Geometry primitives in NYC State Plane (EPSG:2263)

NYC's empirical flood layers (Sandy, DEP, Ida HWMs) are published in
EPSG:2263 (US survey feet). Reprojecting on every spatial query is
wasteful, so the convention here is: load once, reproject to 2263 at
ingest, do all distance/buffer math in feet. WGS84 is reserved for
inputs (lat/lon from the geocoder) and outputs (GeoJSON for the map).

app/spatial.py

@@ -0,0 +1,22 @@
+"""Spatial helpers. NYC works in EPSG:2263 (NY state plane, feet)."""
+from __future__ import annotations
+
+from pathlib import Path
+
+import geopandas as gpd
+
+NYC_CRS = "EPSG:2263"  # ft
+WGS84 = "EPSG:4326"
+
+DATA = Path(__file__).resolve().parent.parent / "data"
+
+
+def to_nyc(g: gpd.GeoDataFrame) -> gpd.GeoDataFrame:
+    if g.crs is None:
+        raise ValueError("layer has no CRS")
+    return g.to_crs(NYC_CRS) if g.crs.to_string() != NYC_CRS else g
+
+
+def load_layer(path: str | Path, layer: str | None = None) -> gpd.GeoDataFrame:
+    g = gpd.read_file(path, layer=layer) if layer else gpd.read_file(path)
+    return to_nyc(g)