APPLICATION OF REMOTE SENSING AND

GEOGRAPHICAL INFORMATION SYSTEM IN

CIVIL ENGINEERING

Date:

INSTRUCTOR

DR. MOHSIN SIDDIQUE

ASSIST. PROFESSOR

DEPARTMENT OF CIVIL ENGINEERING

Resolving Power2

� Resolving power is an index used to represent the limit of spatialobservation.

� In optics, the minimum detectable distance between two image points is calledresolving limit, and the reverse is defined as the resolving power.

� Resolving power is determined by the minimum separation of lines of aregular one-dimensional grating (test pattern) that can be registered in aphotographic image without a merging of the lines. Resolving power ismeasured by means of resolution testers and is usually expressed in mm-1, thatis, number of lines per mm

� There are several methods to measure the resolving limit or resolving power.Two such methods are introduced below.

� (1) Resolving power by refraction and

� (2) Modular transfer function (MTF),

Resolving Power3

For photographic materials currently in use, the resolving power usually lies

in the range 70–300 mm-1

� (1) Resolving limit by refraction:Theoretically an object point will beprojected as a point on an imageplane if the optical system has noaberration.

� However, because of diffraction theimage of a point will be a circle with aradius of about one wavelength oflight, which in called the Airy pattern,as shown in Figure.

� Therefore there exists a limit to resolvethe distance between two points eventhough there is no aberration.

Resolving power by refraction4

Airy Disk

1. Diffraction ring 2. Diffraction ring

Obviously the resolution limit or resolving power is connected to that Airy Disk,

because no detail imaged by the telescope can be smaller than this disk. The

radius of the Airy Disk can be estimated with:

q lin = 1.22 * ( f * λ ) / D (for linear resolving in cycles/mm)

q ang = 1.22 * λ / D (for angular resolving in arc sec)

� The resolving limit depends on how theminimum distance between two Airyimages is defined.

� a. Rayleigh's resolving

power: distance between the left Airypeak and the right Airy peak when itcoincides with the zero point of the leftpeak, that is 1.22u

� Rayleigh's resolution limit [arc sec] = 140/ Aperture Diameter [mm]

� b. Sparrow's resolving limit: thedistance between the two peaks when thecentral gap fades away, that is 1.08u.

� Sparrow's resolution limit [arc sec] = 70 /Aperture Diameter [mm]

Resolving power by refraction5

1/2 of the radius of the Airy

Disk

radius of the Airy

Disk

� Modulation Transfer Function is the most widely used scientific method ofdescribing imaging system (lens).

� The modulation transfer function is, as the name suggests, a measure of thetransfer of modulation (or contrast) from the subject to the image.

� MTF coincides with the power spectrum which is obtained by Fouriertransformation of a point image. Generally speaking, an optical imagingsystem will give a low pass filter as shown in Figure.

Modulation Transfer Function (MTF)6

Modulation Transfer Function (MTF)

[A] is the original test pattern

[B] is the image of the test pattern

[C] is the line profile of the original test pattern where 255=white and 0=black

[D] is the line profile of the image of the test pattern where 255=white and 0= black

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� The resolving power measured on a resolving test chart by human eyes, dependson individual ability and the shape or modulation (contrast) of the chart.

� On the other hand, MTF has no such problems because MTF comes from ascientific definition in which the response of spatial frequency, with respect to theamplitude, considers the optical imaging system as a spatial frequency filter.

� As the spatial frequency is defined as the frequency of a sine wave, MTF showshow much the ratio of the amplitude decreases before and after an opticalimaging system with respect to the spatial frequency as shown in Figure

Resolving Power and Modulation Transfer Function8

� An array of light arranged by order of wavelength is called a spectrum.

� An optical device used to disperse light into a spectrum is call dispersingelement.

� Spectroscopy is defined as the study of the dispersion of light into itsspectrum.

� There are two types of dispersing elements, the prism and the diffractiongrating. Figure shows the types of dispersing elements

Dispersing Elements9

Dispersing Elements

Schematic of a grating spectrometerPrism

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� The optical mechanism of prisms anddiffraction gratings are shownin Figures

Dispersing Elements11

� A prism designed for spectroscopy is called a dispersing prism, which is based on the theory that refractive index is different depending on the wavelength.

(1) Prism12

� The spectral resolution of a prism is much lower than that of a diffractiongrating.

� If higher spectral resolution is required, a layer prism should be produced.

� This can be a problem, because it is rather difficult to prepare homogeneousmaterial and to keep the weight low.

(1) Prism13

� A diffraction grating is a glass orpolished metal surface on which alarge number of very fine, parallelgrooves or slits are cut.

� These parallel elements are used todiffract light, generally bothseparating and spreading differentwavelengths or colors.

� Diffraction gratings aremanufactured on flat or curvedsurfaces according to their use

(2) Diffraction grating

� Diffraction gratings are used in spectrographic equipment(spectroscopes, spectrophotometers, monochromators, spectrographs,spectrometers, and so on) as the principal optical element to separatelight into its component wavelengths (frequencies) by diffracting them atvarious angles.

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(2) Diffraction grating15

(2) Diffraction grating

Ordinary pressed CD and DVD media are every-

day examples of diffraction gratings and can be

used to demonstrate the effect by reflecting

sunlight off them onto a white wall.

This is a side effect of their manufacture, as one

surface of a CD has many small pits in the

plastic, arranged in a spiral; that surface has a

thin layer of metal applied to make the pits more

visible.

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� Types of Diffraction Gratings

� There are two fundamental types of grating masters, ruled and holographic.

� Ruled gratings can be blazed for specific wave lengths and generally have high efficiency. These gratings are often used in systems requiring high resolution.

� Holographic gratings will often have lower scatter since they are generated optically.

� Each can be manufactured on a flat (plane) or concave substrate. Each type of grating has its own advantages.

(2) Diffraction grating17

� A filter can transmit or reflect a specified range of wavelength.

� A filter designed for spectroscopy is called a spectroscopic filter.

� Filters are classified into three types - long wave pass filters, short wave

pass filters and band pass filters from the viewpoint of function, as shownin Figure.

Spectroscopic Filter18

� (1) Absorption filter :a filter which absorbs a specific range of wavelengths, for examples, colored

filter glass and gelatin filter.

Spectroscopic Filter19

� (2) Interference filter:

� a filter which transmits a specific range ofwavelengths by utilizing the interference effectof a thin film.

� When light is incident on a thin film, only aspecific range of wavelengths will pass due tothe interference by multiple reflection in a thinfilm as shown in Figures.

� The higher the reflectance of the thin film, thenarrower the width of the spectral bandbecomes.

� If two of these films, with different refractiveindexes, are combined, the reflectancebecomes very high which results in a narrowspectral band, for example of the order ofseveral nanometers.

Spectroscopic Filter

In physics, interference is a phenomenon in which two waves superimpose to form a

resultant wave of greater or lower amplitude

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� In order to obtain a band pass filterwhich transmits a single spectralband, a short wave pass filter andlong wave pass filter should becombined.

� A dichroic mirror ,which is used forthree primary color separation, is akind of multiple layer interferencefilter, as shown in Figures.

� It utilizes both functions oftransparency and reflection.

Spectroscopic Filter21

� (3) Diffraction grating filter:

� a reflective long wave pass filter utilizing the diffraction effect of a grating,which reflects all light of wavelengths longer than the wavelength determinedby the grating interval and the oblique angle of the incident radiation.

� (4) Polarizing interference filter:

� a filter with birefringent crystallinity plate between two polarizing plates,which pass a very narrow spectral band, for example less than 0.1 mm.

� This utilizes the interference by two rays of light ; a light following Snell's lawand the other not following Snell's law, which pass a light with a narrow bandof wavelength determined by the thickness of the birefringent crystallinityplate .

Spectroscopic Filter22

Spectroscopic Filter

(a) Diagram of a polarizing interference filter: (P) polaroid filters, (Q) quartz

plates; (b) transmission of individual stages (1–6) and of entire filter (7).

Wavelengths are shown at bottom.

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� There are many kinds of spectral measurement devices ,forexample, spectroscopes for human eye observation of thespectrum, spectrometer to record spectral reflectance, monochrometer toread a single narrow band, spectrophotometer forphotometry, spectroradiometer for measurement of spectral radiation etc.

� Figure shows a classification of spectrometers, which are divided mainlyinto dispersing spectrometers and interference spectrometers. The formerutilizes prisms or diffraction gratings, while the latter the interference of light.

Spectrometer24

� (1) Dispersing spectrometer

� A spectrum is obtained at the focal plane after a light ray passes through aslit and dispersing element as shown in Figures are typical dispersingspectrometers ; Littnow spectrometer and Czerny - Turner spectrometerrespectively.

Spectrometer25

� (2) Twin beam interference spectrometer

� A distribution of the spectrum is obtained by cosine Fourier transformation of the interferogram which is produced by the inference between two split rays.

� Figure shows Michelson interferometer which utilizes a beam splitter.

Spectrometer26

� (3) Multi-beam interference spectrometerThe interference of light will occur if oblique light is incident on two parallel semi-transparent plane mirrors. A different spectrum is obtained depending on incident angle, interval of the two mirrors and the refraction coefficient.

Comments….

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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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