FEATURE - New Mapping Perspectives in Peru
by François Riendeau and Claire Gosselin
The ever-increasing capacities of Earth observation (EO) satellites and related technologies have opened new opportunities for countries with incomplete cartographic coverage and/or outdated maps. For a long time, costly aerial photographic surveys were the only way to gather the high-resolution basic information needed to make maps. Today, stereoscopic images from EO satellites covering the entire world are sophisticated enough to comply with many strict cartographic standards for a fraction of the price of aerial survey maps. However, the effective exploitation of this huge and diversified data source requires cartographers to master new tools and techniques.
A technology transfer project wherein Canadian experts trained Peruvian government officials had precisely this objective in mind. Last June, some 80 participants of this joint project met at the Miraflores Park Hotel in Lima. They attended the Final Results seminar. The event was organized to highlight how each of the eight Peruvian agencies involved would benefit from the use of Earth observation (EO) satellite images for large-scale, cartographic purposes. The project served to introduce the satellite image mapping solution developed by VIASAT GeoTechnologies: StereoSat in their respective mapping programs.
StereoSat is an innovative, economical, and efficient EO-based technology designed to produce topographic, thematic, and cadastral data that complements conventional aerial photography-based methods. This approach combines remote sensing image analysis techniques (image fusion, spectral composites, radiometric pre-processing, and such) with digital photogrammetry principles (block triangulation, stereoscopic interpretation, and rigorous geometric correction). The StereoSat branding not only stands for a set of methodologies and applications that capitalize on the increased capabilities of EO satellites, but also reflects nearly 20 years of Company experience in using satellite imagery for cartographic purposes.
Department heads and mapping specialists from Peruvian governmental agencies in the base mapping1, mining2, natural resources3, agriculture4, public works5, as well as municipal7 and cadastral8 sectors attended the seminar. Presentations by VIASAT GeoTechnologies team members and a Canadian Space Agency9 official supplemented those of the Peruvian participants.
The Head of the Department of Photogrammetry at the Instituto Geográfico Nacional (IGN), Captain Percy Guillermo, started the technical session by announcing the launch of the new national topographic mapping program at the 1:25 000 scale, entirely based on satellite imagery. He described how his team, after a relatively short training session by VIASAT GeoTechnologies specialists, had initiated the production of 25 topographic map sheets at 1:25 000 scale using SPOT-5 imagery and StereoSat technology. The Peruvian mapping agency estimates their costs, in both time and money, will be reduced by roughly 30-35 percent by using stereoscopic satellite imagery, rather than aerial photography for the production of these new topographic map sheets.
During the testing phase, the Company lent technical support to IGN photogrammetrists for block triangulation, ground control planning, digital elevation model production, and image processing. Validation, performed entirely by IGN staff, revealed that the quality standards for that map scale had been fully met. In addition to the 1:25 000 topographic maps, a 1:5 000 map was also produced for demonstration purposes (see figure 1).
Another thrilling moment occurred when the Instituto Geológico, Minero y Metalúrgico (INGEMMET) presented the first 1:25 000 geological map ever produced in Peru. Until then, geological maps had been produced at the 1:50 000 scale from aerial photographs.
With StereoSat and using SPOT-5 (5-m resolution) and ASTER (30-m resolution) satellite imagery, INGEMMET was able to produce a larger scale map containing geological/lithological information of even better quality than those previously produced. Mapping specialists may doubt that satellite imagery can produce better results than aerial photography, but Flor Rodriquez, geology specialist at INGEMMET, provides an explanation: the fact that we can simultaneously view detailed spatial features and patterns and a wide spectrum of colours greatly facilitates lithological discrimination.
That is because a set of StereoSat techniques allows the combination, in a single 3D view, of the relatively high spatial resolution of SPOT-5 and the high spectral resolution of ASTER. ASTER has nine very near infrared and shortwave infrared bands to select from, as opposed to a single near infrared band for SPOT-5.
Moreover, the possibility to interpret on the screen and directly produce digital files significantly reduces the time needed for the actual map production and simplifies fieldwork planning. Normally, to produce such a map using the conventional 2D image and photo interpretation method would have taken three months or so. INGEMMET was able to complete the 1:25 000 sheet (see figure 2) in half the time, largely due to the increased accuracy obtained with the 3D interpretations of high spatial/spectral resolution satellite imagery.
Other presentations included one by the Instituto Nacional de Recursos Naturales (INRENA). The 3D interpretation of high-resolution (1-m IKONOS) satellite imagery, made possible with StereoSat, greatly facilitated their task of producing diverse thematic maps including those of land use, forest cover, geomorphological land units and others.
In a specific case, a color composite made of a multispectral SPOT-5 image and a synthetically derived stereoscopic10 RADARSAT-1 image pair helped detect and correct interpretation errors produced from earlier 2D work.
They found that topographic context, which can be easily appreciated using StereoSat techniques, significantly enhanced their satellite image interpretation work.
This joint Canada-Peru project, initiated in October of 2006, aimed to demonstrate the advantages of the StereoSat solution in the Peruvian context through the creation of a set of cartographic products. At that time, Peruvian officials considered their topographic database to be incomplete and/or outdated several agencies were looking for an economical means that could complement analog aerial photography-based methods.
Simultaneously, many scientists, including geologists and geomorphologists, were interested in the potential of EO satellite images, but were frustrated by the fact that they were limited to 2D interpretation. The project started with a series of training/work sessions (see figure 4) using data acquired over high-priority areas in urban and rural settings with varied topographic and climatic conditions. Stereoscopic optical (IKONOS, ASTER and SPOT-5) and radar (RADARSAT-1) images were made available to the Peruvian experts to produce topographic, thematic, and cadastral maps.
StereoSat had previously been successfully used by the Quebec Ministry of Natural Resources and Fauna for 1:100 000 topographic mapping in sparsely inhabited regions between the 51st and 53rd parallels as well as for forest mapping in the North. It had also been tried in sub-Saharan Africa as a tool to provide up-to-date information for water management.
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About the author
Mr. François Riendeau has 14 years of experience in using satellite images for mapping applications. A project supervisor and GIS specialist in the Earth Observation Department at VIASAT GeoTechnologies, he holds a Diploma of Collegiate Studies in Geomatics and a Post-Graduate Diploma in GIS. He has been promoting satellite photogrammetry as a new approach for extracting geographic information in several countries in Africa and South America.
1Instituto Geográfico Nacional (IGN)
2Instituto Geológico, Minero y Metalúrgico (INGEMMET)
3Instituto Nacional de Recursos Naturales (INRENA)
4Ministerio de Agricultura (MINAG)
5Universidad Nacional Agraria La Molina (UNALM)
6Ministerio de Vivienda, Construcción y Saneamiento (MVCS)
7Servicio de Agua Potable y Alcantarillado de Lima (SEDAPAL)
8Comisión para la Formalización de la Propiedad Informal (COFOPRI)
9This project was equally financed by the Canadian Space Agency through its Earth Observation Applications Development Program, the Quebec Ministry of Economic Development, Innovation and Export, and VIASAT GeoTechnologies, a member of GéoQuébec, a not-for-profit organization composed of 11 private companies and 8 educational institutions working with the provincial Government to promote Quebec expertise in geomatics and land governance.
10Synthetic stereoscopy is a technique that allows for the production of an artificial stereoscopic image pair from a single orto-image and an existing digital elevation model.