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Spatial Visualization - technical tips, best practices, and news from the product team

  • September 2, 2013

A note on tile names and directory structure

Jayant Sharma
Sr. Director, Product Mgmt

This is a guest post by Honglei Zhu. 

Important: The tiling scheme described here can, and likely will, change and hence be invalid in future releases.

This note outlines the tile
storage, and file naming conventions, for map tile layers created with FMW
MapViewer. The tiles are stored in a directory structure based on the data
source name, map tile layer name, number of zoom levels, and tile mesh codes.

Tile
Mesh Code and File Storage Structure

Each map tile layer is
in a directory under $DEFAULT_TILE_STORAGE_ROOT and named [datasource.tile_layer_name].
For example, if the default tile storage is /private/tilestore and the tile
layer demo_map is in the datasource is mvdemo, then the tiles will be under
/private/tilestore/MVDEMO.DEMO_MAP. The
first level of subdirectories contains one directory per zoom level. So if
there are 19 zoom levels then the subdirectories will be named 0 through 18. Each
zoom level directory will contain tile (files) or directories. That is, tiles
are stored in a tree-like directory structure with leaf nodes contain only tile
image files. Non-leaf nodes contain only directories. Mesh codes are used in
naming these subdirectories and tiles.

Tile
Mesh Code

A tile mesh code is
represented by X (horizontal) and Y (vertical) integer arrays, in a form of
[meshX0, meshX1, meshX2, … meshXn] and [meshY0, meshY1, meshY2, … meshYn]. At each zoom level, the length for
the X and Y mesh code array is always the same. Different zoom levels, however,
may have different length arrays. The longer the mesh code, the more tile
images that zoom level contains.

For a tile with mesh
code of [meshX0, meshX1, meshX2, … meshXn] and [meshY0, meshY1, meshY2, …
meshYn], its filename and path is:
$DEFAULT_TILE_STORAGE_ROOT/[tile_layer_name]/[zoom_level]/[meshX0]_[meshY0]/ [meshX1]_[meshY1]/ [meshX2]_[meshY2]/…/ [meshXn]_[meshYn].png.

For example, a MVDEMO.DEMO_MAP
tile at zoom level 10, with an X mesh code of [1, 12, 11] and a Y mesh code of
[3, 12, 9] will be at:
/private/tilestore/MVDEMO.DEMO_MAP/10/1_3/12_12/11_9.png

Note:Tile mesh codes are internal to MapViewer and
are may change between releases.
So the following description of how mesh codes
are generated may not be valid in future releases.

The mesh code is computed
from a tile index. The tile index (X,Y) is computed in the Oracle maps client
code from the map tile layer definition as follows:

tileIndexX = (tileMinX–CoordinateSystem.minX) / tileWidth;

tileIndexY = (tileMinY–CoordinateSystem.minY) / tileHeight;

The server then converts the tile index to a mesh code.

Directory
Tree Depth (Mesh Code Length)

When the number of tile
images is small for a given zoom level, all tiles may be stored in its root
directory. When the number gets to millions or even larger, at higher zoom
levels, they are stored in
subdirectories for more efficient file retrieval.

The expected number of
tiles for a given zoom level is calculated using:

maxMeshCodeX = coordinateSystem.width/tileWidth;

maxMeshCodeY = coordinateSystem.height/tileHeight;

total_tiles = maxMeshCodeX*maxMeshCodeY;

A TILING_FACTOR determines
the depth of the directory tree for a given zoom level. The default value of 20
means there will be no more than 400 (20 x 20) sub-directories or tile images
stored at any zoom level. The mesh code length for X equals the base-20 (or
base TILING_FACTOR) logarithm of maxMeshCodeX.

Examples
of converting a tile index (x,y) to Mesh Code

1) Given
that the TILING_FACTOR is 20, then for zoom level 6, the mesh code length is 4.
If the tile index (x,y)=(6063, 7403), then its mesh code is x=[0,15,3,3],
y=[0,18,10,3], and the tile is stored at
:[tile_layer_name]\6\0_0\15_18\3_10\3_3.png

2) If
the TILING_FACTOR is changed to 10, the mesh code length becomes 5. Keeping the
zoom level and tile index the same, the mesh code is x=[0, 6, 0, 6, 3],
y=[0,7,4,0,3], and the tile is stored at:
[tile_layer_name]\6\0_0\6_7\0_4\6_0\3_3.png

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