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All the processes mentioned here can develop into catastrophic events in the case, the population is not prepared. Often 
the impact of the event is underestimated, or the event develops more rapidly than anticipated. 
2.1 Erosion 
Erosion and sedimentation are two processes, which are generally everywhere present on the land surface. Wind and 
water remove the topmost surface and the material will be deposited elsewhere. Both processes can achieve catastrophic 
dimensions. Landslides strike without warning and lahars, a form of volcanic sedimentation, can bury within hours 
whole valleys under meters of sediments. 
Optical sensors, spaceborne or airborne, can be employed to map repeatable the area affected by erosion. The effects 
along coastlines can be very dramatic. Changes in coastline caused by erosion and sedimentation in the magnitude of 
several kilometres have been observed in satellite images in the vicinity of Jakarta, on the island of Java/Indonesia. 
Wind erosion in arid and semi-arid environments are well known and can be monitored using earth observation 
satellites. In the northern parts of France, Belgium, in Denmark and Germany, in the Midwest of the United States wind 
erosion has been found to be quite severe in places. Recently, much attention is drawn to landslides. Here again, we 
have to differentiate between several types of landslides. Large to medium sized landslides can be detected on SPOT, 
IRS and LANDSAT-TM images and in a combination of RADAR-images and Digitial Terrain Models (DTM). Smaller: 
ones can be easily identified on air-photos. The risk of soil creep, which can accelerate during heavy rainfalls are 
extremely difficult to assess. Mapping using airborne laser-scan technology is still expensive and needs to be flown 
repeatedly. The example of subtle terrain changes in the northern part of Germany, however, demonstrate the value of 
this method (KÜHN et al. 1999). Radar interferometry even from satellite orbits can give excellent results in the cm- 
range of displacement (MASSONET et al. 1999). However, vegetation cover, which is abundant in the moderate 
climates and especially in the humid tropics makes it impossible so far, to detect slight topographical changes. The use 
of permanent corner reflectors provides only movements of the particular reflector, but this can be done in the 
magnitude of cm as well. Longer radar wavelengths, however, might not be affected so much by vegetation cover, but 
can only be used for landslides with considerable dislocations. Among all the remote sensing data, optical data, 
especially air-photos can provide the largest amount of information. Combined with a DTM, a thorough field survey and 
further geologic and climatic data, the air-photo interpretation will definitely yield the best results. There has been 
observed a thermal anomalous behaviour prior to sliding recently, but research has not been carried out to explain the 
reasons behind it. 
2.2 Volcanoes 
Volcanoes are distributed around the world along plate boundaries. In smaller clusters they occur also within geological 
plates or continents, mainly along deep seated geological graben structures and fault systems. We differentiate between 
active volcanoes, dormant volcanoes and extinct volcanoes. This distinction alone bears already some risk: Pinatubo, the 
volcano that exploded in the Philippines in June 1991 was regarded as dormant. Its last eruption dated back to the 15 ® 
century. 500 years is a very short interval in the life of a volcano with an age of more than one million years. In this 
regard, we even have to regard our volcanoes in Germany as a potential danger, since some of which show slight 
degassing and are morphological very prominent. Their last eruptions took place only 11.000 years ago. 
There is a number of risks involved by dwelling in the vicinity of an active volcano. Not only the eruptive products, 
pyroclastic flows, tuffs, lava flows and lahars are threatening life and infrastructure, but also associated earthquakes, 
tsunamis and the silent death of CO , -eruptions as happened on Nyos-volcano in Cameroon in 1986. The huge methane 
content in Kivu-Lake at the Zaire-Burundi border is another constant threat to the population living at its shore (TIETZE 
1992). 
In the aftermath of volcanic eruptions the deposition of lahars can have catastrophic dimensions as could be seen on the 
flanks of Pinatubo in the Sto. Tomas basin and the Sacobia watershed, or on Mt. St. Helens. 
Traditionally, volcanoes are best monitored by using geophysical instruments, so-called seismometers, which record the 
seismic events very often associated with volcanic underground movements. Remote sensing methods are suitable to 
observe the surface processes of volcanic eruptions. These include also processes that precede a volcanic eruption: 
thermal remote sensing of volcanic lakes or fumarole fields, topographic changes like bulging and collapse of the 
volcanic edifice could be observed in optical and radar data, including interferometric data. Gas emissions are 
monitored from ground stations in order to detect changes in the gas composition, which could indicate a change in the 
behaviour of a volcano. New instruments on LANDSAT-7 and on the EO-1 polar platform to be launched at the end of 
this century by NASA and NASDA will provide thermal multispectral sensors with a foot-print of 60m X 60m on the 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 115