SVTiles format vector tiles

 SVTiles is a type of storage format customized by SuperMap, which stores the vector tile data with the SQLite database. The svtiles file use metadata and tiles data to manage the vector tiles.

 The name of the SVTiles file is composed of the map name, Hashcode, width and height of tiles, and the .svtiles suffix like China_-678451788_256X256.svtiles.

 From iServer 8C(2017) , vector tile adds style information. When caching vector tiles, it will generate the SVTiles file, and the style information file with ".resources" suffix to store the style attribute information of the vector tiles.

The style information file also uses SQLite database. And it is named in the same way as SVTiles.

.svtiles file and .resource file use the following tables and views to store and manage vetor tiles.

• SVTiles
  • metadata (metadata table)
  • tiles (tile data table)
  • geometries (geometric object table)
  • attributes (attribute information table)
  • tilefeatures (tilefeatures view)
  • tilegeometries (tilegeometries view)
• resource
  • style (style table)
  • Symbol (symbol table)

SVTiles

metadata (metadata table)

Metadata table adopts key-value pairs for storing settings of vector tiles, including name and value. The structure of the table is shown in the figure:

Below shows an examples of metadata table..

Table 1 Example of metadata table

Table 2 shows the fields' description of metadata table.

Table 2 Description of fields in metadata table

tiles (tile data table)

All vector tile data and values for locating tiles are included, as illustrated in the figure:

Table 3 shows the fields' description of tile data table.

Table 3 Description of fields in SVTiles tile table

Example of tile dataset content is shown in the figure below.

geometries (geometric object table)

Records the geometry information of the geographic features in tiles, as illustrated in the figure below:

Table 4 shows the fields' description of geometric object table.

Table 4 Description of fields in geometric object table

Each record corresponds to a clipped geometric object. geometry_data stores pixel coordinates. At different levels, pixel coordinates of a same geometric object are different; at the same level, one geometric object can cover multiple tiles, as shown in figure 1. The tile stores geometric object after clipping, as shown in figure 2.

Figure 1 Geometric object covering 2 tiles

Figure 2 Geometric object after clipping

The default storage format is SuperMapJSON. It can also be WKT, GeoJSON, GML. SuperMapJSON format is as follows:

{

“type”:”REGION”,

“points”:[0,0,256,0,256,256,0,256,0,0,1,1,3,3,2,2,1,1],

“parts”:[5,4]

}

attributes (attribute information table)

Records the attribute information of the geometric objects, as shown in the figure below.

Table 5 shows the fields' description of attribute information table.

  Table 5 Description of fields in attribute information table

The attribute and geometric objects are corresponding to each other. The geometric object of each geographic feature spread in different tiles (2.3), therefore, attributes and geometry has 1:many relationship. The Json format of attr_data is as follows:

{

“NAME”:”Beijing”,

“PostCode”:100000,

“POP”:11510000,

“Country”:”China”

}

search_values are strings separated by comma and it is used for fast query. For instance, search_values for Beijing is “Beijing,100000”. Query statement according to search_values is as follows:

where search_values like ‘%Beijing%’

tilefeatures (tilefeatures view)

The tilesfeatures view generated based on tiles, attributes, geometies.

Table 6 shows the fields' description of tilefeatures view.

 Table 6 Description of values of tilefeatures view

tilegeometries (tilegeometries view)

The tilesgeometries view generated based on tiles, attributes, geometries is used to completely represent the geometric information of tile and geographic features in tile. If attribute information is not required, the query based on tilegeometries will be faster, as shown in the figure below:

Table 6 shows the fields' description of tilegeometries view.

Table 7 Description of fields in tilegeometries view

•   SQL manifest.

metadata

CREATE TABLE metadata (name text, value text);

CREATE UNIQUE INDEX metadata_idx ON metadata (name);

 tiles

CREATE TABLE tiles (resolution double, tile_column integer, tile_row integer,tile_id text, create_time

text);

CREATE UNIQUE INDEX tiles_index on tiles(resolution,tile_column,tile_row);

Create index tiles_id_index ON tiles(tile_id);

 geometries

CREATE TABLE geometries(layer text,fid long,tile_id text,geometry_data text );

CREATE UNIQUE INDEXgeometries_index ON geometries(layer ,fid,tile_id);

 attributes

CREATE TABLE attributes( layer text, fid long, attr_datatext,search_values text);

CREATE UNIQUE INDEX attrbutes_index ON attributes (layer, fid);

 tilefeatures

CREATE VIEW tilefeatures as

SELECT A.*,B.layer,B.fid,B.geometry_data,C.search_values,C.attr_data

FROM tiles as A,geometries as B,attributes as C

WHERE A.tile_id=B.tile_id and B.layer=C.layer and B.fid=C.fid

 tilegeometries

CREATEVIEW tilegeometries as

SELECT A.*,B.layer,B.fid,B.geometry_data

FROM tiles as A,geometries as B

WHERE A.tile_id=B.tile_id

•    Comparison between SVTiles V201401 SVtiles V201310

Functionality comparison is shown in table 8:

Table 8 SVTile functionality comparison between SVTiles V201401 SVtiles V201310

Data table comparison is shown in table 9:

Table 9 Table comparison between SVTiles V201401 SVtiles V201310

•    SuperMapJson

SuperMapJson is a type of Json format defined by SuperMap iServer. This format is composed of 3 parts:

 type: Geometric shape type.

Optional values are POINT, LINE, REGION, which corresponds to MultiPoint, MultLineString, MultiPolygon in WKT separately.

 points: Represents a group of pixel coordinate sequence, each neighboring items representing a pair of coordinates.

parts: Represents the segmentation information of the current geometric object. Each segment represents a child object and the value of segment represents the number of child object points.

Example:

{

“type”:”REGION”,

“points”:[0,0,256,0,256,256,0,256,0,0,1,1,3,3,2,2,1,1],

“parts”:[5,4]

}

In the example above, “parts”:[5,4] represents that the object has two segments, and the coordinates of the two segments are [0,0 ,256,0,256,256,0,256,0,0], [1,1,3,3,2,2,1,1] respectively.

Note: For geometric object with type being POINT, json can contain no parts items. Each coordinate in points of this type of geometric object represents a child POINT.

resource

style (style table)

The style table is used to record each layer's style type and its style description. The fields in the table are shown below:

The meanings of the above fields show below.

Table 10 The fields of style table

Example:

#District@Changchun[zoom<=9.80459446371E-5][zoom>=1.05263157895E-5]{text-placement-type:simple;text-placements:"E,NE,SE,W,NW,SW";line-pattern-file:url(SYMBOLLINE__District@Changchun__64__64__true__-1483079698.png);polygon-pattern-file:url(SYMBOLFILL__District@Changchun__64__64__true__-1483079698.png);point-file:url(SYMBOLMARKER__District@Changchun__64__64__true__-1483079698.png);polygon-opacity:1;polygon-pattern-opacity:1;}

Symbol (symbol table)

The symbol table records the symbol image in tile.

The fields in the table are shown below:

Table 12 The fields of symbol table

The symbol contents in database are shown below.