SUMMARY. Forest fires cause significant economic damage to the environment all over the world. Early warning and the fast extinction of fires are, apart from preventive measures, the only way to avoid major casualties and damage to nature, especially in densely populated regions. Different methods for early detection of fires have been developed in relation to local conditions and financial aspects. A common method is for trained staff to observe the endangered areas. In Germany, several hundred observation towers have been erected in the forests. The staff work up to 12 hours a day, usually under difficult circumstances. In countries with large forest areas aeroplanes are used to watch the forest areas. Satellite data can be applied only to keep large fires under control. Various technical systems for forest fire detection have been tested in recent years and these are described. The Automatic Early Warning System for Forest Fires (AWFS), a system developed and tested in Germany, is described. AWFS was installed on three observation towers in Brandenburg, Germany, and it was tested during the last two forest fire seasons. The testing forest authority confirmed that the system is mature for service and easy to use. AWFS also saves on costs. Further systems are about to be installed in Germany.
The probability of fires in forests and fields is steadily increasing owing to climate changes and human activities. In Europe, up to 10,000 km2 of vegetation are destroyed by fire every year, and up to 100,000 km2 in North America and Russia. According to the prognoses, forest fires - including fire clearing in tropical rain forests - will halve the world’s forest stand by the year 2030. Vegetation fires result in a high human death toll, speed up the extinction of species, and aggravate the greenhouse effect. Approximately 20% of CO2 emissions into the atmosphere are caused by forest fires. In several German states enormous economic damage is caused by forest fires. For instance, in Brandenburg alone, more than 1,000 forest fires are registered every year, of which 90% are caused by human activities. Large fires cause damage of up to 70,000 DM/ha. The annual financial loss caused by forest fires in Germany amounts to a two-digit number of millions. However, preventive and fire extinction measures cost several times this sum.
In order to minimize damage, forest fires must be spotted as soon as possible. Great efforts are therefore made in all regions to achieve early recognition. In principle, different kinds of sensors are suitable for the detection of fires. CCD-cameras find the smoke, infrared (IR) radiometers detect the heat flux from the fire, IR spectrometers identify the spectral characteristics of the smoke gases, and light detection and ranging (LIDAR) systems measure the laser light backscattered by the smoke particles.
Several platforms and instruments for surveillance activities have been tested in recent years. The individual method depends on the character of the area of interest, the financial budget, and specific regional conditions. In the USA, Canada, Russia, Finland, and other countries with large forest areas an early warning system based on aircraft patrolling with IR-cameras is used. However, this is expensive to operate if quick detection is mandatory. Multifunctional satellites such as NOAA and Landsat with CCD- and IR-cameras on board can observe large areas with moderate time and spatial resolution. In the future, the performance of a new class of specialized small fire satellites, such as the German BIRD,1 will be tested. A major intention of BIRD, due to be launched in 2001, is to demonstrate the scientific and technological value and the technical and programmatic feasibility of fire detection under low-budget constraints. A mini-satellite constellation is studied2 within the framework of the European project FUEGO. This will become operational in 2005, providing early fire outbreak detection and high resolution fire-line monitoring. The International Space Station ISS will be also equipped with special cameras to look for fires on earth (experiment FOCUS).
In time-critical local fire events in densely populated areas, ground-based fire watching is the first choice. In Germany several hundred observation towers are manned during the main forest-fire season. The fire-watchers observe the forests for up to 12 hours a day in the most difficult circumstances (extreme temperatures, poor hygienic conditions, isolation, only short breaks of concentration) and report on any smoke formation. Additionally, the authorities have to spend large sums on the construction of observation towers, which also need to be maintained and operated in accordance with relevant legislation and regulations.
Various technical systems have thus been developed to give autonomous alert in the event of fire. Most of these use CCD cameras or IR detectors. Although numerous technical methods have been tested, reliability standards have never been sufficient to develop a product that is suitable for the German market. Camera-based systems such as AWIS from the Netherlands,3 ArtisFire (France), and Firehawk (South Africa) are only in a test phase. Optical systems often imply a high rate of false alarm due to cloud, light reflection, agricultural activities, and industrial plants. IR sensor systems tested in Spain can only detect the fire itself. However, smoke is the relevant feature for the early recognition of fires in densely wooded areas.
The pilot project “Autonomous Early Warning System for Forest Fires” (AWFS) was ordered and supported by the forest authority of Brandenburg. It comprised the installation and testing of a system for the following tasks:
A solution for this complex undertaking was found by further developing know-how from unmanned space missions and consistently adapting it to the problem of forest fire recognition.
System concept
The system (Fig. 1) consists of a digital high resolution CCD-camera with a special filter mounted on a pan-and-tilt unit (PTU). This scans the landscape from the top of an observation tower, completing a full circle in about 8 min. for one full circle. AWFS can also be mounted on the braced poles of mobile phone providers, high buildings, or other suitable locations. The images are resolved with 14 bits and transmitted via optical fibres to the computer unit, which is also located in the tower. There the images are analysed by means of specially developed software. If there seems to be a smoke formation, compressed images and further details (time, position) are reported via ISDN to the control centre, where they are processed by a PC and displayed on screen. The operator receives all the information he needs to make decisions. Currently, one control centre can support up to seven towers. In each tower up to 10,000 digital images with a data volume of 16 Gbytes are produced and evaluated every day.
Hardware components installed in the observation tower
The CCD-camera was originally developed for space missions. Its innovative electronic concept realizes a radiometric resolution of 14 bits (~16,000 different grey scale values). The components are extremely resistant to environmental conditions and are remarkable for their low energy consumption and extremely low noise. Even very weak structures can be resolved under all sorts of lighting conditions. The 70-mm objective with 10° field of view allows 2 m geometric resolution at a distance of 10 km. Tests confirmed that the red-free filter increases the contrast between vegetation and smoke, as red light is poorly reflected by chlorophyll.
The pan-and-tilt unit can be positioned with a relative precision of up to 0.2° and with an absolute precision of 1° after being oriented in the landscape by means of GPS-defined land marks. During the scanning stage several single images are taken within a few seconds for every camera position. Full image information is then transmitted to a controlling PC at the tower base, where the data are evaluated, stored, and passed to an image-processing computer. Both computers work with the MS Windows NT operating system.
The controlling PC covers the following functions:
Data transmission to the PC in the control centre is currently achieved by a wired ISDN-connection. However, it is also possible to use radio transmission or other specific networks.
Image processing software
The image-processing computer uses complex algorithms to identify smoke in real time. It simultaneously calculates the optimum exposure time and sends it to the controlling PC. The image-processing software is the heart of AWFS. First of all, exact matching must be achieved for the images taken from the same camera position, because the towers tend to swing considerably in the wind. Next, the horizon line is determined in the image for orientation purposes. Within a few seconds the smoke is identified by means of dynamic and structural features and its grey scale value. In a first step, typical features of smoke are looked for by analysing a standard difference image. In a second step, the texture is evaluated.4 The respective method is based on a structural analysis of the texture of smoke, which can be clearly discerned from the surrounding structures.
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The control centre is equipped with a PC and appropriate software. Its task is to:
By means of the software developed for the control centre, operators who are unfamiliar with modern PC-technology can easily become acquainted with their scope of duties within a few days. With their knowledge of local conditions and the information provided by the system they are soon able to make qualified decisions regarding the initiation of fire fighting activity.
Practical operation is the only way to demonstrate the performance of an autonomous early-warning system for forest fires. A pilot test was therefore run during the 1999 and 2000 forest fire seasons. In addition to traditional fire watching methods, AWFS was installed on three observation towers in southern Brandenburg, which is the region with the highest forest fire risk in all Germany. With their emission of dust and smoke, the numerous open pits and power plants in this region made the task of fire watching especially difficult. The control centre was located on the premises of the local forest authority. The test results were evaluated monthly and reported in co-operation with the officers responsible.
Forty-five forest and field fires occurred in the test region of about 800 km2. All these fires were detected and indicated by AWFS within the prescribed time limit. Sometimes (especially in the late afternoon), fire indication was given even earlier than by experienced observation tower staff, who presumably were suffering from the effects of tiredness. Fig. 2 presents an example of automatic smoke detection.
False alarms are a specific problem. One must differentiate between alarms due to irrelevant smoke sources (e.g., chimneys) and false alarms proper. Irrelevant smoke sources are usually stationary objects. A facility was accordingly created to allow the operator to permanently mask such sources. To minimize false alarms a two-step decision process was established, with good results. In the first step, the complex software analyses the images. If it finds a suspect feature all relevant information is transmitted to the operator. Most days the software false-alarm rate is clearly less than 1%. A rate of 1% means approximately two false alarms per hour, which the operator can easily cope with. In certain weather conditions the number of false alarms increases owing to light reflection, ascending water vapour (after short but heavy rain showers), dust formation, or low clouds. In the second step, experienced staff evaluate the situations and take the final decisions, which showed a high reliability rate throughout the test phase.
All in all, AWFS offers the following advantages:
The test stage proved to be successful, and the German forest authority intends to introduce the system in several states. In order to be fully prepared for this next step the experience gained will be considered and evaluated over the next few months.
The focal points of future development will be as follows:
Due to its universal basic structure, AWFS can also be used in other areas. The concept of the system (digital camera with high spatial and radiometric resolution, wide range in brightness, real-time image processing, autonomous alarm signal transmission to control centre) and the experience gained so far in detecting complex structures in the natural environment are suitable for various observation tasks, e.g. environmental monitoring or security duties. The system can not only observe sensitive areas but also autonomously raise alarms, transfer data to any other place, and selectively store images.
RESUME. Les incendies de forêts provoquent des dommages économiques considérables à l’environnement de tout le monde. A part les mesures de prévention, l’avertissement précoce et l’extinction rapide sont la seule manière d’éviter les pertes massives et les dégâts à la nature, particulièrement dans les régions à forte densité de population. Il y a divers méthodes pour le répérage précoce des incendies qui ont été développés par rapport aux conditions locales et aux aspects économiques. Selon une méthode commune, des observateurs contrôlent les zones à risque. En Allemagne, plusieurs centaines de tours d’observation ont été érigées dans les forêts. Le personnel travaille jusqu’à 12 h par jour, souvent dans des conditions très difficiles. Dans les pays avec une grande extension de forêts ils emploient des avions pour contrôler les zones intéressées. Les données obtenues par les satellites peuvent être utilisées pour tenir sous contrôle seulement les grands incendies. Divers systèmes techniques pour la découverte des incendies de forêt ont été testés dans les années récentes, et les Auteurs de cet article les présentent. Le “Automatic Early Warning System for Forest Fires” (AWFS) (“Système automatique d’avertissement précoce pour les incendies de forêt), un système développé et testé en Allemagne, est décrit. Le système AWFS a été installé sur trois tours d’observation à Brandebourg, Allemagne, et a été testé pendant les deux dernières saisons d’incendies de forêt. L’administration responsable pour les épreuves pratiques dans les forêts a confirmé que le système est prêt à entrer en service et facile à utiliser. En outre le systême AWFS économise sur les coûts. En Allemagne on est en train d’installer des systèmes supplémentaires.