The Earth Observation division of the South African National Space Agency (Sansa) has launched the latest edition of its mosaic of satellite imagery of the whole of South Africa, using data from the Spot 5 Earth observation satellite. Assembled from imagery taken and collected from January 24 to December 20 last year, it is known as the Spot 5 Mosaic 2011.
“The mosaic can be used for many purposes,” highlighted Sansa Earth Observation production manager Natalie Diemer, “housing and urban planning, agriculture (especially determining crop yields), water and, particularly, dam monitoring, environmental monitoring, forestry monitoring, and so on.”
A total of 952 images were acquired, most during the winter when cloud cover is minimal over most of the country. “Cloud cover is always a problem,” she observed.
These images were composed of 476 multispectral (colour) images with a resolution of 10 m and 476 panchromatic (black and white) images with a resolution of 2.5 m. These were combined to create 476 2.5 m resolution true colour scenes, which were “mosaicked” to create the image of the entire country. This was then divided into squares, called tiles, to produce the final product.
The merging of the two types of image to form the scene is done automatically. “But, sometimes, there is a glitch in the process,” said Diemer. “So every scene needs to be checked manually. My team does this.”
The entire mosaic is supplied on a single disc, and is available in different formats – UTM, tiff (zipped); geographic, tiff (zipped); geographic JP2000 format, compressed; geographic, ecw format (zipped) and degraded to 10 m resolution, geographic, tiff (zipped).
“We send out about 30 discs,” she reported. Each province gets one, with the data for its territory; others go to the Presidency, government departments and government agencies such as South African National Parks.
In addition, Sansa Earth Observation supplies discs containing each scene separately and providing both the final scene and the two images that make it up. These discs are used for scientific research and are supplied, for example, to universities.
About 15 such discs are issued.
The first Spot 5 mosaic of the country was compiled in 2006.