Definition of Remote Sensing
Remote sensing can be defined as the technique of obtaining information about objects through the analysis of data collected by instruments that are not in physical contact with the objects of investigation.
Remote vs In Situ Sensing
Remote sensing differs from in situ sensing in which the measurement instrument is in contact with the object being measured. For example, it is possible to obtain information about soil temperature using either in situ or remote sensing. In situ sensing could be conducted by pushing a thermometer into the soil and recording the soil temperature at selected sample points. Interpolation techniques could be applied to the sample point data to create a continuous soil temperature surface within the study area of interest. This data collection process is expensive, difficult to replicate - especially in remote study areas, and is subject to both measurement and interpolation errors.
Similar temperature information can be collected by remote sensing using optical-mechanical scanners that are sensitive to the thermal infrared portion of the electromagnetic spectrum. Infrared imagery has often been used to assess the energy efficiency of buildings. An infrared image taken in winter when the outside temperature is cold will clearly show where heat is being lost. Windows, doors or areas with poor insulation appear as bright spots on a thermal infrared image because they are at a higher temperature than the exterior surface of well insulated walls. Airborne or satellite sensors that detect infrared radiation could be used to obtain a soil temperature map without having to rely on interpolation between sample points. This method can be cheaper than in situ sensing, and in the case of satellite imagery, can be repeated to obtain multi-temporal images.
There are many types of remote sensors. The human eye and ear are passive remote sensors. Passive remote sensors do not emit energy but simply detect energy that is reflected or propogated back to the sensing device. The eye detects energy in the visible portion of the electromagnetic spectrum while the ear detects sounds sound energy. The photographic camera, used in daylight without flash, is a passive remote sensor as are airborne or spaceborne multispectral scanners such as Landsat MSS, Thematic Mapper or SPOT that have been widely used in environmental remote sensing. Active remote sensors emit energy and then detect the emitted energy that gets reflected back off an object. The photographic camera used with a flash, radar and laser systems used by police to catch speeders, sonar used for bathymetric mapping, and spaceborne radar systems are examples of active remote sensors. In the context of environmental remote sensing, passive remote sensing systems have been the most widely used but active remote sensing systems such as radar offer significant advantages in that they can penetrate clouds and so do not require clear sky conditions.
Stages in the Remote Sensing Process
The remote sensing process involves two stages: data collection and data analysis. Data collection is accomplished through the use of remote sensing devices that record data on photographic film or as digital data. The resulting data provide a synoptic view of a portion of the Earth's surface. but require analysis and interpretation in order to provide meaningful information. This can involve either visual interpretation, which has been the predominant mode of airphoto interpretation, or digital analysis, which has been widely used in processing satellite imagery. Remote sensing remains both an art and a science because there is often no obvious, well-documented choice of data collection devices or analysis and interpretation techniques that are best suited to a particular applied problem situation.
Electromagnetic Radiation (EMR)
Energy is the capacity to do work. Energy can take many forms such as light, heat or sound, and can be transmitted between objects through three processes: conduction, convection and radiation. Conduction requires that the objects be in direct physical contact. Energy is transferred from the high energy object to the low energy object until both objects are at the same energy level. For example, a hot water bottle warms a bed until the hot water bottle and the bed reach the same temperature, after which no further energy transfer occurs. Convection occurs in liquids and gases. Convection is based on currents that distribute energy throughout the volume of liquid or gas. Radiation does not require physical contact or the existence of a liquid or gas.
Environmental remote sensing systems focus on electromagetic energy which is a dynamic form of energy caused by the oscillation or acceleration of an electrical charge. All objects that have a temperature higher than absolute zero (0o K or -273.16o C) emit electromagnetic energy. The wavelength of the emitted energy is a function of temperature.
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Remote sensing can be defined as the technique of obtaining information about objects through the analysis of data collected by instruments that are not in physical contact with the objects of investigation.
Remote vs In Situ Sensing
Remote sensing differs from in situ sensing in which the measurement instrument is in contact with the object being measured. For example, it is possible to obtain information about soil temperature using either in situ or remote sensing. In situ sensing could be conducted by pushing a thermometer into the soil and recording the soil temperature at selected sample points. Interpolation techniques could be applied to the sample point data to create a continuous soil temperature surface within the study area of interest. This data collection process is expensive, difficult to replicate - especially in remote study areas, and is subject to both measurement and interpolation errors.
Similar temperature information can be collected by remote sensing using optical-mechanical scanners that are sensitive to the thermal infrared portion of the electromagnetic spectrum. Infrared imagery has often been used to assess the energy efficiency of buildings. An infrared image taken in winter when the outside temperature is cold will clearly show where heat is being lost. Windows, doors or areas with poor insulation appear as bright spots on a thermal infrared image because they are at a higher temperature than the exterior surface of well insulated walls. Airborne or satellite sensors that detect infrared radiation could be used to obtain a soil temperature map without having to rely on interpolation between sample points. This method can be cheaper than in situ sensing, and in the case of satellite imagery, can be repeated to obtain multi-temporal images.
There are many types of remote sensors. The human eye and ear are passive remote sensors. Passive remote sensors do not emit energy but simply detect energy that is reflected or propogated back to the sensing device. The eye detects energy in the visible portion of the electromagnetic spectrum while the ear detects sounds sound energy. The photographic camera, used in daylight without flash, is a passive remote sensor as are airborne or spaceborne multispectral scanners such as Landsat MSS, Thematic Mapper or SPOT that have been widely used in environmental remote sensing. Active remote sensors emit energy and then detect the emitted energy that gets reflected back off an object. The photographic camera used with a flash, radar and laser systems used by police to catch speeders, sonar used for bathymetric mapping, and spaceborne radar systems are examples of active remote sensors. In the context of environmental remote sensing, passive remote sensing systems have been the most widely used but active remote sensing systems such as radar offer significant advantages in that they can penetrate clouds and so do not require clear sky conditions.
Stages in the Remote Sensing Process
The remote sensing process involves two stages: data collection and data analysis. Data collection is accomplished through the use of remote sensing devices that record data on photographic film or as digital data. The resulting data provide a synoptic view of a portion of the Earth's surface. but require analysis and interpretation in order to provide meaningful information. This can involve either visual interpretation, which has been the predominant mode of airphoto interpretation, or digital analysis, which has been widely used in processing satellite imagery. Remote sensing remains both an art and a science because there is often no obvious, well-documented choice of data collection devices or analysis and interpretation techniques that are best suited to a particular applied problem situation.
Electromagnetic Radiation (EMR)
Energy is the capacity to do work. Energy can take many forms such as light, heat or sound, and can be transmitted between objects through three processes: conduction, convection and radiation. Conduction requires that the objects be in direct physical contact. Energy is transferred from the high energy object to the low energy object until both objects are at the same energy level. For example, a hot water bottle warms a bed until the hot water bottle and the bed reach the same temperature, after which no further energy transfer occurs. Convection occurs in liquids and gases. Convection is based on currents that distribute energy throughout the volume of liquid or gas. Radiation does not require physical contact or the existence of a liquid or gas.
Environmental remote sensing systems focus on electromagetic energy which is a dynamic form of energy caused by the oscillation or acceleration of an electrical charge. All objects that have a temperature higher than absolute zero (0o K or -273.16o C) emit electromagnetic energy. The wavelength of the emitted energy is a function of temperature.
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