geopoly-simplify is a Python tool for simplifying polygon and line (fault) shapefiles representing geological maps across multiple output scales, while rigorously preserving topological integrity — producing outputs that contain no gaps between adjacent polygons, no overlaps, and no broken alignments between the fault network and polygon boundaries.
The tool implements a Modified Visvalingam-Whyatt (MVW) algorithm within a three-stage pipeline:
- Stage 0 — Topology pre-processing: coordinate snapping, hairline overlap/gap correction, and bidirectional fault-polygon vertex alignment.
- Stage 1 — Fault network simplification: MVW simplification of fault lines with all polygon-boundary vertices pinned as hard constraints.
- Stage 2 — Polygon simplification: MVW simplification of polygon units with junction pinning, fault-vertex pinning, boundary preservation, unique geological contact pinning, and thin-body collapse prevention.
The tool was developed and tested on two Australian geological datasets:
- GSWA (Geological Survey of Western Australia) — Ninghan 1:500 000 geological map
- GSSA (Geological Survey of South Australia) — Flinders Ranges geological map
Requirements: Python 3.9 or later.
Clone or download this repository, then install the dependencies:
pip install -r requirements.txtThe core dependencies are:
| Package | Minimum version |
|---|---|
| fiona | 1.9 |
| shapely | 2.0 |
| geopandas | 0.14 |
| numpy | 1.24 |
| pandas | 2.0 |
No separate compilation step is required. All dependencies are available from PyPI.
Simplify a polygon shapefile without a fault network, using the Modified Visvalingam-Whyatt algorithm at a 50 000 m² area threshold:
fromgeopoly_simplifyimportvector_simplify_file_two_stageresult=vector_simplify_file_two_stage(
input_file="geology_polygons.shp",
output_file="geology_polygons_simplified.shp",
method="modified_visvalingam_whyatt",
threshold=50000, # area threshold in m²
)
print(f"Features processed : {result['features_processed']}")
print(f"Features simplified: {result['features_simplified']}")Run the complete topology-preserving pipeline — topology pre-processing, fault simplification, and polygon simplification — at two scales:
fromgeopoly_simplifyimportvector_simplify_file_two_stage# --- Medium scale (e.g. 1:500 000) ---result_500k=vector_simplify_file_two_stage(
input_file="geology_polygons.shp",
output_file="geology_500k.shp",
method="modified_visvalingam_whyatt",
threshold=500000, # 500 000 m² area thresholdfault_file="faults.shp", # fault network aligned with polygonsboundary_preserve="hard", # pin exterior boundary verticesunit_field="CODE", # geological unit identifier fieldpreprocess=True, # run Stage 0 topology pre-processingsnap_decimals=7, # coordinate grid precision
)
# --- Small scale (e.g. 1:2 000 000) ---result_2m=vector_simplify_file_two_stage(
input_file="geology_polygons.shp",
output_file="geology_2m.shp",
method="modified_visvalingam_whyatt",
threshold=2000000, # 2 000 000 m² area thresholdfault_file="faults.shp",
boundary_preserve="soft", # allow exterior vertices to be removedunit_field="CODE",
preprocess=True,
snap_decimals=7,
)
# Inspect contact preservation statisticsprint(f"Contacts preserved (shape OK): {result_500k['n_contact_preserved']}")
print(f"Contacts minimal (2 verts) : {result_500k['n_contact_minimal']}")
print(f"Contacts shape-lost (1 vert) : {result_500k['n_contact_shape_lost']}")
print(f"Contacts diverged : {result_500k['n_contact_diverged']}")See examples/run_simplification.py for a fully-commented walkthrough.
geopoly-simplify uses a Modified Visvalingam-Whyatt (MVW) algorithm embedded in a three-stage topology-preserving pipeline.
Visvalingam-Whyatt core: vertices are ranked by the area of the triangle they form with their two neighbours; those with the smallest effective area are removed first, in a min-heap priority queue, until the remaining vertex count satisfies the threshold.
Junction pinning: every point where three or more features meet receives an infinite area weight and is never removed, preventing T-junction breaks and gaps.
Fault-polygon alignment: fault vertices that coincide with polygon boundary vertices are pinned in both directions, so fault lines and polygon edges stay co-located after simplification.
Geological contact pinning: for every unique pair of adjacent geological units, one representative vertex on the shared boundary is pinned, guaranteeing that the contact relationship is always detectable in the output.
Thin-body protection: narrow polygons (fold stripes, dyke outlines) are pre-simplified at a conservative threshold; surviving intermediate vertices are pinned before the main simplification run, preventing interior collapse and cross-boundary gaps.
Self-intersection prevention: a cross-product edge test vetoes any vertex removal that would create a self-intersecting ring; a
make_valid()post-pass repairs any residual crossings.Collapse fallback cascade: if arc-mode MVW cannot produce a valid ring, the pipeline tries ring-mode MVW (with progressive threshold reduction), then neighbour-clipping, then writes the original geometry as a last resort.
Six available algorithms: Decimation, Douglas-Peucker, Douglas-Peucker TP, Bend Simplification, Visvalingam-Whyatt, Modified Visvalingam-Whyatt. MVW is recommended for geological maps.
- Format: Esri Shapefile (
.shp). Input files are read and written via Fiona; any Fiona-supported driver can be adapted with minor changes. - Coordinate reference system: any projected CRS with linear units in metres is
supported. The
thresholdparameter is expressed in square metres (m²); ensure your data is in a metric projection before running the tool. - Geometry types:
PolygonandMultiPolygonfor the polygon layer;LineStringandMultiLineStringfor the fault layer. - Attribute schema: the polygon layer must contain a field identifying geological
units (default field name
CODE, as used in GSWA datasets). Override with theunit_fieldparameter for other naming conventions.
| Dataset | Source | Coverage | Scale |
|---|---|---|---|
| Ninghan geological map | GSWA (Geological Survey of Western Australia) | Ninghan region, WA | 1:500 000 |
| Flinders Ranges geological map | GSSA (Geological Survey of South Australia) | Flinders Ranges, SA | varies |
If you use geopoly-simplify in your research, please cite:
Joshi, R., Jessell, M., & Lindsay, M. (2025). geopoly-simplify: Topology-preserving simplification of geological map vector data (Version 1.0.0) [Software]. Zenodo. https://doi.org/10.5281/zenodo.20271988
@software{joshi2025geopolysimplify,
author = {Joshi, Ranee and Jessell, Mark and Lindsay, Mark},
title = {{geopoly-simplify}: Topology-Preserving Simplification of Geological Map Vector Data},
year = {2025},
version = {1.0.0},
publisher = {Zenodo},
doi = {10.5281/zenodo.20580179, url = {https://doi.org/10.5281/zenodo.20580179}}The MVW algorithm builds on:
- Visvalingam, M., & Whyatt, J. D. (1992). Line generalisation by repeated elimination of points. The Cartographic Journal, 30(1), 46-51. https://doi.org/10.1179/000870493786962263
- Douglas, D. H., & Peucker, T. K. (1973). Algorithms for the reduction of the number of points required to represent a digitized line or its caricature. The Canadian Cartographer, 10(2), 112-122. https://doi.org/10.3138/FM57-6770-U75U-7727
The tool was developed and validated using datasets provided by:
Geological Survey of Western Australia (GSWA) — Ninghan 1:500 000 geological map. Data accessed under the GSWA open data licence. https://www.dmp.wa.gov.au/Geological-Survey/Geological-Survey-262.aspx
Geological Survey of South Australia (GSSA) — Flinders Ranges geological map. Data accessed under the GSSA open data licence. https://www.energymining.sa.gov.au/industry/geological-survey
This project is licensed under the MIT License — see the LICENSE file for details.