tecznotes

Michal Migurski's notebook, listening post, and soapbox. Subscribe to this blog. Check out the rest of my site as well.

Feb 12, 2007 9:05pm

special on NASA blue marble tiles, aisle six

My iMac spent the weekend tearing through NASA's Blue Marble satellite imagery set, projecting the 500m/pixel base map to Mercator, chopping it up into tiles, and uploading it to S3.

Here's the root-level tile:

I took care to ensure that the slices match those in use by Google, Yahoo!, Microsoft, and OSM for cross-compatibility among any code that uses tiles from those providers.

The general procedure took three steps:

  1. After downloading the full-resolution imagery from NASA and converting the PNG's to VIPS .v format, I extracted tiles at the highest zoom level to individual 256x256 images. This step had to include the mercator projection, which meant extracting squat rectangles near the poles and stretching them vertically to 256 pixels. I used VIPS for this step because its random-access file format is specially designed to support fast region selection from very large bitmaps (eight 21600x21600 pixel images).
  2. For each lower zoom level, groups of four tiles were combined with ImageMagick. This turned out to be a very basic recursive function that spit out a series of ImageMagick commands as text. God bless sh.
  3. Finally, every tile was uploaded to S3 and given a standard name: {zoom level}-r{row number}-c{column number}.jpg. The numbering scheme should be familiar to anyone who's worked with tile providers or Zoomify before.

S3's cheap bandwidth and the public domain status of these images means that you should feel free to use them in your own applications. If you plan to do anything unusually demanding with them, please go easy on my wallet and take a moment to sign up for an AWS account and use the following Python script to copy the complete tileset to a fresh S3 bucket of your own:

DuplicateBlueMarble.tar.gz (4KB)

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