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82 lines
4.0 KiB
Markdown
82 lines
4.0 KiB
Markdown
# PMTiles
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PMTiles is a single-file archive format for tiled data. A PMTiles archive can be hosted on a commodity storage platform such as S3, and enables low-cost, zero-maintenance map applications that are "serverless" - free of a custom tile backend or third party provider.
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[Protomaps Blog: Dynamic Maps, Static Storage](http://protomaps.com/blog/dynamic-maps-static-storage)
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[Leaflet + Raster Tiles Demo](https://protomaps.github.io/PMTiles/examples/leaflet.html) - watch your network request log
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[MapLibre GL + Vector Tiles Demo](https://protomaps.github.io/PMTiles/examples/maplibre.html) - requires MapLibre GL JS v1.14.1-rc.2 or later
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See also:
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* [Cloud Optimized GeoTIFFs](https://www.cogeo.org)
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## How To Use
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### Go
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See https://github.com/protomaps/go-pmtiles
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### Python
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pip install pmtiles
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pmtiles-convert TILES.mbtiles TILES.pmtiles
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pmtiles-convert TILES.pmtiles DIRECTORY
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pmtiles-show TILES.pmtiles // see info about a PMTiles directory
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pmtiles-serve TILES.pmtiles // start an HTTP server that decodes PMTiles into traditional Z/X/Y paths
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See https://github.com/protomaps/PMTiles/tree/master/python/bin for library usage
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### JavaScript
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<script src="https://unpkg.com/pmtiles@0.0.4/pmtiles.js"></script>
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Example of a raster PMTiles archive decoded and displayed in Leaflet:
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const p = new pmtiles.PMTiles('osm_carto.pmtiles',{allow_200:true})
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p.leafletLayer({attribution:'© <a href="https://openstreetmap.org">OpenStreetMap</a> contributors'}).addTo(map)
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## Specification
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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.**
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A PMTiles archive is composed of:
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* a fixed-size 512,000 byte header section
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* Followed by any number of tiles in arbitrary format
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* Optionally followed by any number of *leaf directories*
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### Header
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* The header begins with a 2-byte magic number, "PM"
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* Followed by 2 bytes, the PMTiles specification version (currently 1)
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* Followed by 4 bytes, the length of metadata (M bytes)
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* Followed by 2 bytes, the number of entries in the *root directory* (N entries)
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* Followed by M bytes of metadata, by convention a JSON object
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* Followed by N * 17 bytes, the root directory.
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### Directory structure
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A directory is a contiguous sequence of 17 byte entries. A directory can have at most 21,845 entries.
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An entry consists of:
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* 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.
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* 3 bytes: the X (column) of the entry.
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* 3 bytes: the Y (row) of the entry.
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* 6 bytes: the offset of where the tile begins in the archive.
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* 4 bytes: the length of the tile, in bytes.
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### Notes
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* A full directory of 21,845 entries holds exactly a complete pyramid with 8 levels, or 1+4+16+64+256+1024+4096+16384.
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* A PMTiles archive with less than 21,845 tiles should have a root directory and no leaf directories.
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* Multiple tile entries can point to the same offset; this is useful for de-duplicating certain tiles, such as an empty "ocean" tile.
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* Analogously, multiple leaf directory entries can point to the same offset; this can avoid inefficiently-packed small leaf directories.
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### Implementation suggestions
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* 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.
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* The order of tile data in the archive is unspecified; an optimized implementation should arrange tiles on a 2D space-filling curve.
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* PMTiles readers should cache directory entries by byte offset, not by Z/X/Y. This means that deduplicated leaf directories result in cache hits.
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## License
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The reference implementations of PMTiles are published under the BSD 3-Clause License. The PMTiles specification itself is public domain, or under a CC0 license where applicable.
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