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Cesium Terrain Server

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Ed Mackey edited this page Nov 27, 2013 · 10 revisions

Cesium Terrain Server

This page describes the format and layout of the terrain data hosted at: http://cesiumjs.org/smallterrain/

Note: the above URL will return "403 - Forbidden" because browsing is not supported.

The terrain server is still very much a work in progress, so the format can and will change in the future.

The server currently hosts a mosaic of GTOPO30 and CGIAR SRTM terrain data. SRTM is used from -60 degrees to 60 degrees latitude and all longitudes. GTOPO30 is used north of 60 degrees latitude and south of -60 degrees latitude. Additional sources, including high-resolution NED terrain, will be added in the future.

The data is organized into a simple multi-resolution pyramid of heightmaps according to the Tile Map Service (TMS) layout and the global-geodetic profile. All tiles have the extension .terrain. So, the two root tiles are found at these URLs:

The 8 tiles at the next level are found at these URLs:

The tiles are 65x65 vertices and overlap their neighbors at their edges. In other words, at the root, the eastern-most column of heights in the western tile is identical to the western-most column of heights in the eastern tile.

Terrain tiles are served gzipped. Once extracted, they are at least 8,452 bytes in size. The first and most important part of the file is a simple array of 16-bit, little-endian, integer heights arranged from north to south and from west to east - the first 2 bytes are the height in the northwest corner, and the next 2 bytes are the height at the location just to the east of there. Each height is the number of 1/5 meter units above -1000 meters. The total size of the post data is 65 * 65 * 2 = 8450 bytes.

Following the height data is one additional byte which is a bit mask indicating which child tiles are present. The bit values are as follows:

  • Southwest - bit 0 - value 1
  • Southeast - bit 1 - value 2
  • Northwest - bit 2 - value 4
  • Northeast - bit 3 - value 8

If a bit is set, the corresponding .terrain file can be expected to be found on the server as well. If it is cleared, requesting the .terrain file will return a 404 error.

The child bit mask is followed by the water mask. The water mask is either 1 byte, in the case that the tile is all land or all water, or it is 256 * 256 * 1 = 65536 bytes if the tile has a mix of land and water. Each mask value is 0 for land and 255 for water. Values between 0 and 255 are allowed as well (but not currently present in the data) in order to support anti-aliasing of the coastline.

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