Spanish scientists detect shadows in satellite images

JUNE 2, 2008--Scientists at the University of Malaga have devised a procedure for accurately identifying shadows in high-resolution images captured by satellites.

Jun 2nd, 2008

JUNE 2, 2008--Scientists at the University of Malaga ( internacional.universia.net/espanya/uma/inf_general_ing.htm) have devised a procedure for accurately identifying shadows in high-resolution images captured by satellites, making it possible to obtain more precise information on streets, buildings, vehicles, crops, and other elements detected from space. The results of this research have been passed to a company in the Parque Tecnológico de Andalucía, which is applying them in satellite image processing and detection of urban changes.

Vicente Arévalo, professor in the IT Engineering School at the University of Malaga and coauthor of the study, explained that high-resolution images provided by current satellites and planes "have opened a new era in the field of teledetection and, resolution enhancement also means that shadows, something inherent in any image, take on special significance." For example, shadows fall on buildings, cars, or street furniture, and in an aerial photograph "it is very important to detect what is or is not a shadow to correctly identify the elements that appear in it."

Arévalo pointed out that the identification of shadows enables the subsequent application of specific information-recovery techniques, as well as the preparation of three-dimensional designs. Thanks to the shadows, IT engineers can estimate the height of elements in a landscape, such as a house, among other parameters.

To carry out this study, researchers have used images captured by the QuickBird satellite, also used for capturing aerial photographs of the Google Earth virtual atlas. Images obtained with this satellite have 60-cm/pixel resolution. Once the image has been taken, its color components are analyzed and the so-called "seeds"--small groups of pixels that have a greater probability of being shadows--are identified through circles. To these seeds other surrounding pixels are added that are significant statistically for detecting the shadowy areas of the photo as accurately as possible in a process in which other tools such as edge detectors are also used.

The method has been successfully tested in images obtained under different lighting conditions, in both urban and rural areas. For example, in a field of olive trees, it is easy to quantify the trees and see their size more accurately if shadows are properly identified in the high-resolution images.

However, the scientists' main focus is to detect urban changes, seeing how certain areas of cities change over time. Their studies make it possible to detect things ranging from earth movements or changes

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