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Monday, February 23, 2009

Earth Viewer: NASA Blue Marble Next Generation Imagery Added

Iceland from 39 km above

I have just put a new version of Earth and Moon Viewer into production which adds imagery derived from the NASA Blue Marble Next Generation database, produced from images collected during 2004 by the MODIS instrument on the Terra satellite. The oceans, which are supremely uninteresting in the raw imagery, are replaced by bathymetric shading showing the ocean floor contour.

The original NASA imagery has a resolution of 500 metres per pixel. At the moment, the Earth and Moon Viewer database reduces this resolution to 1 km per pixel due to addressing limitations in the 32 bit operating system running on the server farm. When the planned update to 64 bit is complete (probably in the next two months), the resolution will be increased to the full 500 metres per pixel. The image of the Earth at night is based on data from Defense Meteorological Satellite Program (DMSP) satellites and has about one fifth the resolution of the day imagery, so close-up zooms including the night side may look crude—there are simply no higher resolution data available to my knowledge.

A major change in this update is the expansion of the colour palette. Before, day images were compressed to a palette of 1024 colours and night images to a mere 64 colours, allowing both day and night pixels to be compressed into a 16 bit composite indexed pixel. With the Blue Marble Next Generation imagery, separate day and night image databases are used, each with its own colour map with up to 65,536 colours (the night image does not require that many). This results in much more faithful colour rendition, particular in difficult areas such as shallow waters around coastlines. Click the image of Iceland above, rendered for an observer 39 km above sea level, choose other image databases, and compare for yourself.

No changes have been made to the Web interface used by applications requesting custom images, so this update should be 100% compatible with existing downstream applications. Developers of such applications should, however, evaluate the new image database option and if they deem it as gorgeous as I do, consider changing their imagery selector in the request URL to "img=NASA500m.evif" to request the Blue Marble image database.

As always, if you're interested in using high resolution Earth imagery in your own applications, you should start with the original NASA databases, as they contain more colour information than the derivatives used on this site, which have been compressed in the interest of conserving memory and expediting the rendering of custom images.

Posted at February 23, 2009 21:38