In this monthly feature, we span the globe to examine Our Changing Landscape with time series of medium resolution RapidEye satellite imagery. The RapidEye archive dates back to late 2008 and already contains more than 4 billion square kilometers of data. This month, we look at changes in the sand dunes of Erg Chebbi, Morocco.
The RapidEye Constellation
RapidEye is a constellation of five 5-meter medium resolution satellites each offering five spectral bands of information. The RapidEye constellation offers a daily revisit time to every location on the planet with a huge footprint that is 77-km wide. The data is priced competitively with a base price of $1.28 per square kilometer for all five spectral bands – academics do receive discounts. RapidEye adds a fifth band, the red edge, to the ‘traditional’ multispectral set of blue, green, red and near-infrared (NIR). The additional spectral data available in the red edge band allows users to extract more useful land ‘information’ than can be from traditional 4-band imagery sources. When RapidEye imagery is ordered as a Level 3A Orthorectified product, images from multiple dates are extremely well registered, making it the ideal data source for Our Changing Landscape.
Erg Chebbi, Morocco
If you have ever traveled in Morocco, there is a good chance you have made the trek to the southeastern part of the country to visit what many picture as the Sahara: tall, picturesque piles of golden sand with camels walking slowly over them. In reality, sand dunes only make up a small percentage of the desert landscape, but there is no doubt they are an iconic natural feature. To reach the tallest sand dunes in Morocco, most travelers enter the region through Merzouga which is only several kilometers from the edge of Erg Chebbi. Merzouga is a small city with a few markets, restaurants and several quiet streets lined by pinkish buildings matching the hue of the landscape. Once tourists reach their final destination in Erg Chebbi, they are greeted by 150-meter tall mountains of sand.
An erg is a sea of sand formed by the accumulation of eolian (i.e. wind-blown) sand with little to no vegetation and covering an area larger than 125 square km (sq km). While 85% of the world’s eolian sand is found in areas larger than 32,000 sq km, Erg Chebbi is roughly 20 km long by 5 km wide so it is debatable if the name erg even applies.
Erg Chebbi is covered by sand dune after sand dune. A dune is formed when there is abundant sand, ample and steady wind and obstacles on the landscape to capture the blown sand. Dunes generally move across the landscape in the same direction as the prevailing wind. Sand particles are blown up the side of the dune facing the wind, and when they reach the top of the hill, they tumble down the slip face. In this slow and steady progression, sand dunes roll across the landscape, waxing, waning and shifting shape.
While I was unable to find a vast research on the movement of sand dunes, the studies I did find suggest a migration range of 0.7 to 7.5 meters per year. Further, dunes can elongate their shape with a range of 2.3 to 13 meters per year. As a final note, the migration and elongation of dunes will vary across a single region. It is the intent of Our Changing Landscape to track a small area of Erg Chebbi over a 5 year time period and see if change in the sand dunes can be detected with 5-meter RapidEye imagery.
If you would like to find out more about using RapidEye for your academic studies, engineering projects or any landscape analysis, let us know at [email protected] or (303) 993-3863.