Remote Sensing

January 20, 2006Geog 258: Maps and GIS

Outlines

• Principles of remote sensing• Three key aspects of resolution of

remotely sensed image• Aerial photo vs. satellite image• Classifying aerial photograph• Geometric distortion of aerial photograph• Classifying satellite image

Principles of remote sensing• What you see is the amount of electromagnetic energy

reflected by object; needs energy source, object, sensor• Electromagnetic energy can be divided into different spectral

bands (visible light, NIR, microwave) given its wavelength– Electromagnetic spectrum

• Electromagnetic energy interacts with object differently– Snow reflects most of energy (that’s why it looks bright)– Dry soil absorbs most of energy (that’s why it looks dark)

• How object interacts with electromagnetic energy is a function of wavelengths basis of image interpretation– Spectral reflectance signature

• Electromagnetic radiation interacts with atmosphere as well. Some radiations (in visible, NIR and microwave bands) pass through atmosphere relatively well– Atmospheric window

Electromagnetic spectrum

Spectral signature

Explain why water looks darkish blue; Explain why vegetation looks greenish; Explain why sand looks reddish yellow

Atmospheric window

•It shows how much electromagnetic radiation transmits atmosphere•In which bands, is % transmission high?•Advantage of using those bands? •Compare aerial photo and satellite image in terms of spectral bands

Optical sensorOptical sensor

Infrared sensorInfrared sensor Radar sensorRadar sensor

AAerial photographerial photograph

Different kinds of image

• Pancromatic image• True-color image• False-color image

Pancromatic image

• If airborne cameras use black/white film or satellite sensors use a single band, it produces pancromatic image (gray scale image)

Color composite• Color primaries: RGB (Red, Green, Blue)

• Many colors are formed by combining color primaries in various proportions– Look inside your inkjet color printer. Did you really buy 100

different color cartridges?

• Same principles apply to producing color images taken from airborne cameras or satellite sensors

True-color image• See Table 9.1 (p. 182)• How many bands are used for Landsat 4 and 5?• What’s the spectral range (and corresponding

colors) for band name 3, 2, 1?• Each band collects different images (See Figure

9.27 at p. 182)• To produce color-image, bands 321 are

assigned to RGB respectively• The color of resulting image resembles what

would be observed by human eyes• That’s why it’s called true-color image

False-color image• If three bands are arbitrarily (i.e. doesn’t have to be RGB

bands) assigned to RGB layers, it produces false-color image

SPOT XS1 (green band) SPOT XS2 (red band) SPOT XS3 (Near IR band)

R = XS3 (NIR band)G = XS2 (red band)B = XS1 (green band)

Why does vegetation appear red in this color composite?

Why does vegetation appear dark in green band in pancromatic image?

Why does vegetation appear bright in NIR band in pancromatic image?

Three key aspects of resolutions

• Spatial: pixel (grain) size; minimum distance which can be recorded– Aerial photo has of course higher resolution

• Temporal: how often is the image recorded– For satellite image, it can be regular (satellites are

orbiting the earth in regular time interval)• Spectral: how many bands are used

– Satellite sensors use different # bands; most of them less than 10 (e.g. TM uses 7 bands); sensors with # band > 10 are called hyperspectral (e.g. MODIS)

• Important in determining the fitness of use

Types of sensor systems• Passive

– Rely on external energy source– Optical sensor (reliant on solar energy)

• Active– Sensor itself sends out radiation (like flash camera)– Radar sensor (reliant on microwave)

• Which one can record the image of earth at night, and which one can’t?

• Another reason for the popularity of Radar imaging is the ability of microwave to penetrate atmosphere (e.g. clouds); see Figure 9.4 (p. 164)

• So what would be the advantage of Radar imaging compared to optical imaging?

Aerial photography vs. Satellite imaging(Airborne RS vs. Spaceborne RS)

• Which began earlier? (with the development of what?)• Which would have a higher spatial resolution? (in which

altitude?)• Are they analog or digital? (output format)• Which is more flexible? (can satellite be launched any

time?)• Which would provide geographic coverage in a

systematic manner?• Which is equipped with wider spectral bands? (does air

photo has thermal-infrared sensors?)• How would post-processing be different?

Classifying aerial photo

• Film emulsion – {black-and-white, true-color, color-infrared}

• Camera’s height – {high, mid, low-altitude}

• Camera’s vantage point – {vertical, oblique}

• When color-infrared films were invented, and for which purpose?

Geometric distortion of air photo• Aerial photo gives us perspective view (it distorts

geometry of geographic features)

• Transformation from central to parallel perspective results in planimetrically correct photo or orthophoto

Sources of geometric distortion

• Camera tilts

Scale is not uniform

Sources of geometric distortion

• Variations in terrain

Remember photo is flat (2D) and objects should be seen from the point of infinite height in the planimetrically correct photo.

In the aerial photo,

a’ is displaced outward to a

b’ is displaced inward to b

due to terrain variations

Classifying satellite image• Spatial resolution

– {high, low}• Types of energy source

– {passive, active}• Types of spectral bands

– {pancromatic, multispectral, hyperspectral}• Special purposes

– {earth-observing, meterological, thermal-infrared}

• From the list of earth observing satellite, http://www.ersc.wisc.edu/resources/EOSC.php, place the sensors in classification schemes above

Review questions• What is the difference between aerial

photo, orthophoto, and DOQs?• Discuss advantage and disadvantage of

aerial photo vs. satellite imagery• What is the advantage of using active

sensor systems?• What is the advantage of using spectral

bands beyond visible wavelength ranges (0.4-0.7 micron)?