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Bing Maps provide a world map that users can easily pan and zoom. In order to achieve smooth users interaction, the map is pre-rendered at many detailed levels where each level is cut into tiles of 256 x 256 pixels each. Each tile is given XY coordinates ranging from (0, 0) in the upper left to (2level–1, 2level–1) in the lower right. For example, at level 3 the tile coordinates range from (0, 0) to (7, 7) as follows:

Figure 1: Tile XY coordinates at level 3

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In order to optimize the tiles indexing, the two-dimensional tile XY coordinates are combined into one-dimensional strings called quadtree keys, or “quadkeys” for short. Each quadkey uniquely identifies a tile and it can be used as a key in common database B-tree indexes. To convert tile coordinates into a quadkey, the bits of the Y and X coordinates are interleaved, and the result is interpreted as a base-4 number (with leading zeros maintained) and converted into a string. For instance, given tile XY coordinates of (3, 5) at level 3, the quadkey is determined as follows:

tileX = 3 = 0112

tileY = 5 = 1012

quadkey = 1001112 = 2134 = “213”

Figure 2: Tile Quadkeys

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Finally, tiles that are close to each other have quadkeys that are relatively close together. This is important for optimizing performance, because neighboring tiles are usually requested in groups.

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