SourCeBook oN remoTe SeNSiNg aND BioDiverSiTy iNDiCaTorS

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Sourcebook on Remote Sensing and Biodiversity Indicators or helicopters, as well as on various satellite sensors orbiting the Earth. The airborne sensors are typically used to collect data on demand, when they are needed. Airborne data collection is most valuable for small areas, for specific events, to complement less detailed data, and for validation of data collected by satellite sensors over larger areas. Historical aerial photography archives can also be critical in change detection for periods before the widespread availability of satellite technology, which occurred in the mid-1970s. Remote sensing satellite systems for land cover assessment are operated by a growing number of countries, including Brazil, Canada, China, France, India, Japan, Russia, and the United States. Many satellites monitor the earth, with different sensors gathering different types of environmental data. The sensors acquire images of the earth and transmit them to ground receiving stations located throughout the world. Once these raw images are processed and analysed, indicators of biodiversity change can be assessed. 3.3 Spectral images Satellites view the planet with sensors that provide digital images of several discrete areas of the light spectrum. Human vision is limited to the visible wavelengths from blue to red, but satellite sensors are not so limited. Most satellite sensors also record images in wavelengths invisible to us, such as in the near-infrared and thermal regions (figure 3.1). Some can detect even longer wavelengths, such as in the microwave region. Images from these different wavelength areas or bands can be combined in different ways to produce false-colour images for human viewing, interpretation, and analysis (figure 3. ). Because the images are collected in a digital format, they can be analysed by computer. Just as different objects reflect different visible wavelengths, resulting in the colours the human eye can see, objects also reflect other non-visible wavelengths in different ways that are also important in classification. For example, geologists can distinguish rock and mineral types from space by using images taken in the middle-infrared, although most rocks and minerals look fairly similar in the visible wavelengths. The most commonly used data in land cover mapping are from optical sensors—instruments that detect short-wave solar energy reflected off the Earth’s surface. These sensors analyse three main regions of the spectrum, the visible, near-infrared, and middle-infrared. Most remote sensing of land cover types is dependent on the fact that in these main regions, reflectance differs significantly for different surfaces such as soil, leaves, wood, ash, water, and snow (figure 3.3). In vegetation, the canopy structure affects 4 �� FIguRe 3.1 Spectral chart. Source: Short, N.M. NASA remote sensing tutorial http://rst.gsfc.nasa.gov (accessed April 12, 2007).
A B Chapter 3. The Basics of Remote Sensing FIguRe 3.2 False colour composite. False colour composites of Ichkeul lake in Northern Tunisia A November 14, 2001 and A July 29, 2005. Vegetation on land and in water is indicated by shades of red as infrared reflectance is passed through a red colour filter within an image processor. Dam construction in the region has drastically reduced the inflow of freshwater to the lake causing an increase in salinity and a replacement of reed beds, sedges and other fresh-water plant species by halophytic (salt-loving) plants, thereby diminishing critical stopover habitat for migrating birds. Source: NASA/gSFC/MeTI/eRSDAC/ JAROS and u.S./Japan ASTeR Science Team. 5
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SourCeBook oN remoTe SeNSiNg aND BioDiverSiTy iNDiCaTorS

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