Weather reports are one familiar application of this information-gathering technique. The satellite images, as well as the actual predictions, are obtained through remote sensing of the Earth. The satellites don't gather the information themselves; they simply orbit the Earth and provide platforms from which the sensors can observe large areas of the surface.  Airplanes also provide platforms for remote sensing, and some sensors operate from land.

Remote sensing satellites are launched by government agencies such as NASA (National Aeronautics and Space Administration), and are usually equipped with sensors that serve a particular purpose. For example, the National Oceanic and Atmospheric Administration (NOAA)   maintains the Geostationary Operational Environmental Satellite (GOES), which is a platform for monitoring weather with sensors that collect images of clouds.

Many types of sensors collect electromagnetic information. Microwave radiometers, laser meters, magnetic sensors and cameras all gather different types of data that can be interpreted to derive accurate, large-scale information about the Earth's surface and atmosphere.¹ Because these data and images are digital, they can easily be quantified and manipulated using computers. This makes remote sensing a uniquely versatile tool, since the same data can be analyzed in different ways for different applications. Some of the fields that use remote sensing are agriculture, geology, archaeology, oceanography, and even architecture.

In this Discovery Guide we will discuss remote sensing through the field of forestry. Forests are an important global resource that human populations depend on for wood, air quality, recreation and many other uses. They also serve as habitats for millions of plant and animal species. Both commercial and non-commercial forestry utilize a particularly diverse range of remote sensing applications. Traditionally, obtaining information about forests was achieved by sending a team of scientists into forest to physically sample small areas of that forest.

The scientists then had to extrapolate the data and apply the findings to the entire forest.Using remote sensing, foresters can get more accurate and cost-effective information, and can directly observe as large an area as necessary. Due to the versatility and scale of remote sensing, it is invaluable in all stages of forest management. The forester's task begins with growing healthy forests. Remote sensing is a useful tool for assessment of environmental conditions, either in an existing forest or prior to planting a new one.

Data for climate analysis can be obtained through microwave or radar sensing, which is ideal for gathering information about the atmosphere, because, unlike photographic or video methods, it allows continuous observation regardless of light or weather conditions. This provides an uninterrupted flow of accurate data for interpretation, and long-term observations can be compiled to monitor climate changes.

These data can be used to track rainfall and winds, as well as measure water and ozone content in the air. Analysis of the data can help determine the compatibility of the climate with forest growth, and predict potential problems.

Hydrology is another factor that is critical to forest growth.  Landsat Thematic Mapper (TM) surface hydrology Scanning Multichannel Microwave Radiometer (SMMR)² data provide information about soil moisture, the central component of forest hydrology. Analysis of these data allows for tracking of other factors such as evaporation and runoff. Quantitative analysis of terrain, shown by aerial photography images, is another source of data for hydrological modeling. These analyses enable foresters to determine water availability and to predict droughts and flooding.

The composition and viability of a forest may be determined using a combination of remote sensing and Geographic Information Systems (GIS).
GIS is a decision-making tool based on geographically referenced information. GIS uses different levels of geographical information, such as elevation, hydrology, or location of roads and infrastructure, to create a multi-layered representation of a site.  Some remote sensing methods that are used in combination with GIS are aerial videography and Thematic Mapper sensing. These data are available for large areas and can be interpreted to provide information on forest age, tree species distribution, and even estimated timber volume.

Forests are often at risk of being destroyed by forest fires. Remote sensing can be used in efforts to reduce the risk and minimize damage if a fire occurs. Weather information, such as measurements of precipitation and temperature, allows foresters to calculate risk assessments and isolate the areas most susceptible to fire. Those areas can be closely monitored by satellites, such as high resolution Advanced Very High Resolution Radiometer (AVHRR)³ and Satellite Pour l'Observation de la Terra (SPOT).⁴ Images from these satellites are readily available and small fires show up on them almost immediately.

Remote sensing contributes to fire-fighting efforts, as well. Data on wind direction and speed, and the dryness of surrounding areas can help predict the directions and speed at which a fire spreads. With this information, firefighters can be dispatched with maximum effectiveness and safety, and fires can be put out before they cause much damage.

Radar and thermal sensing allow for constant observation of fires, unaffected by clouds, smoke, or other conditions that hinder aerial observation.

After a fire, damage can be quickly and inexpensively assessed by using AVHRR or Landsat Thematic Mapper data. With accurate information on the area of the burn scar, amount of biomass destroyed and the amount of smoke and air pollution, forest managers can efficiently proceed with recovery and planning.

During every stage of forest management, foresters can use remote sensing data to estimate future urban spread and population growth. Then, forest management can be planned taking into account the future needs of settlements. Urban planning data can also be applied to the management of urban forestry, to create inventories of trees in parks and on streets.

The logical extension of commercial forestry is logging, and the nature of the industry requires long-term planning for cutting and regrowth. The accurate data from aerial photography and satellite images are used for planning and monitoring of these activities.

Before logging can take place, GIS assessments of forest ecosystems are performed to assess the impact on local wildlife species. This is another application of GIS, which usually uses SPOT or AVHRR satellite data to map regions where animal habitats are located. A remotely sensed tree species inventory can be used to identify rare or endangered plant species, as well as the habitats of animal species, based on the type of surrounding land cover.

Once the distribution of species is known, it can be incorporated into detailed and extensive maps, which are used to plan logging and regrowth. By using remote sensing data, foresters can make optimally informed decisions. They can be aware of the species distribution in a forest, the projected yields from logging, which areas contain habitats that cannot be disturbed, and how much land is needed for growth of settlements. After sections of forest are cut down, GIS and aerial photography techniques can be used to assess the speed and success of re-growth.

For ship route planning, Synthetic Aperture Radar (SAR) data is transmitted to ships in real-time. SAR systems provide long-range, high resolution images using extensive electronic processing of data, and can monitor the ocean surface and detect wave height and movement. Scatterometers, high frequency microwave radar sensors designed to sense ocean surface condition, are used to measure wind speed and direction at the surface. Combined data from these sensors provide reliable information on ocean activity and facilitate efficient route planning.

Ships also rely on radar weather predictions and sea ice detection for safe navigation.

Storms are exhaustively mapped using remotely sensed wave and wind information in combination with buoy data, and sea ice tracking is done using microwave sensors in combination with high-resolution satellite imagery such as AVHRR.

Ships themselves are tracked with radar to pinpoint their location and proximity to other ships. Using up-to-date remote sensing information, ships can travel via the most efficient routes, and can avoid hazardous conditions and collisions to transport timber without losses.

Finally, if an accident does take place during shipping, remote sensing can be used to minimize damage.

Rescue personnel use radar and aerial sensing to quickly locate a damaged ship. In the case of an oil spill, remote sensing information can be used to map the extent of the spill and track its spread by monitoring wave movement and wind speed.

By making shipping more effective, remote sensing aids the forestry industry at the end as well as the beginning of the timber cycle. From growing and monitoring healthy forests to transporting the resulting timber to its destination, remote sensing is a valuable tool for forestry. However, forestry is only one example of the vast number of uses of remote sensing. With the use of multiple sensors and varied data collection and interpretation techniques, remote sensing is a versatile tool that can provide data about the surface of the earth to suit to any need.

1. See NASA's Remote Sensing Tutorial for details on sensor technology.
2. More information regarding SMMR and the Nimbus satellite can be found at The Nimbus 7 Spacecraft System site.
3. Carried by NOAA's Polar Orbiting Satellites (POES), the AVHRR is a scanner that senses in the visible, near-infrared and thermal infrared portions of the electromagnetic spectrum.
4. The SPOT satellites, operated by the French Space Agency, Centre National d'Etudes Spatials, carry HRV (High Resolution Visible) sensors that are capable of producing images at a higher resolution than the AVHRR.
   
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