Southern Africa

Southern Africa

Unless you look very closely, you can’t really tell that there is a four hour and fifteen minute difference between one side of southern Africa and the other in this true-color MODIS image acquired November 7, 2002. This is a composite image of southern Africa taken by both the Aqua and Terra MODIS instruments; the Aqua instrument provided the left half of the image, while the Terra instrument provided the right half. Southern Africa is sufficiently wide enough that it is not possible to get its entirety in one instrument’s pass, so images from the two MODIS instruments were combined.

The calibration of the two MODIS instruments is so accurate that it is possible to stitch together separate passes from each instrument into a virtually seamless image. Were there significant differences in the instruments? calibration, geolocation, or surface reflectance, it would not be possible to produce an image at this high quality. That there is no significant difference in reflectance or geolocation for either instrument promotes continuity and proves that both Aqua and Terra data are equally reliable and accurate.

There are only a couple of clues to the fact that this is a composite image. The first is sunglint on the Atlantic and Indian Oceans. In an image by a single sensor, there would only be one glint field (silver streak) on the water’s surface. Further, the direction of each glint field illustrates which MODIS instrument took which half of the image. The glint that shines from the upper left to the lower right is due to Aqua’s orbit orientation, whereas the glint that shines from the upper right to the lower left comes from Terra’s orbit orientation.

The other clues are in the clouds. Careful observation will reveal that the clouds in the center of the image end very abruptly and form shapes with distinct horizontal edges. This is not natural, but a consequence of the time difference when the two images were acquired. When Terra acquired its half of the image, there were more clouds over central southern Africa than there were four hours and fifteen minutes later when Aqua flew overhead, resulting in the odd cloud patterns when the images were stitched. Finally, the direction of the clouds? shadows also points out that two instruments created this image, but this evidence is only visible in the highest resolutions. The clouds’ shadows on the Aqua (left) side of the image lay to the right, while those on the Terra (right) side lay to the left.

Image courtesy Jacques Descloitres, MODIS Rapid Response Team at NASA GSFC