Remote Sensing Remote Sensing requires the following: 1. Electromagnetic Energy Source 2....
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Transcript of Remote Sensing Remote Sensing requires the following: 1. Electromagnetic Energy Source 2....
Definition:
“Remote sensing is the science of acquiring information about the Earth’s surface withoutactually being in contact with it, by sensing andrecording reflected or emitted energy and processing, analyzing and applying that information”
Remote Sensing
Simplest Form of Remote Sensing: Aerial Photography
Cartographers take detailed measurements from aerial photos in the preparation of maps
Geographers interpret aerial photos to determine land-use, and environmental conditions
Aerial photographs are not mapsMaps: Directionally and geometrically accurateAerial photograps: Radial distortion
GIS’s can account for radial distortion
The Electromagnetic Spectrum
Electromagnetic radiation is energy propagated through space between electric and magnetic fields. The electromagnetic spectrum is the extent of that energy from cosmic rays, gamma-rays, X-rays, ultraviolet, visible, infrared and microwave energy
Electromagnetic waves can be classified by:FREQUENCY or WAVELENGTH
Velocity = Speed of light
Electromagnetic Radiation
Consists of electrical field(E) and magnetic field (M)
Travels at speed of light (C)
The shorter the wavelength,the higher the frequency
This is important forunderstanding informationobtained in remote sensing
UV are shortestwavelengths practicalfor remote sensing
We are blind to everything except this narrow band
Microwaves are longestwavelengths used inremote sensing
Remote Sensing requires the following:
1. Electromagnetic Energy Source2. Interaction with a Target3. Sensor to Record Energy4. Transmission, Reception and Processing5. Interpretation6. Application
Electromagnetic Energy SourceIlluminates or provides electromagnetic radiation to the target of interest
A - Source of ElectromagneticRadiation
B - Radiation comes into Contactwith Atmosphere
C - RadiationInteracts withTarget
D - Sensor Collects andRecords ElectromagneticRadiation
E - Recorded Energy Transmittedto Processing Station (for copy)
F - Processed ImageInterpreted toExtract Target Info
G - Application
Radiation Interacts with AtmosphereRadiation interacts with the atmosphere on the way to the target and as the energy travels from the target to the sensor
Interaction with a TargetRadiation interacts with target. The nature of this interaction is dependent on the wavelength of the radiation and the nature ofthe target
SensorA sensor (mounted on satellite/plane/helicopter) collects/records the electromagnetic radiation scattered or emitted by the target
Transmission and ProcessingRecorded energy is transmitted to a processing station to producean image saved in digital format (or hardcopy)
InterpretationVisual interpretation or digital (GIS) interpretation to extractfurther information about the target
ApplicationInformation applied to solve a problem
* Remote sensing is especially important forextracting information from harsh environmentsor difficult terrain
Transmission through the Atmosphere
Radiation emitted from Earth is of a much longer wavelength and is ofmuch lesser energy
Some wavelengths of E-M energy are absorbed and scatteredmore efficiently thanothers
H2O, CO2, and ozone have the strongest absorption spectra
TransmissionLight moves through a surfaceWavelength dependent (eg. leaves)
Atmospheric Windows
In the diagram below, peaks are windows, while troughsidentify wavelengths that are heavily absorbed
WINDOWS ABSORPTION
Scattering
Passive SensorsMeasure naturally-available energy(eg. thermal infrared radiationemitted from the Earth 24 hours per day, but solar reflected radiation only during solar day)
Active SensorsSensor emits radiation toward targetReflected radiation in emitted bandsare detected and measured(eg. microwaves emitted)
When electromagnetic energy strikes molecules or othertiny objects, one of three things happens:
1. REFLECTION2. ABSORPTION
Remote sensing is most concerned with REFLECTIONof radiation (or emission from the Earth).
3. TRANSMISSION
Reflectance
The ratio of the amount of electromagnetic radiation reflected from a surface to the amount originally strikinga surface
• Specular Reflection
Surface is smooth relative to incident wavelength, resulting in mirror-like reflection (reflected in singledirection)
May help or hinder remote sensing depending onwhere the sensor is situated
• Diffuse Reflection
Surface is rough relative to incident wavelength
Energy is scattered more or less evenly in all directions
Many natural surfaces exhibit a great deal of diffusereflection
Surface types yield distinct spectral responses
Examples
Water: Longer visible wavelengths absorbed more thanshorter visible wavelengths (blue-green)
Leaves: Chlorophyll strongly absorbs in blue and red,but reflects green (green colour results)Healthy leaves efficiently reflect near IR
Absorption vs. Reflection differ for differentwavelengths of electromagnetic radiation
Leaves Water
Spectral signatures
Degree to which an object reflects incident electromagneticenergy in different regions of the electromagnetic spectrum
Characteristic signatures can be obtained for specific landsurface classes
Multispectral sensors detect reflectance in more than oneband
Characteristic spectral responses of different surface types. Bands are thoseof the SPOT remote sensing satellite.
Visual Interpretation in Remote Sensing andAerial Photo Interpretation
The following visual elements are considered in identifying objects:
Tone (Hue or Colour)Brightness or colour of elements on an image. Nothing could be discerned without changes in brightness or colour.
ShapeForm, structure or general outline of the objects. Regular shapes usually indicate human presence and land-use.
Size Absolute size and size relative to background objects (in context ofscale of the image)
TextureSmoothness or roughnessArrangement and frequency of tonal variation (eg. A forest isrough while an asphalt or cement surface is smooth)
PatternSpatial arrangement of objects gives a clue to object character(eg. random pattern in forest area vs.
ShadowMay help determine relative height but may also hinderinterpretation obscuring objects within
Association and SiteRelationship between target and other objects: context of anobject may lead to its identificationUnderstanding of stratigraphy alters geomorphologicalinterpretation of landscape features
Images and Photographs
Representation in digital formatby subdividing image into equally-shaped areas called pixels
The ‘brightness’ of each area can be attributed a numericvalue or digital number
Information from narrow wavelengthranges can be storedin channels, also called bands
Often, data from multiple channels can berepresented as one of three primary colourswhich combine according to brightness.We are, thus, no longer blind to these ’s.
Orbits and Swaths
Geostationary orbits:Very high altitude satellites (approximately 36 000 km)Focus on the same area of the Earth at all timesContinual data collection over a specific areaEg. Weather and communications satellites
Near-polar orbitsSatellite travels northward on one side of the Earth and then southwards during the second half of its orbitIn sun-synchronous orbits, ascending path can be on a shadowed side with the descending path on the sunlit side. Passive sensorswould only record data during the descent.
SwathThe area imaged on the surface.Swaths vary from very small areas (helicopters and planes) to hundreds of kilometres (spaceborne satellites)Earth rotates: Satellite swath may cover new area with each passComplete coverage of Earth after one cycle of orbitsAreas at high latitude generally covered more frequently
Spatial ResolutionSize of the smallest possible feature that can be detectedInstantaneous Field of View (IFOV) is the angular cone of visibility of the sensor (See A at right)This, along with altitude (C), determines the area visible on the ground (B)
Examples of Remote Sensing SatellitesEach has multiple channels for specific purposes
1. Weather GOES (Geostationary Operational Environmental Satellite)NOAA AVHRR (Advanced Very High Resolution Radiometer)
2. Land Surface ObservationLandsat (NOAA)SPOT (Système Pour l’Observation de la Terre)IRS (Indian Remote Sensing)MEIS-II and CASI (Airborne Sensors)
3. Marine ObservationCZCS (Coastal Zone Colour Scanner)MOS (Marine Observation Satellite)SeaWiFS (Sea-viewing Wide Field of View Sensor)
Applications of Remote Sensing
There are many applications of remote sensing, most of which arerelated to Geography as a discipline
Agriculture: Crop type, condition and yield, soil characteristicsForestry: Type, health, biomass, burning, species, deforestationHydrology: Sea ice, navigation, oil spills, sea surface temperatureLand Use: Resource management, habitat protection, urban
sprawl, damage assessment, legal boundariesOceans: Currents, winds, waves, phytoplankton concentration,
temperature monitoring, navigation routing, trafficdensity, bathymetry, land-water interface delineation,coastal vegetation
Mapping: Digital Elevation Models (DEM’s), thematic mapping
AVHRR
Visible, NIR, Thermal
1.1 km Resolution - local area coverage (LAC)4 km Resolution - global area coverage (GAC)
Used for meteorological studiesVegetation pattern analysisGlobal modelingBroad spectral bands
LANDSAT Thematic Mapper
Sun-synchronous, near-polar orbit, imaging the same 185 km x 0.474 km ground swath every 16 days
Global coverage between 81 degrees north latitude and 81 degrees south latitude
Particularly useful in determining land use classes
Blue/Green, Green, Red, NIR, MIR, Thermal30 meter resolution 256 brightness values7 spectral bands
NDVI = (NIR - red) / (NIR + red)
Normalized Difference Vegetative Index (NDVI)
RADAR - Radio Detection and Ranging
Passive Microwave Sensors:Applications include meteorology (atmosphere profiles, water andozone content), hydrology (soil moisture) and oceanography (seaice, currents, oil slicks)
Active Microwave Sensors:RADAR - Sensor transmits a microwave (or radio) signal toward atarget and detects the backscattered portion of the signalStrength of backscattered signal discriminates between targetsTime delay between transmitted and reflected signals determines the distance to the target
Non-Imaging (eg. altimeters) or Imaging SensorsImaging Microwave Sensors include RADARSAT (Canada, 1995)RADARSAT, developed by the Canadian Space Agency, is the world’s first, operationally-oriented radar satellite system capable ofrapid delivery of large quantities of data
Image Processing
1. PreprocessingRadiometric and geometric corrections
2. Image EnhancementImproving contrast, and spatial filtering to enhance specificspatial patterns of interest
3. Image TransformationsCombined processing of multiple spectral bands for imageenhancement
4. Image Classification and AnalysisDigital identification and classification of pixels.Classification: Assigns each pixel to a particular class or theme based on desired statistical characteristics (supervised or unsupervised)
Before GIS:
Popularity of stack maps
Limitation:
Restricted to consistent scale, projection and coverage area
Advantage of Digital Overlay:
1. Faster
2. Scale, projection and coverage arealess problematic (Most applications consist of sources collected by different methods and at different scales)
3. Time and error associated with manual integration and redrafting eliminated
Raster or Vector Implementation
RasterImplementationof Overlay
Overview of Overlay Analysis
1. Three maps represented with a common grid
2. Binary maps converted with Boolean operators such as AND and OR
Eg. Suitability AnalysisAND = more than one condition must
occur simultaneouslyOR = identifies areas with either
condition met
Boolean logic: truth tables
Exclusionary approaches rely on boolean logic, where thevalue of the statement, "A is true AND B is true" isdetermined in a truth table indicated the individualpermutations of A and B:
AND B is true B is false
A is true T F
A is false F F
The value of the statement, "A is true OR B is true" underthe same circumstances:
OR B is true B is false
A is true T T
A is false T F
RECLASS OVERLAYBoolean Logicin Raster Overlay
It is often usefulto RECLASS your data before performing OVERLAY
Task: Given vegetation map and elevation map, isolate a vegetation type within a particular altitude range
Map 1: Vegetation Map (VEGMAP)
Map 2: Digital Elevation Model (DEM1)
This requires the use of the AND operation
STEP 1:
Our vegetation of interest is class 6.
Reclass to assign a value of 1 for all values from 6 to just less than 7. All other values are assigned a value of zero.
CLASS6
STEP 2:
Use the Digital Elevation Model to isolate the elevations between 1700 and 1800 m.a.s.l. In the same way, assign a value of 1 to all values from 1700 to just less than 1801 by using a reclassfunction.
DEM1718
Where do the two isolated characteristics coincide ?
Use OVERLAYMultiply file CLASS6 by DEM1718 to produce the output map RESULT.
Only pixels with a value of 1 survive to be represented in the output file.
RESULT
OVERLAY is often used in combination with other operations such as near-neighbour operations
Eg. Produce a map of riparian vegetation cover within 100 metres of rivers and streams
Locate buffer zone 100m from rivers
Overlay with vegetation map
Produce resultantmap of riparian cover
Vector Implementation of Overlay
Produces many new polygons due to overlappingEach new polygon has a unique, new identifierThe identifier is linked to an attribute tableResult is a single layer coverage linked to all attributes