Initial commit

skills/geopandas/references/crs-management.md

@@ -0,0 +1,243 @@
+# Coordinate Reference Systems (CRS)
+
+A coordinate reference system defines how coordinates relate to locations on Earth.
+
+## Understanding CRS
+
+CRS information is stored as `pyproj.CRS` objects:
+
+```python
+# Check CRS
+print(gdf.crs)
+
+# Check if CRS is set
+if gdf.crs is None:
+    print("No CRS defined")
+```
+
+## Setting vs Reprojecting
+
+### Setting CRS
+
+Use `set_crs()` when coordinates are correct but CRS metadata is missing:
+
+```python
+# Set CRS (doesn't transform coordinates)
+gdf = gdf.set_crs("EPSG:4326")
+gdf = gdf.set_crs(4326)
+```
+
+**Warning**: Only use when CRS metadata is missing. This does not transform coordinates.
+
+### Reprojecting
+
+Use `to_crs()` to transform coordinates between coordinate systems:
+
+```python
+# Reproject to different CRS
+gdf_projected = gdf.to_crs("EPSG:3857")  # Web Mercator
+gdf_projected = gdf.to_crs(3857)
+
+# Reproject to match another GeoDataFrame
+gdf1_reprojected = gdf1.to_crs(gdf2.crs)
+```
+
+## CRS Formats
+
+GeoPandas accepts multiple formats via `pyproj.CRS.from_user_input()`:
+
+```python
+# EPSG code (integer)
+gdf.to_crs(4326)
+
+# Authority string
+gdf.to_crs("EPSG:4326")
+gdf.to_crs("ESRI:102003")
+
+# WKT string (Well-Known Text)
+gdf.to_crs("GEOGCS[...]")
+
+# PROJ string
+gdf.to_crs("+proj=longlat +datum=WGS84")
+
+# pyproj.CRS object
+from pyproj import CRS
+crs_obj = CRS.from_epsg(4326)
+gdf.to_crs(crs_obj)
+```
+
+**Best Practice**: Use WKT2 or authority strings (EPSG) to preserve full CRS information.
+
+## Common EPSG Codes
+
+### Geographic Coordinate Systems
+
+```python
+# WGS 84 (latitude/longitude)
+gdf.to_crs("EPSG:4326")
+
+# NAD83
+gdf.to_crs("EPSG:4269")
+```
+
+### Projected Coordinate Systems
+
+```python
+# Web Mercator (used by web maps)
+gdf.to_crs("EPSG:3857")
+
+# UTM zones (example: UTM Zone 33N)
+gdf.to_crs("EPSG:32633")
+
+# UTM zones (Southern hemisphere, example: UTM Zone 33S)
+gdf.to_crs("EPSG:32733")
+
+# US National Atlas Equal Area
+gdf.to_crs("ESRI:102003")
+
+# Albers Equal Area Conic (North America)
+gdf.to_crs("EPSG:5070")
+```
+
+## CRS Requirements for Operations
+
+### Operations Requiring Matching CRS
+
+These operations require identical CRS:
+
+```python
+# Spatial joins
+gpd.sjoin(gdf1, gdf2, ...)  # CRS must match
+
+# Overlay operations
+gpd.overlay(gdf1, gdf2, ...)  # CRS must match
+
+# Appending
+pd.concat([gdf1, gdf2])  # CRS must match
+
+# Reproject first if needed
+gdf2_reprojected = gdf2.to_crs(gdf1.crs)
+result = gpd.sjoin(gdf1, gdf2_reprojected)
+```
+
+### Operations Best in Projected CRS
+
+Area and distance calculations should use projected CRS:
+
+```python
+# Bad: area in degrees (meaningless)
+areas_degrees = gdf.geometry.area  # If CRS is EPSG:4326
+
+# Good: reproject to appropriate projected CRS first
+gdf_projected = gdf.to_crs("EPSG:3857")
+areas_meters = gdf_projected.geometry.area  # Square meters
+
+# Better: use appropriate local UTM zone for accuracy
+gdf_utm = gdf.to_crs("EPSG:32633")  # UTM Zone 33N
+accurate_areas = gdf_utm.geometry.area
+```
+
+## Choosing Appropriate CRS
+
+### For Area/Distance Calculations
+
+Use equal-area projections:
+
+```python
+# Albers Equal Area Conic (North America)
+gdf.to_crs("EPSG:5070")
+
+# Lambert Azimuthal Equal Area
+gdf.to_crs("EPSG:3035")  # Europe
+
+# UTM zones (for local areas)
+gdf.to_crs("EPSG:32633")  # Appropriate UTM zone
+```
+
+### For Distance-Preserving (Navigation)
+
+Use equidistant projections:
+
+```python
+# Azimuthal Equidistant
+gdf.to_crs("ESRI:54032")
+```
+
+### For Shape-Preserving (Angles)
+
+Use conformal projections:
+
+```python
+# Web Mercator (conformal but distorts area)
+gdf.to_crs("EPSG:3857")
+
+# UTM zones (conformal for local areas)
+gdf.to_crs("EPSG:32633")
+```
+
+### For Web Mapping
+
+```python
+# Web Mercator (standard for web maps)
+gdf.to_crs("EPSG:3857")
+```
+
+## Estimating UTM Zone
+
+```python
+# Estimate appropriate UTM CRS from data
+utm_crs = gdf.estimate_utm_crs()
+gdf_utm = gdf.to_crs(utm_crs)
+```
+
+## Multiple Geometry Columns with Different CRS
+
+GeoPandas 0.8+ supports different CRS per geometry column:
+
+```python
+# Set CRS for specific geometry column
+gdf = gdf.set_crs("EPSG:4326", allow_override=True)
+
+# Active geometry determines operations
+gdf = gdf.set_geometry('other_geom_column')
+
+# Check CRS mismatch
+try:
+    result = gdf1.overlay(gdf2)
+except ValueError as e:
+    print("CRS mismatch:", e)
+```
+
+## CRS Information
+
+```python
+# Get full CRS details
+print(gdf.crs)
+
+# Get EPSG code if available
+print(gdf.crs.to_epsg())
+
+# Get WKT representation
+print(gdf.crs.to_wkt())
+
+# Get PROJ string
+print(gdf.crs.to_proj4())
+
+# Check if CRS is geographic (lat/lon)
+print(gdf.crs.is_geographic)
+
+# Check if CRS is projected
+print(gdf.crs.is_projected)
+```
+
+## Transforming Individual Geometries
+
+```python
+from pyproj import Transformer
+
+# Create transformer
+transformer = Transformer.from_crs("EPSG:4326", "EPSG:3857", always_xy=True)
+
+# Transform point
+x_new, y_new = transformer.transform(x, y)
+```