geopandas-spatial-analysis

name	geopandas_spatial_analysis
description	How to perform spatial filtering (point-in-polygon), combine line boundaries, and accurately calculate metric distances between points and boundaries using GeoPandas.

GeoPandas Spatial Analysis: Filtering and Distance Calculations

When analyzing geospatial data, you often need to find points within a specific area (spatial filtering) and determine how far those points are from a specific feature (distance calculation).

1. Spatial Filtering (Point-in-Polygon)

To find which points (e.g., earthquakes) fall within a specific polygon (e.g., a tectonic plate), use spatial joins or the .within() / .intersects() methods.

import geopandas as gpd

# Load data
points_gdf = gpd.read_file("points.json")
polygons_gdf = gpd.read_file("polygons.json")

# Filter for the target polygon (e.g., 'Pacific' plate)
target_polygon = polygons_gdf[polygons_gdf['PlateName'] == 'Pacific']

# Find points strictly within the target polygon
points_within = gpd.sjoin(points_gdf, target_polygon, predicate='within')

2. Combining Boundaries for Distance Calculation

When your boundary data (LineStrings/MultiLineStrings) consists of multiple segments, you must combine them into a single unified geometry before measuring distances. Otherwise, distance calculations will not work as expected across the entire boundary.

# Load boundary data
boundaries_gdf = gpd.read_file("boundaries.json")

# Filter boundary segments associated with the target plate if necessary
target_boundaries = boundaries_gdf[boundaries_gdf['plate_attribute'] == 'Pacific']

# Combine all relevant boundary segments into a single geometry
combined_boundary = target_boundaries.geometry.unary_union

3. Coordinate System Projection (Crucial Step)

Warning: Never calculate distances directly in geographic coordinate systems like EPSG:4326 (WGS 84). The result will be in degrees, which is completely inaccurate for distance measurements.

Before calculating distances, you must project both your points and your combined boundary geometry to a metric coordinate reference system (CRS). For global, flat distance calculations, EPSG:4087 (WGS 84 / World Equidistant Cylindrical) is a standard choice.

# Project the points to EPSG:4087
points_metric = points_within.to_crs(epsg=4087)

# Project the boundary GeoDataFrame to EPSG:4087 BEFORE applying unary_union, 
# OR project the points first, then use a CRS-aware Series for the boundary.
target_boundaries_metric = target_boundaries.to_crs(epsg=4087)
combined_boundary_metric = target_boundaries_metric.geometry.unary_union

4. Distance Calculation and Unit Conversion

Once both the points and the combined boundary are in a metric CRS, use the .distance() method to find the shortest distance from each point to the boundary.

Metric projections calculate distance in meters. To obtain kilometers, divide the resulting values by 1000.

# Calculate distances (in meters)
points_within['distance_m'] = points_metric.geometry.distance(combined_boundary_metric)

# Convert to kilometers
points_within['distance_km'] = points_within['distance_m'] / 1000

# Optional: Round to 2 decimal places
points_within['distance_km'] = points_within['distance_km'].round(2)