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Lecture 4 Content

Electromagnetic wave

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Electromagnetic waveVisible light, radio waves, heat, ultraviolet, and X-rays are all forms of electromagnetic energy

All forms of electromagnetic energy travels at the speed of light which is 3 x 108 m/sec

The type of energy being detected is characterized by its position in the electromagnetic spectrum

The spectrum is commonly divided into units of wavelengths measured in micrometers (μm) which is one millionth of a meter

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Measuring LightMeasuring LightLight can be classified according to the Light can be classified according to the length of the wavelength of the wave

Gamma Rays Ultraviolet Infrared MicrowaveX-Ray TV/Radio

Wavelength

The Electromagnetic Spectrum (EMS)

Visible

˜ 0.4micrometer

s

˜ 0.7

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Measuring Light: BandsMeasuring Light: Bands

Human eyes only ‘measure’ visible lightHuman eyes only ‘measure’ visible lightSensors can measure other portions of EMSSensors can measure other portions of EMS

Bands

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– Electromagnetic Spectrum

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– Electromagnetic spectrum characteristicsVisible portion:

– Region to which the human eye is sensitive– Extremely narrow range of wavelengths– Wavelength: 0.4 μm to 0.7 μm ( the blue and red

regions)– The ultra-violet region has the shorter wavelengths

next to the visible blue region

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Infra-red portion

– Cannot be seen directly with the human eye– Wavelength: 0.7 μm to 0.8 μm – Longer than the red rays in the visible portion– Useful for penetrating hazy weather conditions

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Thermal infra-red portion

– Cannot be detected by the human eye nor by photographic film

– Wavelength: 0.8 μm to 14 μm– Electronic detectors cooled by liquid nitrogen are

used to measure the energy emitted at these wavelengths

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Microwave portion

– Very low intensity– Wavelength: 1mm to 1m – Useful for penetrating hazy weather conditions

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– Emission of energy from a blackbodyArea under the curve represents the total amount of energy emittedSun’s highest energy levels are emitted at the wavelength 0.5 μm

– Atmospheric effectsAtmospheric windows are the regions of the spectrum that are used for remote sensingThe systems operating in wavelengths longer that the visible range makes use of such windows for which the atmosphere has relatively high transparency

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Sensors collect 2D images of the surface Sensors collect 2D images of the surface in a in a swath swath below the sensorbelow the sensor

Example: Landsat has a 185 km swathExample: Landsat has a 185 km swath

SwathSwath

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Two types of imagesTwo types of images

REMOTELY SENSED imagesREMOTELY SENSED images Continuous dataContinuous data

• THEMATIC Images Discrete data

• Measured Values (light)• Quantitative

• Values only indicate class• Qualitative

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Continuous dataContinuous data

• Two types:

•Panchromatic ( 1 Band/layer)

•Multispectral ( 2 or more Bands)

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Viewing continuous imagesViewing continuous images

Each band or layer is viewable as a separate imageEach band or layer is viewable as a separate imageThematic Mapper Band 1

Band 4Band 5

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Blue

Green

Red

NIR

SWIR

Part of spectru

m

Monitorcolor guns

Viewing imagesViewing images

Three bands are viewable simultaneouslyThree bands are viewable simultaneously

Band4

Band3

Band2

Band4

Band5

Band3

Band1

Band2

Band3

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Band CombinationsBand Combinations

Features can become more obviousFeatures can become more obvious

Vegetation

Urban

2,3,1 (RGB)4,3,2 (RGB)4,5,3 (RGB)

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Image Geometric CorrectionImage Geometric Correction

Raw imagery has no reference to the Raw imagery has no reference to the groundground

Location in image onlyDefined by raster grid only

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Correction is the process of giving an image Correction is the process of giving an image a Real World coordinate systema Real World coordinate system

116 41 52.20 W, 33 50 03.23 N(Latitude, Longitude)

Image Geometric CorrectionImage Geometric Correction

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Ground ControlGround Control

Raw images need ground controlRaw images need ground control

116 28 55.20 W, 33 00 22.43 N

116 47 27.20 W, 33 50 50.09 N

116 45 14.54 W, 33 43 05.65 N

116 28 07.20 W, 33 55 43.98 N

• Measured control points are used to correct entire image

Resampling Method

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Discrete data (Thematic images)Discrete data (Thematic images)

Deal with categorical information about an areaDeal with categorical information about an area

Used and created by analytical operationsUsed and created by analytical operations

Used to extract information from multispectral Used to extract information from multispectral imagesimages

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Viewing thematic imagesViewing thematic images

Thematic imagesThematic images

• Each pixel contains a class value

• Each class value is associated with a specific color

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Viewing thematic imagesViewing thematic images

• Each class can have associated attributes:

Name, Color, Area etc.

Exe…

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Vector CoveragesVector CoveragesDifferent means of representing featuresDifferent means of representing features

Raster Vector

Polygons

Lines

Points

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Vector CoveragesVector Coverages

Vectors can also have attributesVectors can also have attributes

Vectors colored according to attributes

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… The End …