Description of the cache file format based on the Mbtiles specification extension

Mbtiles is a specification developed by MapBox that stores tile map data in a SQLite database and can be quickly used, managed, and shared. This specification is formulated by Mapbox, see http://mapbox.com/mbtiles-spec/.

Supermap Iserver to meet the needs of its own applications, not only support the generation and use of tile map data that satisfies the Mbtiles specification, but also expands the content in mbtiles based on the original specification. The expanded. mbtiles file supports any coordinate system, supports arbitrary resolutions, and supports map tile data in JPG and PNG mixed formats.

The expanded. mbtiles file still uses the metadata table "metadata" and the tile data sheet "tiles" to manage tile map data.

The metadata metadata table stores the relevant settings for the map tile data in the form of key-value pairs, including the field "name" and "value" of the two text types, consistent with the Mbtiles 1.1 specification. Table Structure 1 shows the following:

Figure 1 Table structure of the metadata table

An example of the contents of a metadata table is shown in table 1:

Table 1 Examples of meta-data tables

Name Value Name World Type Baselayer Version 1.1 Description World created in 2012-12-17 16:51:22 by Supermap IServer Format Png Bounds -180.0,-90.0,180.0,90.0 Axis_origin -180.0,90.0 Axis_positive_direction Rightdown Crs_wkid 4326 Crs_wkt geogcs["WGS 84", datum["wgs_1984", spheroid["WGS", 6378137, 298.257223563, authority["EPSG", "7030"], authority["EPSG", "6326"]], primem["Greenwich", 0, authority["EPSG", "8901"], unit["Degree", 0.0174532925199433, authority["EPSG", "9122"], authority["EPSG", "4326"] Tile_height 256 Tile_width 256 Resolutions 78271.516964,529.1666666666666 Scales 5.0E-7 Map_parameter {"Scale": 0.00101610071425, "clipregion": {"center": null, "id": 0, "style ... Compatible False

The fields of the metadata table (keys) are illustrated in table 2:

Table 2 Description of each field in the metadata table

Field name Whether you must Whether the iServer extension Description Name Is Whether The English name of the map, if the map name is Chinese, is automatically converted to pinyin for the Chinese name. Type Is Whether The map type whose value can be overlay or baselayer. Version Is Whether The version of the map tile data, the version of the Mbtiles specification is 1.1. Description Is Whether Describe the information. Format Is Whether Tile data format: PNG, JPG, or jpg_png, where Jpg_png is a mixed format supported by the V5 cache. Bounds Whether Whether The transduction range of the map, whose units are consistent with the map units. The format of bounds in Iserver is: left, bottom, right, on, for example: -180,-85,180,85. Axis_origin Whether Is Transduction the location of the origin point. Axis_positive_direction Whether Is The enumerated type (Rightdown, Rightup, Leftdown, Leftup) of the positive direction of the axis in the coordinate system, for example: The x-axis to the right y-axis down rightdown. Crs_wkid Whether Is The coordinate system's EPSG Code (-1000 is the user-defined coordinate system, and 0 is the normal plane coordinate system). Crs_wkt Whether Is The geographic coordinate system information represented by WKT. (wkt: A text markup language developed by the Open Geospatial Consortium (OGC), see: http://docs.geotools.org/stable/javadocs/org/opengis/referencing/doc-files/WKT.html) Tile_height Whether Is The height of the tile, usually 256. Tile_width Whether Is The width of the tile, usually 256. Resolutions Whether Is Resolution set, which corresponds to the resolution of tiles at all levels. Scales Whether Is A collection of scale scales corresponding to the resolution of each level. Map_parameter Whether Is A JSON string of map default parameters that contains all of the map's feature information, such as the name of the map, the map's scale mapscale, and so on. Compatible Whether Is is compatible with the Mbtiles specification, False if the Wkid field is 3857 and if all resolutions in the resolution collection belong to the Mbtiles canonical resolution.

The Tiles tile data sheet includes all tile data and some values for locating tile data. In addition to the MBTiles1.1 specification of Zoom_level, Tile_colum, Tile_row, tile_data Four fields, added resolution field, support any scale, arbitrary resolution. Table Structure 2 shows the following:

Figure 2 Table structure of the Tile data table

Table content for Tile Data Example 3:

Fig. 3 Example of table contents of tile data table

Note: The Zoom_level value is-1 when the tile corresponds to a ground resolution that does not correspond to the ground resolution in table 3.

Read and write tile data according to resolution value or corresponding scale.

Table 3 Mbtiles Each display level parameter list

Display level Map width height (pixels) Number of slices Ground resolution (M/pixel) Map Scale (DPI) 0 256 1 156543.033928 1:5,916,587,10.,909,131 1 512 4 78271.516964 1:2,958,293,55.,454,566 2 1024 16 39135.758482 1:1,479,146,77.,727,283 3 2048 64 19567.879241 1:739,573,38.,863,641 4 4096 256 9783.939621 1:369,786,69.,431,821 5 8192 1024 4891.969810 1:18,489,334,.71,591 6 16384 4096 2445.984905 1:92,446,67.,357,955 7 32768 16384 1222.992453 1:46,223,33.,678,978 8 65536 65536 611.496226 1:23,111,66.,839,489 9 131072 262144 305.748113 1:11,555,83.,419,744 10 262144 1048576 152.874057 1:577791.709872 11 524288 4194304 76.437028 1:288895.854936 12 1048576 16777216 38.218514 1:144447.927468 13 2097152 67108864 19.109257 1:72223.963734 14 4194304 268435456 9.554629 1:36111.981867 15 8388608 1073741824 4.777314 1:18055.990934 16 16777216 4294967296 2.388657 1:9027.995467 17 33554432 17179869184 1.194329 1:4513.997733 18 67108864 68719476736 0.597164 1:2256.998867 19 134217728 274877906944 0.298582 1:1128.499433 20 268435456 1099511627776 0.149291 1:564.249717 21st 536870912 4398046511104 0.074646 1:282.124858 22 1073741824 17592186044416 0.037323 1:141.062429

Tiles Tile Data Table retains a 11-bit significant number (rounded) when resolution stored. Valid number: From the left of the approximate number of the first not 0 of the number, to the exact number, all numbers are called the approximate number of valid digits (0.00123 of the valid number is three-way, the significant number is 3). Examples are shown in table 4:

Table 4 Resolution Approximate calculation example

Resolution actual value Approximate values for database storage 156543.03392804097 156543.03393 0.00029158412279196264 0.00029158412279 1.19432856695587 1.1943285670

The reference code for calculating resolution in Java is as follows:

Public Static String getresolutionstring (double resolution) {

int n = (int) Math. log10 (resolution);

int precision = 11;

if (Resolution < 1.0) {

n--;

}

int scale = precision-1-N;

return New BigDecimal (resolution). Setscale (Scale, BigDecimal. Round_half_up). toString ();

}

Description of the cache file format based on the Mbtiles specification extension