sensors for remote sensing

APPLICATION OF REMOTE SENSING AND

GEOGRAPHICAL INFORMATION SYSTEM IN

CIVIL ENGINEERING

Date:

INSTRUCTOR

DR. MOHSIN SIDDIQUE

ASSIST. PROFESSOR

DEPARTMENT OF CIVIL ENGINEERING

Optical Remote Sensing2

� Optical remote sensing makes use of visible, near infrared and short-wave infrared sensors to form images of the earth's surface by detecting the solar radiation reflected from targets on the ground

� Photography

� (Photogrammetry)

� Thermal Scanner

� Multispectral

Sensor for Remote Sensing3

Wave Length Band for Principal Sensor4

� We shall concentrate the discussion onoptical-mechanical-electronic radiometersand scanners, leaving the subjects ofcamera-film systems and active radar forSeminar assignment !!

� Major elements of Electro-optical Scanner

� Optical System: lenses, mirrors, apertures,modulators and dispersion devices

� Detectors: provides an electrical signalproportional to the irradiance on its activesurface, generally some type ofsemiconductors

� Signal processors: perform specifiedfunctions on the electrical signal to providethe desired output data

Sensor: Optical-Mechanical-Electrical Sensors

A typical electro-optical sensor design.

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� The two broadest classes of sensors are

� Passive (energy leading to radiation received comes from an externalsource, e.g., the Sun; the MSS is an example) and

� Active (energy generated from within the sensor system is beamedoutward, and the fraction returned is measured; radar is an example)


� Another attribute in this classification is whether the sensor operates in a

� non-scanning or

� a scanning mode.

� If the scene is sensed point by point (equivalent to small areas within thescene) along successive lines over a finite time, this mode of measurementmakes up a scanning system.

� And if the entire scene is sensed directly with the sensor then its terms as non-scanning system

� Most non-camera sensors operating from moving platforms image the sceneby scanning.

� For example, A film camera held rigidly in the hand is a non-scanning devicethat captures light almost instantaneously when the shutter is opened, thenclosed. But when the camera and/or the target moves, as with a moviecamera, it in a sense is performing scanning as such.


� Sensors can be

� Non-imaging (measures the radiation received from all points in the sensedtarget, integrates this, and reports the result as an electrical signal strengthor some other quantitative attribute, such as radiance) or

� Imaging (the electrons released are used to excite or ionize a substancelike silver (Ag) in film or to drive an image producing device like a TV orcomputer monitor or a cathode ray tube or oscilloscope or a battery ofelectronic detectors


� Radiometer is a general term forany instrument that quantitativelymeasures the EM radiation in someinterval of the EM spectrum.

� When the radiation is light from thenarrow spectral band including thevisible, the term photometer can besubstituted.

� If the sensor includes a component,such as a prism or diffractiongrating, that can break radiationextending over a part of thespectrum into discrete wavelengthsand disperse (or separate) them atdifferent angles to an array ofdetectors, it is called aspectrometer.


� The term spectro-radiometer isreserved for sensors that collect thedispersed radiation in bands ratherthan discrete wavelengths.

� Most air/space sensors arespectroradiometers.

� Sensors that instantaneously measureradiation coming from the entirescene at once are called framingsystems. The eye, a photo camera,and a TV vidicon belong to thisgroup.


� The optical setup for imaging sensors will be either an image plane oran object plane set up depending on where lens is before the photon rays areconverged (focused), as shown in the illustration.

Sensor for Remote Sensing

For the image plane

arrangement, the lens receives

parallel light rays after these

are deflected to it by the

scanner, with focusing at the

end.

For the object plane setup, the

rays are focused at the front

end (and have a virtual focal

point in back of the initial optical

train), and are intercepted by

the scanner before coming to a

full focus at a detector.

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� Two broad categories of most scanners are defined by the terms

� "optical-mechanical" and

� "optical-electronic",

� Both are distinguished by the former containing an essential mechanicalcomponent (e.g., a moving mirror) that participates in scanning the scene andby the latter having the sensed radiation move directly through the optics ontoa linear or two-dimensional array of detectors


optical-mechanicaloptical-

electronic

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� Another attribute of remote sensors, not shown in the classification, relates to the modes in which those that follow some forward-moving track (referred to as the orbit or flight path) gather their data.

� Cross-track scanners

� Along track scanners


In doing so, they are said to monitor the path over an area out to the sides of the

path; this is known as the swath width.

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� This is sometimes referred to as theWhiskbroom mode from the vision ofsweeping a table side to side by asmall handheld broom.

� The Cross Track mode normally usesa rotating (spinning) or oscillatingmirror (making the sensor an optical-mechanical device) to sweep thescene along a line traversing theground


� Also known as Pushbroom Scanners

� Sense a swath with an linear array ofCCD’s

� Because pushbroom scanners have nomechanical parts, their mechanicalreliability can be very high


� Spatial Resolution

� Spectral Resolution

� Radiometric Resolution

� Temporal Resolution

Sensor Resolutions

The ratio of distance on an image or map, to actual ground distance is

referred to as scale.

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radiometric

� Instantaneous Field of View (IFOV) isthe angular cone of visibility of thesensor (A) and determines the area onthe Earth's surface which is "seen" froma given altitude at one particularmoment in time (B)

Spatial Resolution17

� This is a measure of the area or size of the smallest dimensions on the earth’ssurface over which an independent measurement can be made by the sensor

� It is expressed by the size of the pixel on the ground in m

� A measure of size of pixel is given by the IFOV, which is dependent on thealtitude and the viewing angle of the sensor

� A narrow viewing angle or a lower altitude produces a small IFOV

� If a sensor has a spatial resolution of 20 metres and an image from that

sensor is displayed at full resolution, each pixel represents an area of

20m x 20m on the ground

� For a pushbroom system the number of detectors influences the spatialresolution

� A system with 1,000 detectors that images a 50 km wide swath has a pixelsize of 50m


� http://webhelp.esri.com/arcgiSDEsktop/9.3/index.cfm?TopicName=Cell_size_of_raster_data

Spatial Resolution

In this case, resolution refers to the pixel (cell) size (the area covered on the

ground and represented by a single cell).

A higher spatial resolution implies that there are more pixels per unit area;

therefore, the graphic on the left represents a higher spatial resolution than

the graphic on the right.

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Spatial Resolution versus Scale

The spatial resolution of the data

used in the image on the left is lower

than the spatial resolution of the data

used in the image on the right. This

means the cell size of the data in the

left image is larger than that of the

data in the right image; however, the

scale at which each is displayed is

the same.

The scale of the image on the

left (1:50,000) is smaller than

the scale of the image on the

right (1:2,500); however, the

spatial resolution (cell size) of

the data is the same.

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� The spectral resolution of a sensorcharacterizes the ability of the sensorto resolve the energy received in aspectral bandwidth to characterisedifferent constituents of earth surface

� Spectral resolution is defined as thespectral bandwidth of the filter and thesensitiveness of the detector

� Spectral resolution describes the abilityof a sensor to define fine wavelengthintervals.

� The finer the spectral resolution, thenarrower the wavelength range for aparticular channel or band

Spectral Resolution

Coarse

Fine

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� Many remote sensing systems record energy over several separatewavelength ranges at various spectral resolutions. These are referred to asmulti-spectral, superspectral, and hyperspectral sensors

Spectral Resolution

Panchromatic

Multispectral

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Multispectral and Hyperspectral Resolution

http://auracle.ca/news/

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� The radiometric resolution of an imaging system describes its ability todiscriminate very slight differences in energy. i.e., it is a measure of howmany grey levels are measured between pure black (no reflectance) to purewhite.

� It is measured in bits

Radiometric Resolution

Examples�1 bit (21) – 2 levels

�7 bits (27) – 128 levels IRS 1A & 1B

� 8 bits (28) – 256 levels Ladnsat TM

�11 bits (211) – 2048 levels NOAA –

AVHRR

� In a 8 bit system, black is measured

as 0 and white is measured as 255.

Imagery data are represented by positive Digital Numbers (DN) which vary from

0 to (one less than) a selected power of 2 according to bit system.

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Radiometric Resolution

0

1

27

1 bit (21)

8 bits (28) 8 bits (28)

� A remote sensing system with a radiometric resolution of 6 bits assigns adigital number (DN) of 28 to one surface and 47 to another. What would bethe equivalent DNs for the same surfaces if the measurements were taken witha 3 bit system?

� The DNs recorded by the 3 bit system range from 0 to 7 and this range isequivalent to 0-64 for the 6 bit system

� 0 1 2 3 4 5 6 7 (3 bit)

� 0 9 18 27 36 45 54 63 (6 bit)

� Therefore a DN of 28 and 47 on the 6 bit system will be recorded as 3 and5 on a 3 bit system.

Radiometric Resolution28

� Temporal resolution of a remote sensing system is a measure of how oftendata are obtained for the same area

� Applicable to satellite remote sensing only

� Temporal resolution varies from less than one hour to approximately 30 days.

� Importance of Temporal Resolution� Change in Land Use/ Land Cover

� Temporal Variation

� Monitoring of a Dynamic Event

� Cyclone

� Flood

� Volcano

� Earthquake

Temporal Resolution29

Comments….

Questions….

Suggestions….

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I am greatly thankful to all the information sources(regarding remote sensing and GIS) on internet that Iaccessed and utilized for the preparation of presentlecture.

Thank you !

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nonscanning system

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scanning mode

nonscanning devicethat

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remote sensing7 sensors

noncamera sensors

sceneby scanning