Technical notes

The shared pipeline

Every command implements one small trait (declare the output columns, compute their values for one location) and shares the rest: extract longitude and latitude (cast to float, nulls to NaN), round and de-duplicate into unique locations with integer-scaled keys (so the join never compares floats), enrich the unique set in parallel with rayon, expand the results back to one value per input row, append the columns, and write. A NaN coordinate gets no key and therefore null output.

Geometry: no PROJ, no GDAL

All geometry is hand-rolled in pure Rust, so downstream projects need no extra system libraries. Two projections do the work:

• A spherical LAEA (Lambert Azimuthal Equal-Area) centered on the region, used for planar distances by coast (and for the nearest-boundary fallbacks in sea and place). Planar distance in that projection is accurate for the regional-scale nearest-feature query. The reference R workflow used EPSG:3035 LAEA for the same reason. A single sphere (authalic radius) is used rather than the GRS80 ellipsoid; the error is well under coastline resolution for regional work. Sub-meter accuracy, if ever needed, means an ellipsoidal LAEA.
• The unit sphere, used by nearest. Reference points become (x, y, z) vectors on the unit sphere; nearest-by-chord equals nearest-by-great-circle, and the squared chord converts back to an exact great-circle distance in meters. This is why nearest needs no projection center and has none of a planar projection’s distortion far from a center.

The haversine_m great-circle distance is used for reference and to refine index candidates.

Spatial indexes

Nearest-feature and point-in-polygon queries use rstar R-trees:

• coast indexes projected shoreline segments; a query takes the nearest segment’s planar distance.
• sea and place index feature bounding boxes for the containment test and boundary segments for the nearest fallback.
• nearest indexes reference points in 3D on the unit sphere.

Cropping keeps whole features: a feature is dropped if its bounding box misses the region-plus-margin box, but it is never clipped, so containment stays exact and cropping cannot invent geometry.

Memory and streaming

The input is read whole into memory because the join back touches every row; the enrichment set itself is only the unique locations, which stays small. For very large inputs a streamed two-pass version (collect unique locations, then append columns chunk by chunk) is the natural next step, noted in the source.

Parquet writes

Parquet is written single-threaded (set_parallel(false)), matching ctddump: the parallel column encoder in the pinned Polars version leaks memory per call, and the single-thread path is safe and deterministic.