Monday, October 5, 2015

Remotely sensed data

GIS and Remote Sensing are linked by remotely sensed data, mostly in the form of aerial or satellite images of a specified piece of land are used as input into GIS for manipulation and analysis. This in turn is presented to policy makers for the formulation of policies in the context of development or conservation of resources.

For many applications, remote sensing can be used effectively and efficiently to update GIS data layers. These updated layers in GIS can be used to improve the interpretability and information extraction potential of remotely sensed data.

GIS users require timely input data to optimise their systems for analysis and decision making. Usually, raster data is preferred. GIS users require multiple multiple images from different regions of the electromagnetic spectrum and/or different dates at scales ranging from local to global. Moreover, the data might have a variety of spatial, spectral and temporal resolutions.

Maps are compiled using photogrammetric techniques to process remotely-sensed data and these maps form the base upon which GIS applications are achieved. Remotely sensed data are also used to measure several environmental parameters like: surface and cloud top reflectances, albedo, soil and snow water content, fraction of photosynthetically active radiation, areas and potential yield of crop types, height and density of forest stands, etc. Such data mapped and/or monitored over time form the basis for monitoring.

Environmental planners, resource managers and public-policy decision makers are employing remotely-sensed data within the context of GIS to improve the management of resources.

Remote sensing data are acquired by aerial camera systems and a variety of active and passive remote  sensor systems operating at wavelengths throughout the electromagnetic spectrum. Data acquired by aerial camera systems can be scanned, converted into digital format and input into GIS.

Most satellite scanners are typically electro-mechanical scanners, linear devices or imaging spectrometers that operate in either a 'sweep' (LANDSAT) or 'pushbroom' (SPOT) mode. These are passive systems that record solar radiation reflected from Earth's surface.

Data derived from multi-spectral scanners can provide information on, vegetation types, its distribution and condition, geomorphology, soils, surface waters and river networks.

Short Wave Infra-Red (SWIR) sensors record emitted energy from surfaces and have been particularly useful for monitoring fires and studying areas of geothermal and volcanic activity. Thermal or Long Wave Infra Red (LWIR) sensors are used for mapping ocean temperatures and study of the dynamics of ocean waters and currents. Thermal maps are used to monitor urban areas, industrial sites, manufacturing centers and agricultural fields.

Active systems operating in the visible spectrum use laser technologies (Ex: LIght Detection And Ranging Systems (LIDARS)) mainly for oceanographic and forestry applications. Regardless of the wavelengths they use, active systems DO NOT depend on the sun for image capture.

The following is a list of satellite based sensors currently providing operational raster remotely sensed data for GIS developers and users:

  1. LANDSAT programme: -
      1. It has provided coverage of Earth for almost 25 years
        1. It is the result of NASA Earth Resources Survey Program and several other U.S. government agencies
        2. It was originally known as Earth Resources Technology Satellite in 1972.
        3. Four additional satellites have been placed in orbit since 1972 for providing continuous data for use in a wide range of environmental applications
        4. The first three LANDSATs had a Multi Spectral Scanner (MSS) as the primary sensor while the next two had a high resolution scanner called the Thematic Mapper (TM)
        5. MSS had a spatial resolution of 80m and images in the visible and near-infra Red region while the TM had a resolution of 30m and images in the visible and thermal infra-Red band.
        6. The LANDSAT programme established the operational viability of space-based remotely sensed data
    1. Satellite Pour I'Observation de la Terre:-
        1. SPOT is an operational, commercial remote sensing programme that operates on an international scale. 
        2. SPOT satellites are owned and operated by a french space agency, Centre National d'Etudes Spatiales (CNES). 
        3. Three SPOT satellites have been placed into orbit since 1986.
        4. The mission objectives for SPOT are:
          1. providing remotely-sensed data suited for land cover, agriculture, forestry, geology, regional planning and cartography applications.
        5. Data from the High Resolution Visible sensor (HRV) provide both multispectral coverage with 20m spatial resolution and panchromatic imagery with 10m resolution. This data is particularly well suited for urban and cartographic applications.
    2. Advanced Very High Resolution Radiometer:-
        1. The AVHRR sensor is carried aboard the USNOAA's (United States National Oceanic and Atmospheric Administration's) Polar Orbiting Environmental Satellites (POES)
        2. This program was established to provide data for use in meteorological applications.
        3. The daily coverage provided by AVHRR has resulted in the data being used for many operational land mapping and monitoring programs
        4. AVHRR data are multispectral and the data have a resolution of 1.1km at nadir and an orbital swath of 2600 km.
    3. Marine Observation Satellite
    4. Japanese Earth Resources Satellite
    5. India Remote Sensing Satellite
    6. European Resource Satellite
    8. High Spatial Resolution Satellites