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