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21 February 2004
Ancient desert markings imaged from orbit

Visible from ESA's Proba spacecraft 600 kilometres away in space are the largest of the many Nazca Lines; ancient desert markings now at risk from human encroachment as well as flood events feared to be increasing in frequency. Designated a World Heritage Site in 1994, the Lines are a mixture of animal figures and long straight lines etched across an area of about 70 km by 30 km on the Nazca plain, between the Andes and Pacific Coast at the southern end of Peru. The oldest lines date from around 400 BCE and went on being created for perhaps a thousand years.
     They were made simply enough, by moving dark surface stones to expose pale sand beneath. However their intended purpose remains a mystery. It has variously been proposed they were created as pathways for religious processions and ceremonies, an astronomical observatory or a guide to underground water resources. These lines have been preserved down the centuries by extreme local dryness and a lack of erosion mechanisms, but are now coming increasingly under threat: it is estimated the last 30 years saw greater erosion and degradation of the site than the previous thousand years before them.
     The Proba spacecraft acquired images through the Compact High Resolution Imaging Spectrometer (CHRIS), but its 18.6 metre resolution is too low to make out the animal figures although the straight Nasca Lines can be seen faintly. Clearly shown in the Proba images is another cause of damage to the Lines: deposits left by mudslides after heavy rains in the Andean Mountains. These events are believed to be connected to the El Niņo phenomenon in the Pacific Ocean.
     A team from Edinburgh University and remote sensing company Vexcel UK has been using data from another ESA spacecraft to measure damage to the Nasca Lines, with their results due to be published in the May Issue of the International Journal of Remote Sensing. Their work involves combining radar images from the Synthetic Aperture Radar (SAR) instrument aboard ERS-2. Instead of measuring reflected light, SAR makes images from backscattered radar signals that chart surface roughness.
      Nicholas Walker of Vexcel UK explained: "Although the instrument lacks sufficient resolution to unambiguously distinguish individual lines and shapes, by combining two satellite images using a technique known as SAR interferometric coherence it is possible to detect erosion and changes to the surface at the scale of centimetres. This technique seems to provide an effective means for monitoring the integrity of the markings and we are developing it to include more sensors and data of higher spatial resolution, so as to encourage the establishment of a long term and frequent monitoring programme supporting conservation efforts in the area."

Source: ESA News (20 February 2004)

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