GeoSubSampler

A QGIS plugin for subsampling geological orientations, faults, and polygons. This plugin provides various methods to reduce the complexity of geological datasets while preserving important structural information.

Overview

GeoSubSampler is designed to help geologists and GIS professionals manage large geological datasets by providing intelligent subsampling methods. The plugin supports processing of:

• Structural orientation data (dip/strike measurements)
• Geological polygons (rock units, formations)
• Fault and contact lines

Installation

1. Download the plugin files
2. Place them in your QGIS plugins directory
3. Enable the plugin through QGIS Plugin Manager
4. The plugin will appear as a dockable widget in QGIS

Features

1. Structural Orientation Subsampling

Input Field Configuration

Structural Data Fields

• Layer Selection: Choose the point layer containing structural measurements
• Dip Field: Field containing dip angle values
  • Type: Numeric field from selected layer
  • Description: Angle of dip from horizontal (0-90°)
• Dip Direction/Strike Field: Field containing directional information
  • Type: Numeric field from selected layer
  • Description: Either dip direction (0-360°) or strike direction
• Dip Direction Checkbox: Toggle between dip direction and strike convention
  • Description: Check if using dip direction, uncheck if using strike

Contact/Boundary Data

• Polyline Layer: Layer containing geological contacts or boundaries
  • Type: Line layer
  • Description: Used for first-order subsampling relative to geological boundaries

Stochastic Subsampling

Randomly samples a percentage of orientation points.

Parameters:

• Retain Percentage: Percentage of points to keep (0-100%)
  • Type: Float
  • Range: 0.0 - 100.0
  • Description: Controls what fraction of the original dataset is retained

Grid Cell Averaging

Divides the area into grid cells and averages orientations within each cell.

Parameters:

• Grid Size: Size of grid cells for averaging
  • Type: Float
  • Units: Layer units (meters for projected, degrees for geographic)
  • Description: Larger values create coarser grids with more averaging

Kent Statistics Grid Averaging

Uses Kent distribution statistics for spherical averaging of orientations within grid cells.

Parameters:

• Grid Size (Kent): Grid cell size for Kent statistical averaging
  • Type: Float
  • Units: Layer units
  • Description: Grid size for applying Kent distribution statistics

Kent Statistics with Outlier Removal

Applies Kent statistics while removing the largest outliers within each grid cell.

Parameters:

• Grid Size (Kent Outlier): Grid cell size for Kent averaging with outlier removal
  • Type: Float
  • Units: Layer units
• Outlier Threshold: Threshold for outlier identification and removal
  • Type: Float
  • Description: Statistical threshold for identifying and removing outliers

First Order Subsampling

Retains orientation points based on proximity to geological contacts and angular relationships.

Parameters:

• Distance Buffer: Buffer distance around geological contacts
  • Type: Float
  • Units: Layer units
  • Description: Points within this distance of contacts are prioritized
• Angle Tolerance: Angular tolerance for point retention
  • Type: Float
  • Units: Degrees
  • Description: Tolerance for angular relationships between measurements and contacts

2. Fault/Line Processing

Segment Merging

Merges connected fault segments based on geometric criteria.
WARNING: This is not particulary optimised and may take hours to complete on large (>10k polylines) datasets

Parameters:

• Distance Tolerance: Maximum distance between line endpoints for merging
  • Type: Float
  • Units: Layer units
  • Description: Endpoints closer than this distance are candidates for merging
• Search Angle Tolerance: Maximum angle difference for parallel lines
  • Type: Float
  • Units: Degrees
  • Description: Lines with orientations within this tolerance are considered parallel
• Join Angle: Minimum angle at connection points
  • Type: Float
  • Units: Degrees
  • Description: Rejects connections that create angles smaller than this value

Length-Based Subsampling

Filters fault lines based on minimum length criteria.

Parameters:

• Minimum Fault Length: Threshold length for fault retention
  • Type: Float
  • Units: Layer units
  • Description: Faults shorter than this length are removed

3. Polygon Processing

Minimum Area Filtering

Removes small polygons and fills holes based on area thresholds.

Parameters:

• Minimum Polygon Diameter: Threshold diameter for polygon retention
  • Type: Float
  • Units: Kilometers
  • Description: Polygons with equivalent diameter smaller than this are removed/merged
• Node Tolerance: Tolerance for node snapping during processing
  • Type: Float
  • Units: Layer units
  • Description: Distance tolerance for merging nearby vertices

Stratigraphic Priority Fields

Define the hierarchy for polygon merging decisions:

• Priority 1-5 Fields: Stratigraphic fields in order of importance
  • Type: Attribute fields from polygon layer
  • Description: Used to determine which polygon attributes are preserved during merging
  • Order: Priority 1 has highest importance, Priority 5 has lowest

Lithological Classification

• Lithology Field: Field containing lithological information
  • Type: Attribute field
  • Description: Used for lithological classification during polygon processing

Dyke/Intrusion Handling

• Dyke Field: Field identifying intrusive bodies
  • Type: Attribute field
• Dyke Codes: Comma-separated list of codes identifying dykes/intrusions
  • Format: Text list (e.g., DY1, DY2, INTX-D )
  • Description: Special processing applied to polygons with these codes

Series Processing Options

• Series Checkbox: Enable processing of multiple scales
  • Description: When checked, creates multiple outputs at different scales
• Increment Value: Step size for series processing
  • Type: Float
  • Units: Same as minimum diameter
  • Description: Increment between successive processing scales

Technical Notes

Coordinate System Handling

• Geographic Coordinates: Grid sizes are automatically converted from meters to degrees (÷110,000)
• Projected Coordinates: Grid sizes are used directly in the specified units

File Format Requirements

• Input layers must be saved as shapefiles on disk
• The plugin cannot process temporary or memory layers

Output Naming Convention

Output files follow the pattern: [original_name]_[method]_[parameters].shp

• If a file with the same name exists, a random 5-digit suffix is added

Performance Considerations

• Segment Merging: Very slow for large datasets (>1 second per object)
• Large Datasets: Consider using subsampling methods before applying computationally intensive operations

Error Handling

The plugin includes validation for:

• Missing required fields
• Invalid layer selections
• File access permissions
• Coordinate system compatibility

Dependencies

• QGIS Python API (PyQt, qgis.core)
• GeoPandas
• NumPy
• Random and time modules for statistical operations
• rtree for polyline merging

License

This program is free software under the MIT License.

Author

Ranee Joshi
Email: raneejoshi@gmail.com

Version Information

• Begin Date: 2025-07-28
• Generated by: Plugin Builder
• Plugin Builder: http://g-sherman.github.io/Qgis-Plugin-Builder/

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Note: This plugin requires layers to be saved as shapefiles for processing. Ensure your data is properly saved before attempting to use the subsampling functions.