vector-tile-spec

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

1. Purpose

This specification attempts to create a standard for encoding of tiled geospatial data that can be shared across clients.

2. File format

The vector tile encoding scheme encodes vector data for a tile in a space efficient manner. It is designed to be used in browsers or serverside applications for fast rendering or lookups of feature data.

Vector Tiles use Google Protocol buffers as a container format. It is exclusively geared towards square pixel tiles in Spherical Mercator projection.

3. Internal structure

A vector tile can consist of one or more named layers and containing one or more features.

Features contain an id, attributes, and geometries: either point, linestring, or polygon.

Geometries are stored as an a single array of integers that represent an command,x,y stream (where command is a rendering command like move_to or line_to). Commands are encoded only when they change. Geometries are clipped, reprojected into spherical mercator, converted to screen coordinates, and delta and zigzag encoded.

Feature attributes are encoded as key:value pairs which are dictionary encoded at the layer level for compact storage of any repeated keys or values. Values use variant type encoding supporting both unicode strings, boolean values, and various integer and floating point types.

For example, a GeoJSON feature like:

{
    "type": "FeatureCollection", 
    "features": [
        {
            "geometry": {
                "type": "Point", 
                "coordinates": [
                    -8247861.1000836585, 
                    4970241.327215323
                ]
            }, 
            "type": "Feature", 
            "properties": {
                "hello": "world"
            }
        }
    ]
}

Would be structured like:

layers {
  name: "points"
  features {
    id: 1
    tags: 0
    tags: 0
    type: Point
    geometry: 9
    geometry: 2410
    geometry: 3080
  }
  keys: "hello"
  values {
    string_value: "world"
  }
  extent: 4096
  version: 2
}

The complete and authoritative details on encoding are part of the code comments for the vector tile protobuf schema document.