README.md

---
license: other
license_name: copernicus
license_link: https://copernicus.eu/user-license
task_categories:
  - other
tags:
  - elevation
  - dem
  - copernicus
  - terrain
  - geospatial
size_categories:
  - 100K<n<1M
---

# Copernicus GLO-30 DEM Chunks

Global Digital Elevation Model (DEM) tiles from the Copernicus GLO-30 dataset,
pre-processed into 256x256 chunks in OZCHNK01 merged format for efficient
web serving.

## Source

Copernicus DEM GLO-30 (30-meter resolution) from the European Space Agency.
Original COG tiles downloaded from AWS Open Data:
https://copernicus-dem-30m.s3.eu-central-1.amazonaws.com/

## License

Copernicus Licence (free for most uses). See: https://copernicus.eu/user-license

## Format

Binary merged format (OZCHNK01):

- **Magic**: 8 bytes "OZCHNK01"
- **Version**: 2 (Copernicus = Float32)
- **Grid**: Variable ROWS x COLS (depends on tile dimensions)
- **Index**: Array of [4-byte offset, 4-byte size] per chunk
- **Data**: Concatenated zlib-compressed 256x256 Float32 chunks

### Tile Grid

- Internal COG tiles: 1024x1024 pixels
- Each split into 4x4 = 16 chunks of 256x256
- Edge tiles may have fewer chunks
- Data type: Float32 (4 bytes/pixel)

### File Naming

Files are organized by latitude band:

- Path: `{lat_dir}/{basename}.merged`
- Example: `N00/COPDEM_N00_00_E006_00_DEM.merged`

## Coverage

Global land areas at 30m resolution (~90GB total, ~26,450 tiles).

## Usage

For use with OpenZenith elevation API. Each merged file contains all 256x256
chunks for one Copernicus tile, allowing efficient range requests to fetch
individual chunks.

To read a chunk:

1. Open the merged file
2. Read the 12-byte header (magic + version + rows + cols)
3. Read the index: rows*cols entries of [offset, size]
4. Seek to the chunk offset, read size bytes
5. Decompress with zlib to get 256x256 Float32 pixel data
6. Elevation value = float32 at row*256 + col