Remote Sensing Remote Sensing requires the following: 1. Electromagnetic Energy Source 2. Interaction with a Target 3. Sensor to Record Energy.

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