A simple space-efficient format for individual tiles of map data

License: MIT

RSTiledata.h

typedef enum {
    RSTiledataBuilding = 1,
    RSTiledataHighwayMotorway = 2,
    // [...]
    RSTiledataHighwayService = 7
} RSTiledataThingType;

int RSTiledataThingTypeIsClosedWay(RSTiledataThingType thingType);

Datapack

A datapack is a LevelDB database with items of the form:

key:    uint64le(zoom) + uint64le(x) + uint64le(y)
value:  Tiledata

For disk locality, tiledata keys will probably later become varint(zoom) + interleaved_uint64be_bits(x, y).

Tiledata format

Given

• zoom (e.g. 17)
• tile_x, tile_y
• resolution_bits (assumed to be 8 for now)

...we can decode this tiledata:

*
  thing_type : varint
  num_nodes : varint
  +
    delta_x : signed varint
    delta_y : signed varint

...where x and y are quantized values in this space:

(0, 0): this tile's upper-left corner
(1, 1): this tile's lower-right corner

Before each tile, x and y are set to 0.

To convert x to the (zoom, x, y) space, divide by (1 << resolution_bits) and add tile_x.

Example

To simplify the example, we assume resolution_bits is 2.

Consider: one building (Xs) and one tertiary highway (\s):

\ 
----
-XX-
-X--
----  \

Tiledata:

0106020204000002010000020100050200001E0A

More verbosely:

01              building=*
06              6 nodes             ------------------
  02 02          1  1    1  1       | Signed varints:
  04 00          3  1    2  0       |   0   00
  00 02          3  2    0  1       |   -1  01
  01 00          2  2   -1  0       |   1   02
  00 02          2  3    0  1       |   -2  03
  01 00          1  3   -1  0       |   2   04
                                    |   3   06
05              highway=tertiary    |   4   08
02              2 nodes             |   5   0A
  00 01          0 -1    0 -1       |   6   0C
  0E 0A          7  4    7  5       |   7   0E