WMTS Overview

SuperMap iServer provides WMTS (Web Map Tile Service), which complies with WMTS implementing specifications, OGC(Open Geospatial Consortium) making. WMTS is a standard of the caching technology that OGC proposes, that is, the cache in the server is cut into a map of certain sizes of tiles, only providing the service of a single tile pre-defined to the client, putting more data processing operations such as layers overlaying and others on the client, thus to ease the pressure of data processing on GIS server, to improve the user experience.

The WMTS services of SuperMap iServer provide the map tiles based on the following aspects:

1. Tile matrix set
2. Scale
3. TopLeftCorner
4. See

SuperMap iServer now supports the following WMTS version:

• WMTS 1.0.0

Tile matrix set

WMTS uses a tile matrix set to show the map split, as shown in Figure 1. A tile is a matrix image containing geographic data, a map to be cut into multiple tiles according to a fixed scale, forming a tile matrix, a tile matrix set composed of one or more tile matrices. Different tile matrix has different resolution, each tile matrix using a tile matrix identifier (generally the number of the tile matrix, the layer with the lowest resolution to be Layer 0, upward increasing successively) to identify.

Figure 1 tile matrix set

In figure 2, each tile matrix has

1. its tile size as a scale;
2. its width(TileWidth) and its height(TileHeight) defined by the number of pixels, that is, its tile size. The tile size SuperMap iServer now provides is 256*256;
3. the upper left corner coordinate of bounding box(TileMatrixminX, TileMatrixmaxY);
4. the width and the height of a matrix(MatrixWidth, MatrixHeight) defined by a tile as a unit, such as the number of tiles.

Figure 2 Tile matrix

Each tile in the tile matrix is identified by the row number and the column number of each tile(TileRow, TileCol), the row number and the column number being counted from the upper left tile in tile matrix, (0, 0) as the beginning row number and the beginning column number, successively to the right increase and down increase, as shown in Figure 2.

Scale

The server of WMTS only provides the service of limited number of coordinates and scales, in order to improve the interoperability between the client and the server, WMTS proposing a concept of well-known scale set. The well-known scale set is an agreement among the servers, composed of a public coordinate reference system and a group of public well-known scale sets. The definition of well-known scale set is only an agreement mechanism, not necessary in respect of the interoperability in the technology.

Please refer to Coordinate reference system supported by iServer OGC services for more information.

SuperMap iServer WMTS server provides tile data that based on scale collection and refer to wellKnownScaleSets. Please refer to WellknownScale for more contents about universal scale collection.

Scale=1:Ground resolution (a)*Screen resolution (pixel/inch)/0.0254 (m/inch)    

This method can be simplified for: Scale=0.0254/(a*dpi).

The ground resolution (a) is to the actual distance of a pixel,the unit is meter. The screen resolution (dpi) is to the number of pixels every inch screen of the length contains and general default is 96 dpi, namely, each inch has 96 pixels. 0.0254 (m/inch) means the unit conversion between the meter and inch.

For the WMTS 1.0.0 standard service, the resolution is defined by the pixel size (0.28mm=0.00028m). It converts into screen resolution, namely, the pixel number in every inch is: 1inch/(0.00028m/0.0254(m/inch))=0.0254/0.00028≈90.714.

The default screen resolution to output maps of SuperMap iServer is 96, which is defined by electronic map data specification. It is different with that in WMTS 1.0.0. When using local caches (such as MBTiles) to publish WMTS services, it may be different. So SuperMap iServer provides wmts-china interface instance. This interface adopts the map scale defined in electronic map data specification and a common screen resolution 96.

TopLeftCorner

In WMTS, TopLeftCorner is the character sequence that describes the upper-left coordinates of TileMatrixSet; which is made up of X and Y. In the geographic coordinate system, that the longitude is before the latitude does not conform to the international practice. Although there is no standard to stipulate thst the latitude should be in front of longitude, in general, latitude is in front of longitude.

In the WMTS services provided by SuperMap iServer, the TopLeftCorner of geographic coordinate system is "Y X"; The TopLeftCorner of projected coordinate system, custom coordinate and plane-coordinate system is "X Y".

Table 1 TopLeftCorner order of common coordinate system

See

Please refer to: WMTS relevant specifications of OGC(Open Geospatial Consortium), http://www.opengeospatial.org.