering)
  • Numeric: Returns numbers (for calculations)
  • String: Returns text (for labeling)

Example:

-- Boolean expression for filtering
population > 50000 AND status = 'active'

-- Numeric expression
area / 10000  -- Convert m² to hectares

-- String expression
upper(name) || ' (' || country || ')'

See: Expression Cheat Sheet

F​

Feature​

A single geographic entity (point, line, or polygon) with associated attributes.

Examples:

  • Point Feature: A building location, tree, fire hydrant
  • Line Feature: A road segment, river, pipeline
  • Polygon Feature: A parcel boundary, lake, administrative zone

In QGIS: Rows in the attribute table represent features.

Filter​

A query or condition that selects a subset of features from a layer.

Types in FilterMate:

  1. Attribute Filter: Based on data values
  2. Geometric Filter: Based on spatial relationships
  3. Combined Filter: Both attribute AND geometric

Result: Temporary view showing only matching features (original data unchanged).

G​

Geodesic​

Calculations that account for Earth's curvature (spherical/ellipsoidal).

Use When:

  • Large geographic areas (countries, continents)
  • High-accuracy distance measurements
  • Working across multiple UTM zones

Contrast: Planar (flat Earth approximation, faster but less accurate at large scales)

In FilterMate: Available as "Round (Geodesic)" buffer type.

Geometric Filter​

A filter based on spatial relationships between features in different layers.

Example: "Find all buildings within 100m of a river"

  • Target: Buildings layer
  • Reference: Rivers layer
  • Predicate: Within Distance
  • Buffer: 100 meters

See: Geometric Filtering

Geometry​

The shape and location of a geographic feature (point, line, polygon, etc.).

Types:

  • Point: Single coordinate (0D)
  • LineString: Connected points (1D)
  • Polygon: Closed shape with area (2D)
  • MultiPoint/MultiLineString/MultiPolygon: Collections

Common Issues:

  • Invalid geometry: Self-intersecting, holes outside boundary
  • NULL geometry: Missing spatial data
  • Empty geometry: Valid but contains no coordinates

Repair: QGIS Processing → Fix Geometries

GeoPackage (GPKG)​

An open, standards-based file format for storing geospatial data.

Advantages:

  • Single-file (vs Shapefile's multiple files)
  • No attribute name length limits
  • Supports multiple layers
  • Better performance than Shapefile

Recommended Format: Best for QGIS workflows and data sharing.

File Extension: .gpkg

GIST Index​

A spatial index type used in PostgreSQL/PostGIS for fast spatial queries.

Purpose: Dramatically speeds up spatial operations (10-100× faster).

FilterMate: Automatically creates GIST indexes on filtered views.

Manual Creation:

CREATE INDEX idx_geom ON my_table USING GIST(geom);

H​

History Manager​

FilterMate's built-in undo/redo system for filter operations.

Features:

  • Tracks up to 100 filter operations
  • Undo/redo buttons in toolbar
  • Session-based (not saved with project)
  • Alternative: Save important filters as Favorites

See: Filter History

L​

Layer​

A collection of geographic features of the same type (all points, all polygons, etc.).

Examples:

  • Roads layer (lines)
  • Buildings layer (polygons)
  • Trees layer (points)

In QGIS: Appears in Layers Panel, can be styled and queried independently.

M​

Materialized View​

A database query result stored as a physical table for fast access.

FilterMate Use: PostgreSQL backend creates materialized views for filtered data.

Benefits:

  • Sub-second query times on large datasets
  • Spatial indexes automatically created
  • Refreshable when data changes

Performance: 10-50× faster than on-the-fly queries.

O​

OGR​

Open source library for reading/writing vector geospatial data formats.

FilterMate Backend: Used for Shapefiles, GeoPackage (when not Spatialite), and other file formats.

Performance: Slowest backend but most compatible.

When Used:

  • Shapefile layers
  • GeoPackage layers (non-Spatialite)
  • File geodatabases
  • GeoJSON files

See: OGR Backend

P​

Planar​

Calculations assuming a flat Earth surface (2D Cartesian plane).

Use When:

  • Small geographic areas (<100 km²)
  • Local coordinate systems (UTM, State Plane)
  • Speed is priority over precision

Accuracy: ±0.1% error for areas <10km across, ±1% for <100km.

In FilterMate: Default buffer type "Round (Planar)".

PostGIS​

PostgreSQL extension adding spatial database capabilities.

Features:

  • Advanced spatial functions
  • Spatial indexing (GIST)
  • Coordinate transformations
  • Topology operations

FilterMate: Best backend for large datasets (>50k features).

See: PostgreSQL Backend

Predicate​

See Spatial Predicate

Primary Key​

A unique identifier field for each feature in a database table.

Examples: id, gid, fid, objectid

Importance: Required for PostgreSQL layers in QGIS to enable editing.

FilterMate: Displays primary key in layer info panel.

psycopg2​

Python library for connecting to PostgreSQL databases.

FilterMate Requirement: Must be installed for PostgreSQL backend support.

Installation: See Common Mistakes - PostgreSQL Unavailable

R​

Reference Layer​

In geometric filtering, the layer containing features you're filtering against (not the layer being filtered).

Example:

Goal: Find buildings near roads
Target Layer: Buildings (being filtered)
Reference Layer: Roads (spatial reference)

In FilterMate: Selected in "Reference Layer" dropdown in Geometric Filter section.

S​

Shapefile​

Legacy vector data format widely used in GIS (despite limitations).

Limitations:

  • Multiple files required (.shp, .shx, .dbf, .prj, etc.)
  • 10-character attribute name limit
  • 2GB file size limit
  • Limited data types

Modern Alternative: GeoPackage (GPKG)

FilterMate: Supported via OGR backend (slower than PostgreSQL/Spatialite).

Spatial Index​

A database structure that speeds up spatial queries by organizing features by location.

Types:

  • GIST (PostgreSQL/PostGIS)
  • R-Tree (Spatialite)
  • Quadtree (some file formats)

Performance Impact: 10-100× faster spatial queries.

FilterMate: Automatically created by PostgreSQL and Spatialite backends.

Spatial Predicate​

A test for a specific geometric relationship between two features.

Common Predicates:

PredicateMeaningExample Use
IntersectsTouch or overlapBuildings on parcels
ContainsA completely wraps BPoints in polygons
WithinA completely inside BParcels in city boundary
CrossesLines intersectRoads crossing railways
TouchesShare boundary but don't overlapAdjacent parcels
DisjointDon't touch or overlapIsolated features
OverlapsPartial overlap (same dimension)Overlapping zones

Visual Guide: See Spatial Predicates Reference

Spatialite​

SQLite extension adding spatial database capabilities.

Characteristics:

  • File-based (single .sqlite file)
  • Good performance for medium datasets (<50k features)
  • No server setup required
  • Spatial indexing via R-Tree

FilterMate: Middle-ground backend between PostgreSQL and OGR.

See: Spatialite Backend

Subset String​

QGIS mechanism for filtering layer features using SQL-like expressions.

FilterMate: Applies filters by setting subset strings on layers.

Example:

-- Subset string applied to layer
"population" > 100000 AND "status" = 'active'

View Current Subset: Layer Properties → Source → Provider Feature Filter

T​

Target Layer​

The layer being filtered (features will be selected from this layer).

Example:

Goal: Find buildings near roads
Target Layer: Buildings ← This layer gets filtered
Reference Layer: Roads

In FilterMate: Selected in main "Layer Selection" dropdown.

U​

UTM (Universal Transverse Mercator)​

A projected coordinate system dividing Earth into 60 zones, each 6° wide.

Properties:

  • Units: meters
  • Good accuracy within each zone
  • Minimizes distortion for local areas

Example CRS:

  • EPSG:32633 - UTM Zone 33N (Central Europe)
  • EPSG:32736 - UTM Zone 36S (East Africa)

Use: Ideal for local/regional analysis requiring metric measurements.

V​

Vector Layer​

Geographic data represented as points, lines, or polygons (vs raster images).

Components:

  • Geometry: Shape and location
  • Attributes: Data table with properties
  • Style: Visual appearance on map

FilterMate: Works exclusively with vector layers (not rasters).

W​

WGS84​

World Geodetic System 1984 - the standard geographic coordinate system.

EPSG Code: 4326

Properties:

  • Units: degrees (latitude/longitude)
  • Global coverage
  • Used by GPS

Range:

  • Latitude: -90° to +90° (South to North)
  • Longitude: -180° to +180° (West to East)

FilterMate Behavior: Automatically converts to EPSG:3857 for metric buffer operations.

Symbols & Icons​

🔄 Reprojection Indicator​

Appears in logs when FilterMate automatically transforms layer CRS.

Example: 🔄 Reprojecting from EPSG:4326 to EPSG:3857

🌍 Geographic CRS Indicator​

Shown when FilterMate detects geographic coordinates and converts for metric operations.

Example: 🌍 Geographic CRS detected, using EPSG:3857 for buffer

⚡ PostgreSQL Performance Icon​

Indicates PostgreSQL backend is active (fastest option).

⏱️ Spatialite Performance Icon​

Indicates Spatialite backend (good performance).

⚠️ Performance Warning Icon​

Shown when using OGR backend on large datasets.

✓ Valid Expression​

Green checkmark indicating expression syntax is correct.

✗ Invalid Expression​

Red X indicating expression has syntax errors.

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