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Ocean Water Polygons

OpenStreetMap does not store ocean surfaces as polygons. Coastlines are tagged as natural=coastline linestrings, and it is up to consumers to derive water areas from them.

The osmdata.openstreetmap.de project pre-computes water polygons from OSM coastlines daily. The generator can load these shapefiles to render oceans and seas.

Why

Without water polygons, coastal areas show no ocean.

The --water-shp option is optional -- inland users can skip it entirely to save space.

Setup

Download the WGS84 shapefile (once, ~540 MB):

wget https://osmdata.openstreetmap.de/download/water-polygons-split-4326.zip
unzip water-polygons-split-4326.zip

This creates water-polygons-split-4326/water_polygons.shp and associated files.

Usage

./nav_generator input.pbf output features.json --zoom 6-17 \
    --water-shp path/to/water-polygons-split-4326/water_polygons.shp

The generator reads the PBF bounding box and uses OGR spatial filtering to only load water polygons that intersect the extract area. A regional extract like Languedoc-Roussillon loads ~4 polygons, not the entire world.

How it works

  1. The PBF header provides the bounding box of the extract
  2. GDAL/OGR opens the shapefile and applies a spatial filter (SetSpatialFilterRect)
  3. Matching polygons are injected into the feature store as layer="water" with the standard water color (#aad2df)
  4. The tile processor clips, simplifies, and renders them like any other water feature

Dependencies

Requires GDAL (libgdal-dev on Debian/Ubuntu). Already included in the CMake build.

Notes

  • Use water-polygons-split-4326 (WGS84), not the 3857 (Mercator) variant
  • The shapefile is ~540 MB unzipped but is gitignored
  • Lakes, rivers, and reservoirs come from the PBF data, not from this shapefile
  • Island holes in ocean polygons are handled by the existing GEOSDifference logic in the tile processor