Introduction to Remote Sensing

Inel 5995

Dr. Ramón Vásquez/Prof. Vidya Manian

Key concepts of remote sensing• Spatial Differentiation

– observed spectral differences in the energy reflected or emitted from features. Eg., differences in “color”objects - multispectral remote sensing

– spectral signature - spectral response of a feature. Eg., corn has different responses during planting, emerging and maturing. Yet useful at specific times and places

• Radiometric Differentiation– detection of differences in brightness of objects and

features– contrast at a specific spectral region - R.S. instrument

should be capable of recording this contrast– sensitivity of instrument, and existing contrast in scene

are important issues

• Spatial differentiation– smallest area that can be separately recorded as an entity

on an image - spatial detail

– Pixels - distinct, discrete units identifiable on the image

– spatial detail influenced by choice of sensor and altitude at which image is recorded

– spatial complexity-some landscape can be represented at coarse levels, some require finer levels

• Geometric transformation– RS image represents landscape in a specific geometric

relationship determined by design of RS instrument, specific operating conditions, terrain relief

– Ideal RS instrument creates accurate correspondence between points on ground and corresponding representation on image

– In reality, RS images have positional errors due to sensor optics, motion of scanning optics, terrain relief, earth curvature.

– These can be removed or reduced but should be taken into account

• Interchangeability of pictorial and digital formats– RS outputs are digital arrays representing

brightnesses of areas of the earth’s surface – Pictorial images in digital form - systematically

dividing the image into tiny areas of equal size and shape, then representing the brightness of these areas by discrete values

• RS instrumentation acts as a system– many components of RS acts as a system, not

isolated from one another– Eg., upgrading quality of a camera lens is useful

when the quality of film is also improved

• Role of the atmosphere– All energy reaching the RS instrument passes

through a portion of earth’s atmosphere– The sun’s energy is altered in intensity and

wavelength by particles and gases in the atmosphere. These changes appear degrading image quality and reduces accuracy of interpretation

Electromagnetic radiation

– All objects except those at absolute zero, emit EM radiation

– Understanding of EM radiation and its interaction with surfaces, the atmosphere and instruments

– Familiar form of EMR - visible light

• The electromagnetic spectrum– generated by - changes in energy levels of electrons, acceleration

of charge, decay of radioactive substances, thermal motion of atoms and molecules

– Source - nuclear reaction in sun, passes atmosphere, reaches earth. Some reflected upwards from the surface - this forms basis for photographs and similar images

– some absorbed at the surface of the earth and re-radiated as thermal energy (can also be used for RS images - differ from aerial photographs formed from reflected energy

• Man-made radiation generated by imaging radars are also used for RS

• Em radiation consists of electrical field (E)-varies in magnitude in a direction perpendicular to direction of propagation. A magnetic field (H), at right angles to E, propagated in phase with E

Has three properties• 1. Wavelength: distance from one wave crest to next

(measurement units in Table 2.1)

• Frequency: number of crests passing a fixed point in a given period (Hertz, one cycle per second and multiples of the hertz, Table 2.2)

• Amplitude: height of each peak. Measured as energy levels (spectral irradiance expressed in watts per square meter per micrometer, as energy level per wavelength in interval)

• Speed of electromagnetic energy ( c) is constant at 299,893 km per second.

• c = ; wavelength (), frequency ()• characteristic of EM energy can be specified using either freq.

(Hz, kHz, MHz) or wavelength (Angstrom units, microns, micrometers ( 1 millionth of a meter m, nanometers, millimeters)

• Major divisions of the EM spectrum• arbitrarily defined (Table 2.3)- there are no sharp breaks as in

Fig. 2.3. (subdivisions for convenience, may vary)• The optical spectrum from 0.13 to 15

• Speed of electromagnetic energy ( c) is constant at 299,893 km per second.

• c = ; wavelength (), frequency ()

• characteristic of EM energy can be specified using either freq. (Hz, kHz, MHz) or wavelength (Angstrom units, microns, micrometers ( 1 millionth of a meter m, nanometers, millimeters)

• Major divisions of the EM spectrum• arbitrarily defined (Table 2.3)- there are no sharp breaks as in Fig. 2.3.

(subdivisions for convenience, may vary)

• The optical spectrum from 0.13 to 15 m, those wavelengths that can be reflected and refracted with lenses and mirrors

• The reflective spectrum from 0,38 to 3.0 m, portion of solar spectrum used directly for RS

• Ultraviolet spectrum

• For RS begin with UV region (short-wavelength region-limit of human vision)

• near UV (UV-A) (0.32-0.40 m), far UV (UV-B) (0.32-0.40 m) and extreme UV (UV-C) (<0.28 m)

• Near UV induce fluorescence (emission of visible radiation) in some materials

• UV is largely scattered by earth’s atmosphere, not generally used in RS

• Visible Spectrum

• Isaac Newton - optical light can be divided using prisms, or, in our time, diffraction gratings into 3 segments - additive primaries 0.4-0.5 m-blue, 0.5-0.6 m - green, 0.6-0.7 m-red

• all other colors formed by mixing the 3 primaries; equal proportions form white light

• A “blue”object reflects blue light, etc.

• Intermediate colors - when object reflects 2 or more of the additive primaries “yellow” (red and green) - useful for exposure of photographic films

• colors in films, paintings formed by combinations of 3 subtractive primaries defines colors of pigments and dyes

• Fig. 2.4 - Each of the 3 subtractive primaries absorbs a third of the visible spectrum; Yellow absorbs blue light and reflects red and green

• Mixture of equal proportions of pigments of the 3 subtractive primaries yields black (complete absorption of visible spectrum)

• The Infrared spectrum

• from 0.72-15 m, 40 times as wide as visible region

• Near IR and mid IR radiation - closest to visible, behave analogous to visible, can use films, filters and cameras

• Far IR - wavelengths well beyond the visible extending to wave region. Near IR is essentially solar reflected from earth’s surface, far IR is emitted by the earth. It consists of “heat”or “thermal energy”, also called emitted infrared.

• Microwave energy

• Longest wavelengths used in RS are from about 1 mm to 1 m. Shortest wavelengths in this range have much in common with the thermal energy of the far infrared

• Longest wavelengths merge into the radio wavelengths (commercial broadcasts)

• Radiation Laws

• Isaac Newton: dual nature of light: light is a stream of minuscule particles (corpuscles) that travel in straight lines

• Max Planck discovered that EM energy is absorbed and emitted in discrete units called quanta, or photons. A constant (h) relate frequency () to radiant energy (Q): Q=h

Aerial photography

• Images formed as light passed through a pinhole opening in a dark enclosure

• “camera obscura”-dark chamber-project image onto a screen

• Developed to popular cameras after development of photographic emulsions

• Aerial photography-most reliable and widely used; source for landscape information; production of topographic maps

• Aerial camera-1) a lens to focus light on the film, 2) light-sensitive film to record image, 3) shutter that controls entry of light into the camera, and 4) camera body-light-tight enclosure to hold film, lens and shutter

• Aerial cameras also include – film magazine, drive mechanism, and lens cone

• Lens - formed from a glass disk ground into a shape with nonparallel curved surfaces

• Change in optical density causes refraction of light rays; the sizes, shapes, arrangements, and compositions of lenses control bending of light, maintain color balance and minimize optical distortions.

• Quality of lens-quality of glass, precision with camera• Imperfections-spherical aberration (not a problem for

modern cameras)• Aerial cameras use compound lenses, formed from many

separate lenses; to correct for errors in single components• Optical axis – joins centers of curvature of the two sides of

the lens• Image principle plane-passing through the center of the lens

is the center of refraction within the lens, intersects with the optical axis at the nodal point

• Parallel light rays reflected from an object at infinite distance, pass through lens and focus at the principal focal point ( image formation of distant object)

• Plane passing through the focal point parallel to the image principal plane is the focal plane

• For aerial photographs – scenes are at large distances, focus can be fixed at infinity, with no need to change focus of the lens

• Focal length-distance from the center of the lens to the focal point (in inches or mm.)

• Focal length not identical at all wavelengths – source of chromatic aberration ( corrected by lens design, else individual colors of an image are out of focus in photo)

• Radiation used to form the image is brought to a common focal point

• Field stop – controls field of view of a lens –a mask positioned just in front of the focal plane

• Aperture stop – positioned near center of a compound lens; consists of a mask with a circular opening of adjustable diameter

• Controls the intensity of light at the focal plane; controls only brightness of image without changing its size ( FOV)

• Relative aperture f = Focal length / Aperture size• Same unit of length for focal length and aperture size, f is the f

number• Large f number – aperture opening is small relative to focal

length• Small f number – opening is large relative to focal length• Specification of aperture as “f4”has meaning for cameras of all

sizes; one fourth of focal length for any size camera• Standard sequence: f1, f1.4, f2.8, f4, f5.6, f8, f11, f16, f32, f64,….

Changes amount of light by a factor of 2 as the f stop is changed by one position

• Change from f2 to f2.8 halves the amount of light entering the camera, from f11 to f8 doubles the amount of light; a given lens uses only a portion of the range of apertures mentioned above

• The Shutter – controls the length of time film is exposed to light; are metal blades positioned between elements of the lens forming “intralens” shutters

• Focal plane shutter – consists of a metal or fabric curtain positioned just in front of the film, near the focal plane; curtain has number of slits

• Shutter speed – selected by operator produces desired exposure• Between-the-lens shutter is preferred in aerial cameras• This “intralens”shutter subjects the entire negative to illumination

simultaneously and presents a clearly defined perspective ; allows use of image negative as basis for precise measurements

• The Film Magazine – light tight container holds supply of film; includes a supply spool, holds 100s of feet of unexposed film, a take-up spool to accept exposed film

• The Lens cone- supports the lens and filters and holds them in correct position

• It is detachable to allow use of different lenses• Manufacturer alights lens with other components – for geometric

accuracy of photos• Common focal length for typical aerial cameras = 150mm(6in.), 300 mm,

450 mm• The Drive Mechanism – advances the film after each exposure; a vacuum

platen holds film during exposure• Vacuum platen – (aerial cameras) a flat plate positioned at the focal

plane; a vacuum pump draws air through small holes in the plate to hold the film flat and stationary during exposure

• Vacuum is released after exposure; allows film to advance for next exposure

• Kinds of aerial cameras: civilian – metric or cartographic cameras-less geometric and optical error –high quality images– Calibrated at laboratories– Knowledge of internal geometry (focal length, flatness of focal plane,

etc) enables photogrammetrists to make accurate measurements

• Strip cameras – film is moved in front of a fixed slit that serves as a shutter; speed of film movement coordinated with speed and altitude of aircraft; image is a long continuous strip

• Panoramic cameras – records a very wide field of view– Lens with a narrow FOV scans across a wide strip of land, image

formed by side-to-side motion of lens as aircraft moves forward– Photographs – show long narrow strip of terrain extending

perpendicular to flight track from horizon to horizon– Have serious geometric distortions (forward motion of aircraft and side-

to-side scan of lens) – require correction before use– Useful due to large areas, but only central part are suitable for detailed

interpretation : why ??

• Black-and-White Aerial Films – 1700s and 1800s photosensitive chemicals (silver nitrate)-darkens when exposed to light.– First practical means of recording the image – a silver coated metal

plate treated with iodine vapor

– 1839 – date for beginning of photography

– Early – long exposure times, required bright light, recorded on metal or glass plates – heavy and fragile

• Aerial photography – taken using balloons or kites for elevation of camera – no standard equipment

• George Eastman – modern lightweight photographic equipment – roll film (Kodak camera in 1888)-mass production and standardization

• Modern films – sensitive to both visible and nonvisible portions of spectrum and represent reflectances in much more specific spectral regions

• Major components – film base, or support, (thin 40 to 100m), flexible, transparent material that holds a light sensitive coating (polyester film – flexible lightweight strips, strong for winding and unwinding)

• Base should resist change in size as temp. and humidity vary – precise image size is needed to measure distances in images

• Base-coated with a photographic emulsion (silver nitrate)-light changes it to metallic silver (dark) –darker with increased exposure

• Modern emulsions – have small crystals of silver halide (silver bromide 95% and silver iodide 5%) suspended in a gelatin matrix (5 m thick)

• Gelatin holds crystals in suspension, allows spreading evenly on base (also it is transparent and porous –absorbs halogen gases –allows photographic chemicals to contact crystals)

• Silver halide crystals – small irregular in shape with sharp edges, favor interception of photons that pass into the emulsion – finer size->finer detail that can be recorded – coarse grains-> less detail but greater sensitivity to light; spatial

resolution of film inversely related to its speed

• Recent from Kodak-film emulsions with flat grains – increased surface area; – grains are parallel to film surface, expose large surface areas to light,

increasing speed without decreasing resolution – color prints only

• Protective super-coat (from scratches)– film emulsion coated with thin layer of clear gelatin

• Dust and moisture could affect – use cotton gloves when handling film !!

• Subbing layer – below emulsion; ensure emulsion adheres to base; back side of base has an antihalation backing – absorbs light passing through emulsion and base to prevent reflection

back to emulsion – anticurling agent – counteracts curling effect of emulsion that coats

upper side of film)

– If absent – images of bright objects surrounded by halos due to reflection!!

• Shutter opens – allows light strikes silver halide crystals (small area contains many thousands of crystals); changes to metallic silver, more light more crystals affected;

• Image is only latent; processing is required• Development – bathing exposed film in alkaline chemicals

that reduces exposed silver halide grains; – Developer - Minimally affected crystals completely changed to Ag

(silver) – amplifying pattern– Areas exposed to most intense light have greater density of Ag in

final image

• Acid stop – allows exact control of time film is in contact with developer to counteract effect of alkaline developer

• Fixer – applied to dissolve, remove unexposed silver halide grains

• RESULTING IMAGE – NEGATIVE (brightness reversed from original values in scene)

• Film speed – measure of sensitivity of emulsion to light; fast film low intensity of light for proper exposure; slow film - more light - > aperture opened wider (or longer exposure time)

• DIN and ASA ratings – assess speed of films for hand-held cameras

• Aerial films – aerial film speed (AFS) and aerial exposure index (AEI)

• Contrast – range of gray tones recorded by film

• High contrast – largely in black and white

• Low contrast – representation largely in grays with few really dark and bright tones ( preferable –why ??)– Fine grained emulsion – have low contrast (slower films –higher

spatial resolution and low contrast than coarser-grained fast films)

– Emulsions on photographic papers -> higher contrast than emulsions on films (interpreters prefer film transparencies)

• Spectral sensitivity – records spectral region to which film is sensitive: – Figure: spectral sensitivity curve for Kodak Tri-X Aerographic Film

2403 – features of B/W films

• Sensitive throughout visible spectrum and UV radiation;• Due to scattering of shorter (UV and blue) wavelengths,

filters are used with B/W aerial films– Figure: B/W view of scene in accordance with our view of

panchromatic film – sensitive to visible spectrum (HVS)

• Orthochromatic –films with sensitivity in blue, green and peak sensitivity in green– Figure: spectral sensitivity curve for Kodak Infrared Aerographic

Film 2424, B/W IR film– Desirable to exclude visible radiation; film used with a deep red filter

that blocks visible radiation but allows IR radiation to pass– Figure: image by IR B/W film different from the visible spectrum

image– Eg., living vegetation is much brighter in near IR portion than in

visible portion (hence are bright white on B/W IR image)

• The Characteristic Curve – in the negative –pattern of dark and light related to patterns of metallic silver formed in processed film (bright scenes are darker)

• Intermediate shades of brightness as crystals in emulsion are much smaller than the human eye can resolve

• Shades of gray – variations in abundance of tiny grains of silver in processed film– Crystal present (black) or – absent (dark), – shades of gray (variation in abundance of crystals – proportional to

brightness of original scene)

• Shine light intensity – Io, • through a small area of the negative – fraction of a mm in

diameter, • brightness of light measured on other side – I is a measure of

darkness of that region of the film, I is < Io: Opacity: I/Io

Density: darkness of film log10 (opacity)

• Effect of light on emulsion of a film = product of intensity ,i (“brightness”equivalent to irradiance) and time t.

E = i x t• Used to compensate for use of a fast shutter speed by opening the

aperture to allow more light to enter:– Figure: Characteristic curve –relationship between brightness in scene and

density on the film – (or relationship between density of a negative and log10 of exposure) – has

a typical S-shape

• Toe – lower part is curved• Straight-line segment – center part (is of interest)• Shoulder – upper portion is curved• If exposure is increased by a certain amount, curve allows

prediction of corresponding increase in image density• Image scientists – with suitable controls, can use measurements

from photography to learn about brightnesses in scene– Figure: difference in slope translates a given brightness range (BR) into

different density ranges (DR) in final processed image

• Steep slope – small range in exposure -> big range in density (is a high contrast image- displays large range in brightness)

• Shallow slope – range in scene brightness ->small range in image brightness – low contrast

• Toe and shoulder – no consistent relationship; for very high and very low exposures, film will not produce predictable densities compared to straight line segment– Used for artistic effect in photographs(!!) scientists avoid use of these

image measurements based on very high or very low image densities – have unknown relationship to brightness in scene

• Photographic films used with nonphotographic sensors – knowledge of characteristic curve important in RS

• Sensors record large range of brightnesses – may exceed capability of film; Very dark areas or very bright areas or both will be represented in nonlinear part of the curve; only a small portion showing actual brightness

• Hence, DIGITAL IMAGE ANALYSES that does not rely on film images are ADVANTAGEOUS

• Optical distortions- caused by inferior camera lens, malfunction, minor significance in modern photography

• Tilt -caused by displacement of focal plane from a truly horizontal position due to aircraft motion

• Isocenter – focus of tilt, located at or near principal point• Image areas on the upper side of tilt are displaced further

away from ground than is the isocenter (smaller scales than nominal scale); image areas on the lower side of tilt are displaced down (scales larger than nominal scale)

• For measurements – select distances as lines that pass close to the principal point; errors caused by upward tilt compensate for errors caused by downward tilt (eliminates large errors in areas further from principal point)

• Relief displacement - important source of positional error in vertical aerial photography– Objects positioned directly beneath the center of the camera lens

photographed so that only top of object is visible– For other objects top and sides are visible (lean outward from central

perspective of camera lens

• Difference in apparent location –due to height (relief)-source of positional error

• Amount of displacement depends on (1) height of object (2) distance of object from nadir

• Relief displacement is basis of measurement of height of objects

• Coverage by multiple photographs• Series of flight lines –provides complete coverage of a

specific region• Each flight line consists of individual frames-camera

operator view the area to be photographed through a viewfinder attached to the camera

• Drift – plane’s course deflected by crosswind, position of photos drifted

• Crab – caused by correction of flight path to compensate for drift without change in orientation of camera

• If forward overlap is 50% or more, the image of the principal point of one photograph is visible on the next photograph in flight line – conjugate principal points

• In coverage of large areas - several parallel strips of photography each strip is called –flight line

• Sidelap between adjacent flight line is about 5% to 15%• As pilots and other crew members collect photographs, there

may still be – gaps (holidays) due to equipment malfunction, navigation errors, cloud cover

• Number of photos = Length of flight line / (gd of photo) x (1 – overlap)

• gd – ground distance represented on a single frame (same units as length of flight line) – If flight line = 33 mi., – each photograph represent = 3.4 mi. on a side and forward overlap is

= 0.60, then 33/[3.4 x (1-0.60)] = 33/(1.36) = 24.26 or about 25 photographs are required

• Stereoscopic Parallax – difference in appearances of objects due to change in perspective

• two photographs of the same area taken from different camera positions -> displacement of images of objects from one image to the other

• Distant objects display little or no observable parallax• Used for measuring distance or height• Overlapping aerial photographs record parallax due to shift in

position of camera as aircraft motion carries camera forward between successive exposures.

• If forward overlap = 50%, entire ground area can be viewed in stereo using 3 adjacent frames (a stereo triplet)

• Displacement due to stereo parallax is always parallel to flight line

• Estimation of height of objects - tops of tall objects, nearer to camera show more displacement than do shorter objects

Measurement of stereoscopic parallax

• X – distance between two principal points

• Y – distance between separate images of the base of the object as represented on the two images

• P = X – Y

• B = top-to-top distance

• A = base-to-base distances

• dp = A - B

• H = Flying height of aircraft

• h = Height of object

• h = H x dp / P + dp

• In practice, parallax measurements made using parallax wedge, or parallax bar – devices that permit accurate measurements of small amounts of parallax

• Mosaics – join series of vertical aerial photographs that show adjacent regions on ground to form a mosaic

• Uncontrolled mosaics placing photos together to provide continuous coverage of an area, without concern for preservation of consistent scale and positional relationships (rough alignment at edges) – used as aerial index

• Aerial index – for identifying particular photographs needed for a specific purpose; small scale overview of a particular region

• Cannot be used for measurements of distance or area – lack geometric accuracy

• Controlled mosaic – individual photographs assembled in a manner that preserves correct positional relationships between the features they represent

• Region near principal point – most accurate region is cut out and used for mosaic

• Locational control is provided by ground survey or from accurate maps (controlled mosaic – more expensive)

• Orthophotos and Orthophotomaps – aerial photographs are not planimetric maps – have geometric errors- effects of relief displacement

• Stereoscopic photographs used to generate a corrected form of aerial photograph known as orthophotos – without error tilt and relief displacement

• Orthophotoscope – project a corrected version of a small portion of an image; exposes each small section individually in a manner that corrects for the elevation of that small section

• Scans an entire image piece by piece to generate corrected version of entire image

• Projection orientation is adjusted to correct for tilt• Instrument continuously varies projection distance to correct

for relief displacement• Orthophotomaps – used as maps, show correct planimetric

position and preserve consistent scale throughout image

• Orthophotomaps – can be compiled more quickly and cheaply than usual topographic maps

• Photogrammetry- science of making accurate measurements from photographs, applies principles of optics and knowledge of interior geometry of camera and its orientation to reconstruct dimensions and positions of objects within photograph

• Analysis of stereo aerial photography –estimate topographic elevation (by estimating stereo parallax for any array of points within a region ) for topographic mapping

• Analytical stereoplotters – instrument designed in 1920s, reconstruct orientations of photographs at the time taken. Operators view image in stereo and by visually maintaining constant parallax, trace lines of uniform elevation

• Quality of information depends on quality of photography, accuracy of data, operator’s skill

• Now possible to automatic match of corresponding points on stereo pairs and thereby identify lines of uniform parallax with limited operator assistance

• Digital photography - electronically scanned to record patterns of blacks, grays and whites as digital values – represent brightness of point within image

• Bypass digitization step – replace film in focal plane with an array of light-sensitive detectors that directly record image in digital form

• Softcopy photogrammetry – extend automation of photographic process to conduct analysis in digital domain

• GPS – global positioning systems – acquire accurate positional information and use of data recorded from the aircraft’s navigational system to record the orientations of photographs

• reconstruct geometry of image using above• Digital or soft copy photogrammetry does not require the physical

(hardcopy) form of photograph• Image is used as input for a series of mathematical models that

reconstruct orientation of each image to create planimetrically correct representations– Advantage of speed and accuracy and also creates output data easily

integrated into other production and analytical systems including GISs

• Sources of aerial photography – can be acquired by (1) user, (2) purchased from organizations (archival imagery)

• Produced upon request by specialized firms

• Customers – individuals, governmental agencies or other businesses that use them

• Photography customized to meet specific needs of customers –date, scale, film and coverage

• “do-it-yourself”aerial photography – many small cameras (small format 35 mm) are suitable, charter aircraft (low cost)

• At low altitude and with clear atmosphere – ordinary films produce satisfactory results

• Use a high-wing aircraft to ensure clear view of landscape

• If camera can use filters, possible to use other films (IR or color IR) – aim camera away from sun !!

• EROS Data Center (EDC), Sioux Falls, South Dakota, operated by USGS – repository of aerial photographs and satellite images acquired by NASA, USGS and other agencies – computerized data base

• Earth Science Information Centers (ESICs)– maps and aerial photographs; information about federal programs and agencies; collect data pertaining to maps and photographs held by state and local governments; Reston, Virginia

• Aerial Photography Summary Record System (APSRS) – maintained by ESIC; computer-based information system recording detailed information about aerial photography help by numerous federal, state and private organizations

• National Aerial Photography Program (NAPP) – USGS manages NAPP

• Flight lines are oriented N-S direction centered on each of 4 quadrants systematically positioned within USGS 7.5 minute quadrangles, with full stereoscopic coverage at 60% forward overlap and 27% sidelap

• 10 frames acquired in 2 flight lines, provide full stereoscopic coverage of each quadrangle

• Camera of 6 in. focal length is used at flying altitude of 20,000 ft above terrain, provide coverage at 1:40,000; Black-&-White or Color film is used