MBTiles and SMTiles format map tiles

SuperMap iServer supports the map tiles that conform with MBTiles standard and an extended format based on MBTiles, SMTiles. MBTiles is a kind of standard made by MapBox that stores map tile data to SQLite database to use, manage and share.

Please refer to http://mapbox.com/mbtiles-spec/ for more information about MBTiles.

• MBTiles
• SMTiles
• Description of MBTiles and SMTiles formats
  • metadata (metadata table)
  • tiles ( tile view)
  • map (tile index table)
  • images (tile data table)

MBTiles

MBTiles formats require the map coordinate system is: Web Mercator, namely the PCS_WGS_1984_WORLD_MERCATOR, EPSG Code: 3857. MBTiles creates and organizes the map tiles according to TMS of  OSGeo, supporting the fixed scale set in Web Mercator. As shown in table 1, the initialization display level 0 in this scale contains a 256*256 tile, and define the origin point is (-20037508.34,-20037508.34), that is, the left bottom of global. MBTiles supports PNG and JPG.

Table 1 Parameter list of MBTiles display levels

The map tile storage format of MBTiles *.mbtiles file name is composed by map name, Hashcode, tile width and height, tile format, T (transparent) and .mbtiles suffix, such as China_69470548_256X256_PNG_T.mbtiles and China_69470548_256X256_PNG.mbtiles.

SMTiles

In order to meet the application requirements, SuperMap iServer not only supports to create and use the MBTiles tile map data, but also supports the SMTiles whihc is extended on MBTiles. Compared to MBTiles format, SMTiles supports any coordinate system and scale. The start point of tile should be any specified point. The directory of row increase from origin to left bottom. In addition, map tile format supports PNG, JPG, PNG and JPG.

The map tile storage format of SMTiles *.smtiles file name is composed by map name, Hashcode, tile width and height, tile format, T (transparent) and .smtiles suffix, such as China_69470548_256X256_PNG_T.smtiles and China_69470548_256X256_PNG.smtiles.

Description of MBTiles and SMTiles formats

MBTiles and SMTiles storage mechanism are basically the same. They manage the map tile data through "metadata", tiles, map datasheet, and image datasheet.

metadata (metadata table)

It adopts the key value to store the settings of map tile data, including "name" and "value" fields, in accordance with MBTiles 1.1 specification, as shown below:

The contents of the metadata table are as shown below:

Table 2 Example of metadata table

The keys of the metadata table are as shown below:

Table 3 Description of fields in metadata table

tiles ( tile view)

Contain all tile data and values that used to locate.

MBTiles format supports four fields: zoom_level, tile_colum, tile_row and tile_data in MBTiles1.1 standard. Based on MBTiles, SMTiles adds resolution field to support scale and resolution. zoom_level are -1, as shown below.

Notes: when the ground resolution can not correspond to the resolution in table 3, zoom_level is -1.

Using the tiles view can reduce the redundant tiles. In sea or clear land, it may be several pieces with small scale, but in large scale, such as 1:10000, it may be millions of the single color blue tiles. Through splitting tile index and tile storage, MBTiles use view to relate the two. So the index of these tiles point to the same tile, which can reduce the redundancy of true color.

map (tile index table)

Include the values and tile id which are used to locate. Except the four fields in MBTiles1.1: zoom_level, tile_colum, tile_row and tile_id, it adds the resolution field. Support any scales or resolution. create_time field identifies the time of creating tile. If the tile is the pure color, tile_id is composed of the hexadecimal values of image RGB. For example, the red image tile_id is "ff0000". Others' tile_id is the cluster of resolution_x_y.

Figure 4 map structure chart

images (tile data table)

Include tile id and tile data.

According to resolution or corresponding scale, read tile data. For more information about MBTils scale and resolution, please refer to Table 1.