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High-resolution Remote Sensing: Detailed Information for Participatory Research
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High-resolution imagery, either airborne or from space, together with advanced software, open the way to precision mapping, whereby 1:1,000 to 1:5,000 scale images can be routinely processed and provide detailed information on land-use and high precision digital elevation models (DEMs). This can be compared to a very efficient alternative to land surveying, and can also be used for participatory research. For example, unlike a Landsat TM imagery, high-resolution images may serve as a practical information input for a local community to recognize and localize such key elements farm boundaries, group of houses, trees and trails. High-resolution
imagery can also be useful for understanding the behavior of models and the loss of information as we go to smaller scales.

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Once a camera taking a series of images (e.g. air photos) is properly positioned in space, orthoimages and DEMs can be produced. Using precise ground control points (determined with GPS), along with bridging techniques to tie overlapping images, the software matches the features in the overlap area and computes elevation at each match. A DEM typically has 5 m pixels with 1 m elevation precision. The orthoimage is derived from the original image by transforming it
geometrically, using a DEM, to perfectly overlay a map. Its precision is typically 1 to 3 m, with a 1 m pixel size. Value-added products such as chromo stereo images can be produced to increase the usability of orthoimages for participatory research, as alternatives to physical models. In the case of Rio Tascalapa Watershed in Honduras illustrated below, ground control points were taken with decimeter accuracy with a Dual Frequency Leica 3323 GPS. ERDAS Orthomax v. 8.2 on a Sun SparcStation 10 was used for processing.

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In the past, 1:1,000 scale Styrofoam models have been built and used in the field to stimulate discussions on watershed management by the community. With the new technique, users can delineate features (e.g. soil units, land-use, micro-watersheds) directly on a chromo stereo image, therefore producing georeferenced information that can be readily transferred to a GIS. Detailed maps showing transportation networks, buildings and topography can be used to validate smaller scale maps as well as for cross-scale modelling.

Chromostereo Image of the Rio Tascalapa watershed, Honduras
(better viewed with ChromaDepth MD glasses, http://www.chromatek.com/)

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Contact : Grégoire Leclerc, Javier Puig, William Bell, GIS Unit
International Centre for Tropical Agriculture (CIAT), A.A. 6713, Cali, Colombia.
Phone: +1 415 833 6625/+57 2 445 0089 Fax: +1 415 833 6626.
E-mail: g.leclerc@cgnet.com | http://www.ciat.cgiar.org/