Digital Earth > tsmApi > File Formats > Tile Sets

Tile Sets:

A TerraVision dataset is a multiresolution format for representing one or more very large images (typically many gigabytes in size). The common name for this representation is a Pyramid. These are used to store the raw terrain data for the TerraVision system, i.e. the texture imagery and the elevation data. Each level in the Pyramid hierarchy is a Simple Tile Set (or Pyramid Level) which is typically at one half the resolution of the prior level. The base of the Pyramid is typically level 0; level 1 represents images at twice the inter-pixel spacing of level 0; and so on.

For example, if level 0 is at 1 meter spacing with 1024x1024 pixels, then level 1 is at 2 meter spacing with 512x512 pixels, level 2 is at 4 meter spacing with 256x256 pixels, and level n is at 2^n meter spacing (with 1024/2^n x 1024/2^n pixels).

Note that resolution is the inverse of inter-pixel spacing, so that larger level numbers represent larger inter-pixel spacing, and hence lower resolution.

Each Simple Tile Set is divided into many small rectangular segments, called "tiles". Tiles are the basic storage unit for the image data component of a tile set. A tile is a relatively small rectangular part of the image, typically 128 x 128 pixels. Currently, all tiles in a tile set must be the same size. A Simple Tile Set can be represented either as a collection of individual files, each containing the raw pixel data for a single tile, or the entire tile set can be represented as a Tiled TIFF image where all of the tiles are contained in the single image file and can be accessed independently. The Tiled TIFF format (.tiff) offers a number of advantages over the previous multiple-file (.tile) representation, including:

• Manageability: tile directories are less cluttered because we require only one file for each mosaic, compared to the many thousands of tile files that could be contained in a single directory previously.

• Available Tools: there are a number of freely-available libraries to simplify the generation of TIFF files, e.g. libtiff.

• Accessibility: OI mosaics can be viewed and manipulated using a number of standard desktop tools, e.g. imgview, xv, vstiff, etc.

• Extensibility: the TIFF format is extremely versatile, and in particular supports a number of different image representation and compression schemes (such as JPEG).

In particular, this last point allows TerraVision to support various storage formats for the data within a Simple Tile Set. Currently, we support the use of raw (lrbt-raster) data and JPEG-compressed data. Each different format is stored as a separate TIFF file on disk, e.g. tiles_lrbt.tiff or tiles_jpeg.tiff.

(For details on the Tiled TIFF format, please refer to Section 15 of The TIFF Revision 6.0 Specification.)

The following table illustrates the directory and file organisation of a Pyramid on local disk, for both the raw tile and TIFF file representations:

ntc-1-utm-2scene.128.oi/
   0/
      tiles_lrbt.tiff
   1/
      tiles_lrbt.tiff
   ...

ntc-1-utm-2scene.128.oi/
   0/
      tiles/
         p0p0.tile  p0p1.tile
         p0p2.tile  ....
   1/
      tiles/
         p0p0.tile  p0p1.tile
         p0p2.tile  ....
   ...

The following figure illustrates all of the above. It shows two Pyramids, both of which have 3 levels (Simple Tile Sets), and the division of these into individual tiles is shown by the red dotted lines:

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Last updated: Monday, 22-Jan-2001 10:35:47 PST.