high level overview of spec

README.md

@@ -7,6 +7,32 @@ PMTiles is a single-file archive format for tiled data. A PMTiles archive can be
 See also:
 * [Cloud Optimized GeoTIFFs](https://www.cogeo.org)
 
+## Specification
+
+A detailed specification is forthcoming. 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.
+
+### Design considerations
+* Directories are recursive, with a maximum of 21,845 entries per directory.
+* Deduplication of tile data is handled by multiple entries pointing to the same offset in the archive.
+
+### Details
+* The first 512 kilobytes of a PMTiles archive are reserved, and contain the headers as well as a root directory.
+* All integer values are little-endian.
+* The headers begin with a magic number, "PM"
+* 2 bytes: which specify the PMTiles specification version, right now always 1.
+* 4 bytes: the length of metadata (M bytes)
+* 2 bytes: the number of entries in the root directory (N)
+* M bytes: the metadata, by convention a JSON object.
+* N * 17 bytes: the root directory.
+
+### Directory structure
+
+A directory is a sequence of 17 byte entries. 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 data is a child directory, not data.
+* 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 data.
 
 ## License