uni'wissen 01(3)-2011_ENG

them. By measuring this radiation, the scientists can draw inferences on the state of the objects. Passive sensors use sunlight reflected from the surface of the earth or the objects. Aerial images created with film and digital cameras are the best example of this method. However, most passive systems only work during the day. The production of night images with a passive sensor requires the use of thermal scanners that mea- sure infrared radiation. Laser Beams from an Airplane Forest scientist Barbara Koch and her team, on the other hand, are specialized in a form of active remote sensing: airborne laser scanning. With this method, devices send pulses of light from an airplane to the earth’s surface and measure how long it takes for the reflected beam to return to the airplane. This interval, the so-called time of flight, is then converted into distance. Measuring systems inside the air- plane calculate its position in space and use the time of flight to locate the precise position of the reflection in a system of spatial coordinates. Thus, individual spatial information is collected for each pulse of light. Since the laser systems send out up to 400,000 signals per second, the researchers receive spatial information on up to ten points per square meter enabling them to produce a relatively precise image of the earth’s surface with its buildings and vegetation. “The advantage of this method is that the laser can be used day and night,” says Koch. The only drawback is that it is not weather-proof like a radar, which can break through the clouds with its long radio waves. “I can’t see through a thick layer of clouds with a laser.” How airborne laser scanning works: Instruments send pulses of light (yellow) from an airplane to the earth’s surface and measure how long it takes for the reflected beam (red) to return to the air­ plane. The data can be used to produce a relatively precise image of the earth’s surface with its buildings and vegetation. Diagram: Klaas The scientists can use this method to quickly gain comprehensive information on areas which would be too time-consuming and expensive to explore from the ground. This explains why air- borne laser scanning has become an increas- ingly popular method for taking inventory of for- ests, since the images capture data that is of great importance for forest science. The data is used primarily to make topographical images of the earth’s surface. If the laser pulse sent out from the airplane reaches the ground there is only a single reflection, but if it hits vegetation it sends back many echoes: from the treetops, from the branches, from the trunk, and from the ground. Due to holes in the forest canopy, the ground echoes are usually still so numerous that it is possible to collect detailed information on the form of the terrain under the trees, including trenches and gorges. However, extracting information from the im- ages is no simple matter. Barbara Koch illus- trates this by presenting an image with many red dots divided into two clouds. “The dots hanging in the air are reflections from trees, the others are from the ground,” she explains. “On an aerial photograph I can always locate forests, streets, and paths intuitively. With a laser image I don’t see much at first glance.” To interpret this data, Koch and her team developed algorithms that classify the clouds of dots automatically. This saves time and money. In this way, the system can provide data on several layers of vegetation at once, allowing the scientists to make infer- ences on the biodiversity of the area. “Forest stands consisting of several levels will generally accommodate more plant and animal species than single-level stands.” It is not yet possible to 1717