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PMTiles version 2

Note: this is deprecated in favor of spec version 3.

PMTiles is a binary serialization format designed for two main access patterns: over the network, via HTTP 1.1 Byte Serving (Range: requests), or via memory-mapped files on disk. All integer values are little-endian.

A PMTiles archive is composed of:

  • a fixed-size 512,000 byte header section
  • Followed by any number of tiles in arbitrary format
  • Optionally followed by any number of leaf directories

Header

  • The header begins with a 2-byte magic number, "PM"
  • Followed by 2 bytes, the PMTiles specification version (currently 2).
  • Followed by 4 bytes, the length of metadata (M bytes)
  • Followed by 2 bytes, the number of entries in the root directory (N entries)
  • Followed by M bytes of metadata, which must be a JSON string with bounds, minzoom and maxzoom properties (new in v2)
  • Followed by N * 17 bytes, the root directory.

Directory structure

A directory is a contiguous sequence of 17 byte entries. A directory can have at most 21,845 entries. A directory must be sorted by Z, X and then Y order (new in v2).

An entry consists of:

  • 1 byte: the zoom level (Z) of the entry, with the top bit set to 1 instead of 0 to indicate the offset/length points to a leaf directory and not a tile.
  • 3 bytes: the X (column) of the entry.
  • 3 bytes: the Y (row) of the entry.
  • 6 bytes: the offset of where the tile begins in the archive.
  • 4 bytes: the length of the tile, in bytes.

All leaf directory entries follow non-leaf entries. All leaf directories in a single directory must have the same Z value. (new in v2).

Notes

  • A full directory of 21,845 entries holds exactly a complete pyramid with 8 levels, or 1+4+16+64+256+1024+4096+16384.
  • A PMTiles archive with less than 21,845 tiles should have a root directory and no leaf directories.
  • Multiple tile entries can point to the same offset; this is useful for de-duplicating certain tiles, such as an empty "ocean" tile.
  • Analogously, multiple leaf directory entries can point to the same offset; this can avoid inefficiently-packed small leaf directories.
  • The tentative media type for PMTiles archives is application/vnd.pmtiles.

Implementation suggestions

  • PMTiles is designed to make implementing a writer simple. Reserve 512KB, then write all tiles, recording their entry information; then write all leaf directories; finally, rewind to 0 and write the header.
  • The order of tile data in the archive is unspecified; an optimized implementation should arrange tiles on a 2D space-filling curve.
  • PMTiles readers should cache directory entries by byte offset, not by Z/X/Y. This means that deduplicated leaf directories result in cache hits.