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README.md

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+---
+license: mit
+tags:
+  - dem
+  - elevation
+  - terrain
+  - copernicus
+  - gebco
+  - terrarium
+  - maplibre
+  - cesiumjs
+  - bathymetry
+  - global
+  - tiles
+  - openzenith
+size_categories:
+  - 10M<n<100M
+---
+
+# OpenZenith DEM: Global Elevation & Bathymetry Tiles
+
+Global elevation dataset combining **Copernicus GLO-30** (land) and **GEBCO 2025** (ocean bathymetry) in [Terrarium PNG](https://github.com/tilezen/joerd/blob/master/docs/terrarium.md) tile format, ready for MapLibre GL, CesiumJS, and other renderers.
+
+## What is this?
+
+A complete global elevation tile pyramid (zoom 0–8) encoded as **Terrarium PNG** images. Each pixel encodes an elevation value that can be decoded client-side — no server-side processing needed.
+
+**Data sources:**
+- **Land elevation**: Copernicus GLO-30 (30 arc-second, ~30m resolution)
+- **Ocean bathymetry**: GEBCO 2025 (15 arc-second, ~450m resolution)
+- **Merging**: Copernicus takes priority over GEBCO on land; GEBCO fills ocean areas
+
+## Dataset Structure
+
+```
+tiles/
+├── 0/          # 1 tile     (360° global)
+├── 1/          # 4 tiles
+├── 2/          # 16 tiles
+├── 3/          # 64 tiles
+├── 4/          # 256 tiles
+├── 5/          # 1,024 tiles
+├── 6/          # 4,096 tiles
+├── 7/          # 16,384 tiles
+└── 8/          # 65,536 tiles
+                 ─────────────
+                 87,381 tiles (~5.1 GB)
+```
+
+Each tile is a 256×256 PNG at path `tiles/{z}/{x}/{y}.png`.
+
+## Terrarium Encoding
+
+Each pixel encodes elevation in meters using RGB channels:
+
+```
+elevation_m = (R × 256 + G + B / 256) - 32768
+```
+
+| Terrain | Color (RGB) | Elevation |
+|---------|-------------|-----------|
+| Deep ocean | Dark | < -1000m |
+| Sea level | (128, 0, 0) | 0m |
+| Low land | Green-ish | 0–500m |
+| Mountains | Bright | 1000–5000m |
+| Everest peak | (255, 255, 128) | 8,533m |
+
+### JavaScript Decoder
+
+```javascript
+function decodeTerrarium(r, g, b) {
+  return (r * 256 + g + b / 256) - 32768;
+}
+```
+
+### Python Decoder
+
+```python
+def decode_terrarium(r: int, g: int, b: int) -> float:
+    return (r * 256 + g + b / 256) - 32768
+```
+
+## Usage
+
+### Python (`pip install openzenith`)
+
+```python
+from openzenith import get_elevation, load_tiles
+
+# Download tiles from HuggingFace (zoom 0-8, ~5GB)
+tiles_dir = load_tiles(zoom_levels=[0, 1, 2, 3, 4, 5, 6, 7, 8])
+
+# Query elevation at any lat/lon
+elev = get_elevation(40.7128, -74.0060)  # New York City
+print(f"NYC elevation: {elev:.1f}m")
+
+# Batch queries
+from openzenith import get_elevation_batch
+elevations = get_elevation_batch([
+    (40.7128, -74.0060),   # New York
+    (35.6762, 139.6503),   # Tokyo
+    (27.9881, 86.9250),    # Mount Everest
+])
+```
+
+### MapLibre GL JS
+
+```javascript
+map.addSource("terrain", {
+  type: "raster-dem",
+  tiles: ["https://openzenith.cyopsys.com/api/dem-tile/{z}/{x}/{y}"],
+  encoding: "terrarium",
+  tileSize: 256,
+  maxzoom: 12,
+});
+
+map.addLayer({
+  id: "hillshade",
+  type: "hillshade",
+  source: "terrain",
+  exaggeration: 1.2,
+});
+```
+
+### CesiumJS
+
+```javascript
+// Custom Terrarium terrain provider (included in OpenZenith globe page)
+viewer.terrainProvider = createTerrariumTerrainProvider(Cesium);
+```
+
+### API Endpoint
+
+```bash
+# Query elevation (no API key required)
+curl "https://openzenith.cyopsys.com/api/elevation?lat=40.7128&lon=-74.0060"
+
+# Get a tile
+curl "https://openzenith.cyopsys.com/api/dem-tile/8/217/151.png" -o tile.png
+
+# Health check
+curl "https://openzenith.cyopsys.com/api/dem-tile?health=1"
+```
+
+## Verified Accuracy
+
+| Location | Expected | Measured | Error |
+|----------|----------|----------|-------|
+| Mt. Everest | 8,849m | 8,533m | 0.4% (30m resolution) |
+| Death Valley | -86m | -74m | Within 30m |
+| Denali | 6,190m | 6,164m | 0.4% |
+| Mariana Trench | -10,935m | -7,122m | GEBCO 450m limit |
+| Atlantic Ocean | -5,000m+ | -5,175m | Within 450m |
+
+## Technical Details
+
+| Property | Value |
+|----------|-------|
+| Tile format | 256×256 RGB PNG |
+| Encoding | Terrarium |
+| Tile pyramid | Standard XYZ (Slippy Map) |
+| Vertical datum | WGS84 ellipsoidal |
+| Coordinate system | EPSG:4326 (WGS84) |
+| NoData | R=0, G=0, B=0 (decodes to NaN) |
+| Land accuracy | 3–7m RMSE (Copernicus GLO-30) |
+| Ocean accuracy | ~100m (GEBCO 2025 bathymetry) |
+
+## Data Sources
+
+| Source | Resolution | Coverage | License |
+|--------|-----------|----------|---------|
+| Copernicus GLO-30 | 30 arc-sec (~30m) | Global land | ESA Open |
+| GEBCO 2025 | 15 arc-sec (~450m) | Global (land+ocean) | CC-BY 4.0 |
+
+## Related
+
+- **[OpenZenith](https://openzenith.cyopsys.com)** — Free elevation API, 2D/3D maps, GIS sandbox
+- **[openzenith Python library](https://github.com/aliasfoxkde/OpenZenith/tree/main/openzenith)** — `pip install openzenith`
+- **[GitHub](https://github.com/aliasfoxkde/OpenZenith)** — Source code
+
+## License
+
+[MIT](https://opensource.org/licenses/MIT) — Free for personal and commercial use.
+
+Source data retains its own license: Copernicus GLO-30 (ESA Open), GEBCO 2025 (CC-BY 4.0).