Basic Terms and Concepts
Remote sensing may be defined as the acquisition of information about an object or event on the basis of measurements taken at some distance from it. In practice the term is normally used to describe the collection and analysis of data made by instruments carried in or above the earth's atmosphere.
A sensor is a device which detects and measures a physical parameter, such as radiation, and converts it into a form which can be stored or transmitted. In other words, it is the device which “sees” the objects or terrain towards which it is pointed.
Electromagnetic radiation (EMR) is a type of energy which appears in such forms as X-rays, visible light, microwaves and radio waves. While these forms of EMR may initially seem to be separate phenomena, they are in fact part of a continuous spectrum. This can be understood best by considering how a prism separates white light into different colours; each colour represents a different wavelength of light. Visible light is the only segment of EMR which human vision can detect.
A given sensor can detect EMR only over a limited range of wavelengths: this range is referred to as a spectral band. The width of the spectral band, i.e. the extent of the limited range of wavelengths detected, is referred to as spectral resolution. Some sensors are comprised of a number of detectors, each of which is sensitive to a different spectral band. These are called multispectral or multiband sensors. By our looking at the earth in two or more bands simultaneously, it is possible to discriminate a wider range of features. The combination of typical responses coming from a specific target seen by a sensor in various spectral bands is called the spectral signature of that target.
Sensors may be classified according to a number of different criteria. For example, there are imaging and non-imaging sensors. As their name implies, imaging sensors produce a two-dimensional “picture” while non-imaging sensors produce point measurements or profiles. Sensors are also described as being either active or passive. Active sensors transmit radiation to “illuminate” the surface and to receive and measure the amount of radiation which is reflected back. Passive sensors, in contrast, measure naturally produced radiation which is either reflected solar energy or emitted terrestrial energy.
In order to provide a view of the earth's surface a sensor must be mounted on a platform which is simply the device or vehicle from which the sensor operates. Although stationary platforms, either attached or tethered to the ground, are sometimes used for specialized applications, aircraft and satellites are the most commonly used platforms for remote sensing. A general rule is that the higher the altitude of the platform, the larger the area that can be “seen” by the sensor; however, the ability to discriminate small objects will be reduced.
The level of spatial detail which can be observed or recorded by a sensor is referred to as its spatial resolution. For a given sensor/platform system, spatial resolution is usually described in terms of the smallest unit area which can be distinguished from its neighbours. In an imaging sensor system, the individual elements which make up the image are called pixels, a term derived from “picture elements”. The area on the earth's surface represented by a pixel normally corresponds to the spatial resolution of the sensor, i.e. the ground resolution cell size.
Data from sensors may be stored in analog or digital formats. In an analog system variations in the strength of the original input signal (e.g., the brightness variation in an image) are represented by continuous variations in some other medium such as voltage or film density. A digital representation, in contrast, divides the original signal into discrete ranges, each of which is assigned a numerical value. The range of the original signal as represented by a single numerical value is termed the radiometric resolution of the sensor system. Digitally recorded data, unlike analog data, can be processed easily by computers and can be copied repeatedly without negatively affecting the original or copied data. For human interpretation, however, an analog display such as a photograph or television picture is more useful. With appropriate equipment, it is possible to covert data from one format to the other.
A final concept which should be mentioned is the timeliness of the remotely sensed information. The term real time is used to describe data that is available for display or analysis at the same time and rate at which it is acquired. Most commonly, there is some delay between the time the sensor “observes” the surface and the time the data is available for use. If this delay is short, for example, a few hours, the data is said to be near real time. When the data has been collected considerably in advance of being analyzed, it is referred to as historical or archival data. Timeliness is a particularly important consideration for fisheries applications because of the dynamic nature of marine resources and ocean processes.
Fisheries Applications
Although direct detection of fish stocks would appear to be the most obvious goal for remote sensing, it is in fact the most difficult to achieve. Visual fish spotting from aircraft is used successfully for locating a number of pelagic species such as anchovy, swordfish, menhaden and tuna. In this case, a trained observer is the “sensor” and direct radio communication is maintained with vessels in the area. If a camera is also carried onboard the aircraft, photographs can be taken for subsequent stock assessment. Different species can be distinguished on the basis of their colour, behaviour and schooling patterns. Fish spotting is limited by the range of the aircraft and is only feasible when the probability of fish detection is reasonably high and the economic return derived from the catch justifies the expense of aerial surveillance.
A modified type of fish spotting makes use of the phenomenon of bioluminescence which is the emission of light by certain types of plankton when they are disturbed by the movement of fish. This phenomenon has been recognized by fishermen for centuries and is regularly used to locate fish when bioluminescent organisms are abundant. Sensitive low-light level television (LLLTV) systems equipped with image intensifier tubes can be used to amplify the relatively small amount of biologically produced light. Information derived from aircraft-mounted LLLTV systems can be used to direct vessels towards schools of fish. Attempts have also been made to image bioluminescence from an orbiting satellite while scanning the night side of the earth.
While the direct detection of fish is not always feasible, their indirect detection may be possible by observation of sea surface phenomena associated with species distribution. This may simply involve mapping the distribution of fishing activities within a given area. Changes in ocean colour from blue to green may also serve as an indicator of increasing plankton abundance. The green colour is associated with the presence of chlorophyll, the light retaining pigment of phytoplankton. While ocean colour has long been used locally by fishermen to locate fish species, aircraft and satellite imagery can record colour variations over a much wider area and in a more precise manner. Techniques have been developed to quantify biological productivity on the basis of chlorophyll distribution and abundance.
Water temperature is another important factor in determining species distribution and thermal sensors can be used to produce maps of the sea surface temperature (SST). Such mapping can be used to identify cold water upwellings of nutrient-rich water and to locate boundary areas between warm and cold waters where certain species are known to congregate.
In addition to resource detection, remote sensing can be valuable in characterizing the marine and coastal environments. This may involve such activities as updating navigational charts with coastline and bathymetric data; mapping the distribution and types of coastal wetlands; identifying marine plants and sediment types in the intertidal zone and in shallow waters; and monitoring the condition of coral reefs. While the above applications are related to relatively static or slowly changing conditions, remote sensing can also be used to observe more dynamic phenomena on a regular, repetitive basis. Examples in this category include turbidity patterns (due to both organic and inorganic materials), currents, freshwater and saltwater mixing, and wind and wave regions. Long term monitoring of these phenomena can provide a better understanding of the physical environment which supports biological activity and establishes a baseline against which divergent or unusual events can be measured.
Improved weather forecasting, aided in part by remote sensing, can mean greater safety for fishermen at sea. Pollution from coastal or offshore sources which can negatively affect fishing grounds can be monitored by remote sensing. The intensity and type of fishing activity also can be remotely sensed. This information can be used to determine the rate of resource exploitation and to assist in the enforcement of fishing regulations.
The examples cited above illustrate some of the remote sensing applications which may be of interest to fisheries personnel. It must be stressed, however, that remote sensing can seldom be used in isolation; it must be integrated with other sources of information. Geographical Information Systems are a valuable integrative tool.
