Remote sensing technique has emerged as an effective tool for systematic survey, analysis, and better management of natural resources (land, soil, water, forests, mountains) along with the monitoring of desertification, flood, drought, and landform change. It provides a vast scope to explore, identify, and analyze the natural resources of undeveloped regions. It documents the dynamic changes in physical processes and resulting landforms, usually by satellite images. This paper provides a general overview of remote sensing. While this technique has been used on beaches, valleys, and other landforms, the main concern of this paper is its role in geography.
Key Words: Remote Sensing, Geography, Application
Despite advances in geographical studies, the methods of traditional geography have become insufficient to apprehend its reality and complexity, considering technological and scientific changes that have happened in the last 30 years. However, this does not mean that these changes are not useful for geographic research. This has become evident now that Geographical Information Systems (GIS) are developing spatial studies to appeal to such technologies as remote sensing and computer sciences (MEC, 1999).
This paper focuses on a particular research tool for geographic research known as remote sensing. This benefits the study of geography in many ways, especially as a research tool, a tool for collecting high quality data, and a tool that aids in the reasoning process. It achieves these tasks by virtue of its spacial and temporal coverage (Rhoads, 2004; Doreen, 2009). Geographers increasingly use remotely sensed data to obtain information about the earth’s land surface, ocean, and atmosphere because it supplies objective information at a variety of spatial scales (local to global), provides a synoptic view of the area of interest, allows access to distant/inaccessible sites, provides spectral information outside the visible portion of the electromagnetic spectrum, and facilitates studies of how features change over time. This data may be analyzed independently or in conjunction with other digital data layers (e.g. in a GIS).
General Overview of Remote Sensing
Although this paper is mainly concerned with remote sensing used for geography, the field of remote sensing is very wide in data acquisition methods, data processing procedures, and various techniques and applications. Therefore, it is useful to provide a general overview about several important topics regarding remote sensing of the surface of the earth. The text also attempts to give the reader an understanding of the capabilities and limitations of remote sensing. Very few equations and formulas will be given in the text, as the focus will be on understanding the basic ideas.
Remote sensing is defined according to certain functions. It involves acquiring the information of an object’s property by a device not in contact with that object under study. This involves the utilization at a distance of any device for gathering information pertinent to the environment, such as measurements of force fields, electromagnetic radiation, or acoustic energy for aircraft, spacecrafts, or ships. The technique employs such devices as a cameras, lasers, radio frequency receivers, radar systems, sonars, seismographs, gravimeters, magnetometers, and scintillation counters. Some examples of remote sensing applications are given in the areas that have importance for the geographers. Due to the wide scope covered, the subjects could not be covered in detail and the interested reader should turn to the relevant literature (Lillesand & Kiefer, 2000; Sabins, 2007; Jenson, 2007; Longley et. al., 2005; Shukla & Pathak 2009).
As humans, we are intimately familiar with remote sensing in that we rely on visual perception to provide us with much of the information about our surroundings. As sensors, however, our eyes are greatly limited by sensitivity to only the visible range of electromagnetic energy, viewing perspectives dictated by the location of our bodies, and the inability to form a lasting record of what we view. Because of these limitations, humans have continuously sought to develop the technological means to increase our ability to record the physical properties of our environment.
Beginning with the early use of aerial photography, remote sensing has been recognized as a valuable tool for viewing, analyzing, characterizing, and making decisions about our environment. In the past few decades, remote sensing technology has advanced on three fronts: from predominantly military uses to a variety of environmental analysis applications that relate to land, ocean, and atmosphere issues; from analogue photographic systems to sensors that convert energy from many parts of the electromagnetic spectrum to electronic signals; and from aircraft to satellite platforms.
1.1 Modern Advantages of Remote Sensing
Remote sensing technology is becoming more important in geography due to attention being paid to the latest information, planning, and management for public and private interests. It is most useful for natural resource management, sustainable development, environmental degradation, and disaster management. Its satellite data are used as basic inputs for the inventory of natural resources and development processes like agriculture, soil, forestry, and geology (Chavez et al., 1977). There are other important technologies that are available to geographers as well, such as maps, aerial photography/photogrammetry/pictometry, SAR, LiDAR, SONAR, and GIS. The next section discusses the technologies mentioned above along with the similarities and differences between them and the field of remote sensing.
1.1.1 Maps: According to the International Cartographic Union, a map is “a conventionalised image representing selected features or characteristics of geographical reality, designed for use when spatial relationships are of primary importance.” This definition declares that in every map there is scientific accuracy and a process of selection present (symbolization, abstraction, generalization). However, the International Cartographic Union adds that “a map shows us the world as we know it,” and what we know is “a very complex subject that is comprised of: The limits of matter, technology, and our measurement tools; what we believe that exists; what we think to be important; and what we want and aspire to.” Thus, a map is subjective, for we always decide what to put on it and how to represent it. A remote sensing image, in contrast, is an objective recording of the electromagnetic reaching the sensor. Another important difference is that a map is a projection of the earth on paper without any relief displacements, while in a remote sensing image it is a projection of relief displacements and geometrical distortions.
1.1.2 Aerial Photography/Photogrammetry/Pictometry: These systems gather data about the upper surface of the earth by measuring the electromagnetic radiation from airborne systems. The major differences are detailed below:
Aerial photos are taken by an analogue instrument (the film of a photogrammetric camera), then scanned to be transformed to digital media. The advantage of a film is its high resolution (granularity), while the advantage of the CCD is that we measure quantitatively the radiation reaching the sensor (radiance values, instead of a gray-value scale bar). Thus, remote sensing data can be integrated into physical equations of energy-balance.
An aerial photograph is a central projection, with the whole picture taken at one instance. A remote sensing image is created line after line, so the geometrical correction is much more complex, with each pixel needing to be treated as a central projection.
Aerial photographs usually gather data only in the visible spectrum, while remote sensing sensors can be designed to measure radiation along the electromagnetic spectrum.
Pictometry is the name of a patented aerial image capture process of the Pictometry International Corp., USA. It produces imagery showing the fronts and sides of objects and locations on the ground. Images are captured by low-flying airplanes, depicting up to 12 oblique perspectives as well as an orthogonal view of every location flown. These perspectives can then be stitched together to create composite aerial maps that seamlessly cover large areas. Pictometry imagery can be overlaid with various shape files because every pixel is georeferenced to its exact location on the earth. This allows pictometry imagery to be integrated into many existing GIS software applications for use in many areas. Direct measurements can be made on pictometry imagery that includes area, distance, height, elevation, pitch, and bearing (http://www.pictometry.com).
1.1.3 SAR: Synthetic Aperture Radar (SAR) provides imagery during night or in bad weather as well as during the day. SAR images can be utilized for earth resource mapping and environmental monitoring, which require broad-area imaging at high resolutions. Synthetic aperture radar complements photographic and other optical imaging capabilities because of the minimum constraints on the time-of-day, atmospheric conditions, and unique responses of terrain/cultural targets to radar frequencies.
Synthetic aperture radar technology can provide terrain structural information to geologists for mineral exploration, oil spill boundaries on water to environmentalists, ice hazard maps to navigators, and reconnaissance-targeting information to military operations.
1.1.4 LiDAR: Light Detection and Ranging (LiDAR) is another technique that offers several advantages over the conventional methods of topographic data collection. This technique provides data with higher density, higher accuracy, less time for data processing, light independence, and minimum ground control points required. Due to these characteristics, LiDAR is complementing conventional techniques in some applications while completely replacing them in several others. Various applications where LiDAR data are being used are geomorphology, glacier studies, forest biomass mapping, and generation of the digital elevation model.
1.1.5 SONAR: The SONAR can also be considered as remote sensing by studying the surfaces of the sea (bathymetry and sea bed features) from a distance. The SONAR is an active type of remote sensing but with sound waves instead of electromagnetic radiation (like Radar, it does not depend on an external source of waves). Both systems transmit waves through an interfering medium (water, air) that adds noise to the data. For corrections, these must be applied to the raw data collected. In remote sensing, however, RADAR is considered to be almost weather-independent, and atmospheric disturbances affect mainly passive remote sensing. To make these necessary corrections, both systems depend on calibration from field data (be it salinity, temperature, pressure measured by the ship while surveying, or measurements of the atmospheric profile parameters by a meteorological radiosonde).
There are some notable differences between SONARs and RADARs. SONARs are mainly used to produce the bathymetry of the sea, while remote sensing techniques focus more on identification of the material’s properties than on its height.Echo-sounders (single or multi-beam) can be compared to Airborne Laser Scanning – both of them create point (vector) data containing X, Y, Z that need to be further processed in order to remove noise (spikes). An added complexity when dealing with bathymetry (as opposed to topography) is the need for tide corrections.
Another major difference is that in remote sensing the results of the analysis can be compared easily to the field (aerial photos, maps, field measurements), while in SONAR the underlying bottom of the sea is hidden from us, and we depend totally on the data gathered.
1.1.6 GIS: GIS is a combination of hardware and software that enables: The collection of spatial data from different sources (remote sensing being one of them). It relates spatial/tabular data, performs spacial/tabular analysis, and designs the layout of a map.
A GIS software can handle both vector and raster data. Remote sensing data belong to the raster type and usually require special data manipulation procedures that a regular GIS does not offer. However, after a remote sensing analysis has been done, its results are usually combined within a GIS or into a database of an area for further analysis (possibly overlaying with other layers). In the last few years, more and more vector capabilities have been added to remote sensing software, and some remote sensing functions are inserted into GIS modules.
General Remote Sensing Applications:
Each application itself has specific demands for spectral resolution, spatial resolution, and temporal resolution of the satellite sensor. There can be many applications for remote sensing in different fields. Some of them are described below.
Agriculture plays a dominant role in the economies of both developed and undeveloped countries. Satellite and airborne images are used as mapping tools to classify crops, examine their health, examine their viability, and monitor farming practices. Agricultural applications of remote sensing include crop type classification, crop condition assessment, crop yield estimation, mapping of soil characteristics, mapping of soil management practices, and compliance monitoring (farming practices).
Forests are a valuable resource for providing food, shelter, wildlife habitat, fuel, and daily supplies (such as medicinal ingredients and paper). Forests play an important role in balancing the earth’s CO2 supply and exchange, acting as a key link between the atmosphere, geosphere, and hydrosphere. Forestry applications of remote sensing include the following:
Reconnaissance mapping: Objectives to be met by national environment agencies include forest cover updating, depletion monitoring, and measuring biophysical properties of forest stands.
Commercial forestry: Of importance to commercial forestry companies and to resource management agencies are inventory and mapping applications. These include collecting harvest information, updating inventory information for timber supply, broad forest type, vegetation density, and biomass measurements.
Environmental monitoring: Conservation authorities are concerned with monitoring the quantity, health, and diversity of the earth’s forests.
Geology involves the study of landforms, structures, and the subsurface to understand physical processes that create and modify the earth’s crust. It is most commonly understood as the exploration and exploitation of mineral/hydrocarbon resources to improve the standard of living in society.
Geological applications of remote sensing include the following: Bedrock mapping, lithological mapping, € structural mapping, sand and gravel exploration/ exploitation, mineral exploration, hydrocarbon exploration, environmental geology, geobotany, baseline infrastructure, sedimentation monitoring, event/monitoring, geo-hazard mapping, and planetary mapping.
Hydrology is the study of water on the earth’s surface, whether flowing above ground, frozen in ice or snow, or retained by soil. Examples of hydrological applications include wetlands monitoring, soil moisture estimation, snow pack monitoring, measuring snow thickness, determining the snow-water equivalent, ice monitoring, flood monitoring, glacier dynamics monitoring (surges, ablation), € river/delta change detection, drainage basin mapping, watershed modelling, irrigation canal leakage detection, and irrigation scheduling.
1.2.5 Sea Ice:
Ice covers a substantial part of the earth’s surface and is a major factor in commercial fishing/shipping industries, Coast Guard operations, and global climate change studies. Examples of sea ice information and applications include ice concentration, ice type/age/motion, iceberg detection, surface topography€¬€ tactical identification of leads, navigation, safe shipping routes, ice condition, historical ice, iceberg conditions, dynamics for planning purposes, wildlife habitat, pollution monitoring, and meteorological change research.
1.2.6 Land Cover and Land Use:
Although the terms ‘land cover’ and ‘land uses’ are often used interchangeably, their actual meanings are quite distinct. Land cover refers to the surface cover on the ground, while land use refers to the purpose the land serves. The properties measured with remote sensing techniques relate to land cover from which land use can be inferred, particularly with ancillary data or a priori knowledge.
Land use applications of remote sensing include € natural resource management, wildlife habitat protection, baseline mapping for GIS input, urban expansion, logistics planning for seismic/exploration/resource extraction activities, damage delineation (tornadoes, flooding, volcanic, seismic, fire), legal boundaries for tax/property evaluation, target detection, and identification of landing strips, roads, clearings, bridges, and land/water interface.
Mapping constitutes an integral component of the process of managing land resources, with mapped information the common product of the analysis of remotely sensed data.
Mapping applications of remote sensing include the following:
·€ Planimetry: Land surveying techniques accompanied by the use of a GPS can be used to meet high accuracy requirements, but limitations include cost effectiveness and difficulties in attempting to map large or remote areas. Remote sensing provides a means of identifying planimetric data in an efficient manner, so imagery is available in varying scales to meet the requirements of many different users. Defence applications typify the scope of planimetry applications, such as extracting transportation route information, building/facilities locations, urban infrastructure, and general land cover.
·€ Digital elevation models (DEMs): Generating DEMs from remotely sensed data can be cost effective and efficient. A variety of sensors and methodologies to generate such models are available for mapping applications. Two primary methods of generating elevation data are stereogrammetry techniques using airphotos (photogrammetry), VIR imagery, radar data (radargrammetry), and radar interferometry.
·€ Baseline topographic mapping: As a base map, imagery provides ancillary information to the extracted planimetric detail. Sensitivity to surface expression makes radar a useful tool for creating base maps and providing reconnaissance abilities for hydrocarbon/mineralogical companies involved in exploration activities. This is particularly true in remote northern regions where vegetation cover does not mask the microtopography and where information may be sparse.
1.2.8 Oceans & Coastal Monitoring:
The oceans provide valuable food-biophysical resources, serve as transportation routes, are crucially important in weather system formation and CO2 storage, and are an important link in the earth’s hydrological balance. Coastlines are environmentally sensitive interfaces between the ocean and land, and they respond to changes brought about by economic development and changing land-use patterns. Often coastlines are also biologically diverse inter-tidal zones and can be highly urbanized. Ocean applications of remote sensing include the following:
·€ Ocean pattern identification:€ Currents, regional circulation patterns, shears, frontal zones, internal waves, gravity waves, eddies, upwelling zones, and shallow water bathymetry.
·€ Storm forecasting: Wind and wave retrieval.
·€ Fish stock and marine mammal assessment: Water temperature monitoring, water quality, ocean productivity, phytoplankton concentration, drift,€ € aquaculture inventory, and monitoring.
·€ Oil spill: Predicting the oil spill extent and drift, strategic support for oil spill emergency response decisions, and identification of natural oil seepage areas for exploration.
·€ Shipping:€ Navigation routing, traffic density studies, operational fisheries surveillance, and near-shore bathymetry mapping.
General Observations on Remote Sensing in Geography
Higgitt & Warburton (1999) have argued that remote sensing techniques provide fresh insights in geography in four main ways:
They provide new applications for geography.
They provide new and improved accuracy of measurement.
They provide new data that allow the investigation of ideas that were previously untestable.
They involve the development of data processing capability.
Application of Remote Sensing in Geography
Geographic applications of remotely sensed data typically take one of four explanatory forms:
Remote sensing images have specific uses within various fields of geographical study.
Remote sensing data possess advantages over conventional data and can provide multispectral, multidata, and multisensor information. This data is very useful in the agricultural fields for the crop type classification, crop condition assessment, crop yield estimation, and soil mapping.
In geology, remote sensing can be applied to analyze large, remote areas. Remote sensing interpretation also makes it easy for geologists to identify an area’s rock types, geomorphology, and changes from natural events such as a flood, erosion, or landslide.
The interpretation of remote sensing images allows physical- and biogeographers, ecologists, agricultural researchers, and foresters to easily detect what vegetation is present in certain areas, its growth potential, and sometimes what conditions are conducive to its being there.
Additionally, those studying urban land use applications are also concerned with remote sensing because it allows them to easily pick out which land uses are present in an area. This can then be used as data in city planning applications and in the study of species habitat.
Remote sensing data has proven to be an important tool in geography. Multi-temporal satellite data help to delineate the various change of the earth surface. Remote sensing has progressively expended applications in various fields such as urban-regional planning, utilities planning, health planning, geomorphology, and resource planning. Because of its varied applications and ability to allow users to collect, interpret, and manipulate data over dangerous areas, remote sensing has become a useful tool for all geographers, regardless of their concentration.
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