Geospatial Analysis

Overview

When working with geographic data (earthquakes, plate boundaries, etc.), using geopandas with proper coordinate projections provides accurate distance calculations and efficient spatial operations. This guide covers best practices for geospatial analysis using modern Python standards.

Critical Rule: Never calculate distances directly in geographic coordinates (EPSG:4326). Always project to a metric coordinate system first.

When to Use

• You need accurate distance calculations between geographic features.
• You are performing spatial filtering, joins, or overlays on plate boundaries, earthquake epicenters, or any other geospatial datasets.
• Your workflow requires projecting data to a metric CRS (e.g., EPSG:4087 or local UTM zones) before analysis.

Prerequisites

• Python 3.8+ with geopandas, shapely, and pyproj installed.
• On Windows (PowerShell), ensure the conda or venv environment is activated before running scripts:

# Activate environment (adjust path as needed)
conda activate geo-env
# or
.\.venv\Scripts\Activate.ps1

• Verify package availability:

python -c "import geopandas; print(geopandas.__version__)"

Procedure

1. Understand Coordinate Systems

2. Load Geospatial Data

From GeoJSON files:

import geopandas as gpd

gdf_plates = gpd.read_file("plates.json")
gdf_boundaries = gpd.read_file("boundaries.json")

From regular data with coordinates:

from shapely.geometry import Point
import geopandas as gpd

data = [
    {"id": 1, "lat": 35.0, "lon": 140.0, "value": 5.5},
    {"id": 2, "lat": 36.0, "lon": 141.0, "value": 6.0},
]

# Use points_from_xy for better performance
gdf = gpd.GeoDataFrame(
    data,
    geometry=gpd.points_from_xy([row["lon"] for row in data], [row["lat"] for row in data]),
    crs="EPSG:4326"
)

3. Perform Spatial Filtering

Find points within a polygon:

# Get the polygon of interest
target_poly = gdf_plates[gdf_plates["Name"] == "Pacific"].geometry.unary_union

# Filter points that fall within the polygon
points_inside = gdf_points[gdf_points.within(target_poly)]

print(f"Found {len(points_inside)} points inside the polygon")

Combine multiple boundary segments:

# Combine multiple boundary segments into one geometry
all_boundaries = gdf_boundaries.geometry.unary_union

# Or filter first, then combine
pacific_boundaries = gdf_boundaries[
    gdf_boundaries["Name"].str.contains("PA")
].geometry.unary_union

4. Calculate Distances (Project to Metric CRS First)

# 1. Load your data
gdf_points = gpd.read_file("points.json")
gdf_boundaries = gpd.read_file("boundaries.json")

# 2. Project to metric coordinate system
METRIC_CRS = "EPSG:4087"
points_proj = gdf_points.to_crs(METRIC_CRS)
boundaries_proj = gdf_boundaries.to_crs(METRIC_CRS)

# 3. Combine boundary segments if needed
boundary_geom = boundaries_proj.geometry.unary_union

# 4. Calculate distances (returns meters)
gdf_points["distance_m"] = points_proj.geometry.distance(boundary_geom)
gdf_points["distance_km"] = gdf_points["distance_m"] / 1000.0

Find the furthest point:

furthest = gdf_points.nlargest(1, "distance_km").iloc[0]

print(f"Furthest point: {furthest['id']}")
print(f"Distance: {furthest['distance_km']:.2f} km")

5. Filter by Attributes

# Filter by name or code
pacific_plate = gdf_plates[gdf_plates["PlateName"] == "Pacific"]
pacific_plate_alt = gdf_plates[gdf_plates["Code"] == "PA"]

# Filter boundaries involving a specific plate
pacific_bounds = gdf_boundaries[
    (gdf_boundaries["PlateA"] == "PA") |
    (gdf_boundaries["PlateB"] == "PA")
]

# String pattern matching
pa_related = gdf_boundaries[gdf_boundaries["Name"].str.contains("PA")]

6. Common Workflow Pattern (Earthquakes and Plate Boundaries)

import geopandas as gpd
from shapely.geometry import Point

# 1. Load data
earthquakes_data = [
    {"latitude": 35.652834, "longitude": 139.839478, "magnitude": 4.5},
    {"latitude": 36.204824, "longitude": 138.252924, "magnitude": 5.2}
]
gdf_plates = gpd.read_file("plates.json")
gdf_boundaries = gpd.read_file("boundaries.json")

# 2. Create earthquake GeoDataFrame
gdf_eq = gpd.GeoDataFrame(
    earthquakes_data,
    geometry=gpd.points_from_xy(
        [eq["longitude"] for eq in earthquakes_data],
        [eq["latitude"] for eq in earthquakes_data]
    ),
    crs="EPSG:4326"
)

# 3. Spatial filtering - find earthquakes in specific plate
target_plate = gdf_plates[gdf_plates["Code"] == "PA"].geometry.unary_union
earthquakes_in_plate = gdf_eq[gdf_eq.within(target_plate)].copy()

# 4. Calculate distances (project to metric CRS)
METRIC_CRS = "EPSG:4087"
eq_proj = earthquakes_in_plate.to_crs(METRIC_CRS)

# Filter and combine relevant boundaries
plate_boundaries = gdf_boundaries[
    gdf_boundaries["Name"].str.contains("PA")
].to_crs(METRIC_CRS).geometry.unary_union

# Calculate distances
earthquakes_in_plate["distance_km"] = eq_proj.geometry.distance(plate_boundaries) / 1000.0

# 5. Find the furthest earthquake
furthest_eq = earthquakes_in_plate.nlargest(1, "distance_km").iloc[0]

7. Performance Tips

1. Filter before projecting: Reduce data size before expensive operations.
2. Project once: Convert to metric CRS once, not in loops.
3. Use .unary_union: Combine geometries before distance calculations to avoid O(N x M) complexity.
4. Copy when modifying: Use .copy() when creating filtered DataFrames to avoid SettingWithCopyWarning.
5. Use points_from_xy: Use gpd.points_from_xy() instead of list comprehensions with Point() for significantly faster GeoDataFrame creation.

# Good: Filter first, then project
small_subset = gdf_large[gdf_large["region"] == "Pacific"]
small_projected = small_subset.to_crs(METRIC_CRS)

# Avoid: Projecting large dataset just to filter
# gdf_projected = gdf_large.to_crs(METRIC_CRS)
# small_subset = gdf_projected[gdf_projected["region"] == "Pacific"]

Pitfalls

Do NOT:

• Use manual haversine formulas or custom point-in-polygon code.
• Calculate distances directly on geographic coordinates (EPSG:4326).
• Iterate over individual boundary vertices for distance checks.
• Use pyproj for basic distance calculations when geopandas provides a vectorized API.
• Use outdated geopandas syntax (e.g., using gdf.set_crs when gdf.to_crs is required for transformation).

Verification

• Run the provided workflow on a sample earthquake and plate boundary dataset to ensure distances are returned in meters/kilometers.
• Verify that filtering before projection reduces processing time compared to projecting first.
• Confirm that gdf.crs is correctly set to EPSG:4326 before calling .to_crs().
• Validate that unary_union is used for boundary distance calculations to ensure O(N) performance.

Quick check command:

python -c "import geopandas as gpd; gdf = gpd.GeoDataFrame({'x':[0],'y':[0]}, geometry=gpd.points_from_xy([0],[0]), crs='EPSG:4326'); p = gdf.to_crs('EPSG:4087'); print('CRS OK:', p.crs)"

Expected output:

CRS OK: EPSG:4087

Related Skills

• raster-analysis
• spatial-joins
• time-series-geospatial