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    Photogrammetry

    VARUN SINGH

    ASSISTANT PROFESSOR, CEDMNNIT ALLAHABADE-mail:[email protected],

    [email protected]

    mailto:[email protected]:[email protected]
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    Introduction

    Definition of Photogrammetry: the art, science, and

    technology of obtaining information about physicalobjects and the environment by photographic and

    electromagnetic images.

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    Mapping from aerial photos is the best mapping procedure yet

    developed for most large projects.

    Used successfully for maps varying in scale from

    1:1,000,000 to 1:120 with contour intervals as small as 3

    meters.

    Topographic mapping is the most common form. SOI

    updated and done this way.

    Used to reconstruct a scaled 3-dimensional optical model

    of the lands surface using a stereoplotter.

    Basic Information

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    Basic Information (Contd..)

    Uses: Aerial photos

    Aid: geological investigations, soil surveys, landsurveys, tax mapping, reconnaissance andmilitary intelligence, urban and regionaldevelopment, transportation system

    investigations, quantity estimates, shore erosion,etc.

    Mathematical methods have been developed tomake precise 3-dimensional measurements from

    photos. Phototriangulation: 3-dimensional positioning of survey

    stations.

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    Basic Information (Contd..)

    Photogrammetry can not be used successfully over

    the following types of terrain.

    Desert or plains areas, sandy beaches, and snowthe

    photograph as uniform shades with little texture.

    Deep canyons or high buildings that conceal ground

    surface.

    Areas covered by dense forest.

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    Basic Categories

    Metrical photogrammetry obtaining measurements from

    photos from which ground positions, elevations, distances,

    areas, and volumes can be computed and topographic or

    planimetric maps can be made

    Photo interpretation evaluation of existing features in a

    qualitative manner

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    Types of Photogrammetry

    Aerialseries of photographs of an area of terrain in

    sequence using a precision camera.

    Terrestrial photos taken from a fixed and usually

    known position on or near the ground with the

    camera axis horizontal or nearly so.

    Close range camera close to object being

    observed. Most often used when direct

    measurement is impractical.

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    History

    The first use of photogrammetry was by Arago, aFrench geodesist, in 1840. This included

    topographic and terrestrial.

    The first aerial photogrammetry was by the French

    in 1849 using kites and balloons. Laussedat (French)father of photogrammetry.

    1st in N. America Deville, Surveyor General of

    Canada.

    SOI and GSI adopted photogrammetry as mapping

    process

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    History

    Airplanes brought great change to photogrammetry. 1stused in 1913.

    Used extensively in WWIphoto interpretation.

    Used in WWIImapping for recon and intelligence.

    WWII1960used often, expensive and accuracy

    problems for engineering design. After mid 60sadvent of computer and plotting has

    made photogrammetric mapping accurate andaffordable.

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    Photogrammetry for Engineering

    Defined: Photogrammetry is the process of

    measuring images on a photograph.

    Modern photogrammetry also uses radar imaging,

    radiant electromagnetic energy detection and x-ray

    imagingcalled remote sensing.

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    Basic Categories of Photogrammetric Interpretation

    Metrical Photogrammetry obtaining measurements fromphotos from which ground positions, elevations, distances,areas can be computed and topographic or planimetric mapscan be made.

    Photo interpretation evaluation of existing features in aqualitative manner timber stands, water pollution, soils,

    geological formations, crops, and military interpretation.

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    Camera System

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    Aerial Camera

    A camera used for vertical aerial photography for mapping

    purposes is called aerial survey camera.

    At present there are only two major manufacturer of aerial survey

    cameras

    Leica-Helava System(LH System)- RC 30 cameraZ/I Imaging-(RMK-TOP) Camera

    Modern aerial survey cameras produce negatives measuring23cmsx23cms(9 x 9in) Up to 600 photographs may be recorded in a

    single film roll.

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    Lens Cone

    The most important and expensive single component within the camera is the

    lens. This is interchangeable, and the manufacturers produce a range of cone,each of different focal length.

    Focal Leng this the most important property of the lens since, together with the

    flying height it determine the scale of the photograph. It also determines the angle

    of view of the camera. The longer the focal length, the narrower the angle of view.

    Lens are generally available in the following standard focal lengths.

    Narrow angle (f=610mm)

    Normal angle (f=305mm)

    Wide angle (f=210,152mm)

    Super wide angle (f=88mm)

    The lens is responsible for projecting an optical image on to the film. In an

    ideal lens all the light rays passing through one central point. Hence the

    projection is called the Central Projectio n

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    Camera classifications

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    Camera Lens

    Lens distortion anderrors cause the lightrays to deviate

    IMAGE or EXPOSURE PLANE

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    lens of high geometric quality

    capable of exposing in rapid succession a large

    no. of photographs while moving in an aircraft

    at high speed

    must have short cycling times, fast lenses andefficient shutters.

    Single lens camera classified according to field of

    view :

    Normal angle camera

    Wide angle camera

    Super wide angle camera

    Requirements of aerial mapping camera

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    Types of Aerial Photography

    Vertical Photographs:

    This photograph is taken with the optical axis of the camera in truly

    verticalor very nearly vertical (with tilt < 3o to 5o )

    Tilt :

    The angle formed between the optical axis of the camera

    and a plumb line.

    Types of photographs

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    900

    Lens

    Film

    VERTICAL

    PHOTOGRAPH

    Plumb Line

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    Types of photographs

    Oblique Photographs:

    This photograph is taken with the optical axis of the camera

    intentionally tilted at an angle >5o.Oblique photographs can be

    categorized into

    Low Oblique : These photos are tilted to such an angle so as notto allow the horizon to be seen on the photo.

    ( angle about 30o)

    High Oblique: These photos are tilted at such an angle so as to

    allow the horizon to be seen on the photo.

    ( angle about 60o)

    Panoramic photograph

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    30o

    Plumb line

    LOW OBLIQUE

    PHOTOGRAPH

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    HIGH OBLIQUE

    PHOTOGRAPH

    Plumb

    line

    Horizon

    60o

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    3/4/2014

    sidelap endlap

    STEREOSCOPIC COVERAGE

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    Sensor Attitude/Orientation

    Oblique

    More Oblique

    Vertical

    2

    3

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    Camera System

    Cameras used for air survey, as with all other survey

    equipment, are precision-built, and their lenses are of suchhigh quality that aberrations are practically negligible

    From the engineering point of view the most popular lens is

    the wide-angle combined with a photograph format size of

    230 mm230 mm

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    Map Vs Scale

    Map is the orthographic projection whereas an aerial

    photograph is a central projection i.e. perspective projection Map has a single constant scale whereas it varies from point

    to point depending upon their elevations on an aerial

    photograph.

    The amount of details on a map are selective but in an aerial

    photograph a wealth of detail is there. Due to symbolic representation the clarity of details is more

    on maps than on a photograph.

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    Photogram

    A photogram is a photographic image made without a camera

    by placing objects directly onto the surface of a light-sensitivematerial such as photographic paper and then exposing it to

    light. The usual result is a negative shadow image that shows

    variations in tone that depends upon the transparency of the

    objects used. Areas of the paper that have received no light

    appear white; those exposed through transparent or semi-transparent objects appear grey

    Ph (C d )

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    Photogram (Contd..)

    Ph t (C td )

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    Photogram (Contd..)

    Generation of a photogram: A spatially extended light source (1) illuminates

    objects (2 and 3) that are placed directly in front of a sheet of photosensitive

    paper. Depending on the object's distance to the paper their shadows look

    harder (7) or softer (5). Areas of the paper that are in total shadow (6) stay

    white; they become grey if the objects are transparent or translucent; areas

    that are fully exposed to the light (4) are blackened.

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    Geometry of Vertical Photograph

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    Geometry of Vertical Photograph

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    G t f Ph t h

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    Geometry of Photographs

    Photo axis: the right-angled xy-axis formed by joining the

    opposite fiducial marks of the photograph. This is the axis

    from which photo coordinates are measured. The x-axis

    approximates to the direction of flight

    Optical axis: the line LpP from the lens centre at 90 to the

    plane of the photograph.

    Geometry of Photographs

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    Geometry of Photographs

    Principal distance:the distance Lp = f , from the lens to the

    plane of the photograph. The principal distance may be

    referred to as the focal length. Vertical axis:the line LvV in the direction of gravity, so 90

    to a level datum plane.

    Tilt:the angle between the vertical and optical axes (see

    also principal line).

    G t f Ph t h (C td )

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    Geometry of Photographs (Contd..)

    Principal point (PP): the point p where

    the optical axis cuts the photograph, andcoincides with the origin of the photo

    axes.

    Plumb point: the point v where the

    vertical axis cuts the photograph.

    Isocentre:the point i, where thebisector of the angle of tilt cuts

    the photograph.

    Geometry of Photographs (Contd )

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    Geometry of Photographs (Contd..)

    Plate parallels:the lines at 90to the principal line; they are level lines.

    Isometric parallel:the plate parallel passing through the isocentre and

    forming the axis of tilt of the photograph.

    Flying height:the vertical height of the lens above ground at exposure.

    It is the height of the lens above datum (e.g. MSL) minus the mean

    height of the terrain.

    Swing: the angle s measured in the plane of the photograph,

    clockwise from the +y axis to the plumb point. It defines the directionof tilt relative to the photo axes.

    Geometry of Photographs

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    Geometry of Photographs

    The photographic nadir is defined

    by the intersection with the

    photograph of the vertical line thatintersects the ground nadir and the

    center of the lens (i.e. the image of

    the ground nadir)

    The ground nadiris defined as thepoint on the ground vertically

    beneath the center of the camera

    lens at the time the photograph

    was taken.

    Perspective geometry of vertical photographs

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    Perspective geometry of vertical photographs

    points v and V (ground and photo

    nadir point),

    i and I (ground and photo

    isocentre)p and P (ground and photo principalpoint)

    photographic plane

    horizontal plane

    principle plane

    Principle line

    (UP)

    Perspective geometry of vertical photographs

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    Perspective geometry of vertical photographs

    * The figure shows camera

    axis SP of a camera,

    perpendicular to thephotographic plane ABCD,

    tilted at angle from the

    vertical at exposure so that

    the plane of the photograph

    itself is inclined by an angle to the horizontal plane

    CDEF, representing a level

    ground. S represents

    perspective center as

    defined by the inner or rearnode of the lens system.

    Perspective geometry of vertical photographs

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    Perspective geometry of vertical photographs

    Perspective Axis: Line

    CD where the two

    plane meet is called

    the perspective axis or

    horizontal trace.

    Perspective geometry of vertical photographs

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    Perspective geometry of vertical photographs

    Horizontal lines drawnin the photo plane are

    called plate parallels

    or plate horizontals.

    Line iI is the bisector

    of tilt angle . It meets

    photo plane at i and

    ground plane at I.These points are

    known as photo

    isocentre and ground

    isocentre respectively.

    Perspective geometry of vertical photographs

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    Perspective geometry of vertical photographs

    Principal plane: A

    plane containing P, V,and S is called the

    principal plane. Photo

    principal line (VP) and

    ground principal lines(vp) are contained in

    this plane.

    Perspective geometry of vertical photographs

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    Perspective geometry of vertical photographs

    Isomertric Plate

    Parallel: For a truly verticalphotograph taken from exposurestation S, various photo plate

    parallels are lines I'I", P'P", etc.

    The plate parallel through I is

    also called isometric plateparallel.

    E t i O i t ti

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    Exterior Orientation

    Exterior Orientation

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    Exterior Orientation

    Details on Aerial Photographs

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    Details on Aerial Photographs

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    48

    Details on Aerial Photographs

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    Details on Aerial Photographs

    Scale and its variation due to ground relief

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    Scale and its variation due to ground relief

    Ground Coordinates from vertical photograph

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    p g p

    Ground Coordinates from vertical photograph

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    p g p

    Image displacement due to ground relief

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    Image displacement due to ground relief

    Image displacement due to ground relief

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    g p g

    Image displacement due to ground relief

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    g p g

    Image displacement due to ground relief

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    g p g

    Image displacement due to ground relief

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    g p g

    Image displacement due to ground relief

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    Amount function of:

    Flying height Distance from nadir point to image point

    Elevation of ground point

    Position of point w.r.t. principal line and axis of tilt

    Relief displacement Will be less on upward side of photo

    Identical along axis of tilt

    Greater on downward half of photo

    g p g

    Image displacement due to ground relief

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    g p g

    FLYING HEIGHT OF A VERTICAL PHOTOGRAPH

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    FLYING HEIGHT OF A VERTICAL PHOTOGRAPH

    FLYING HEIGHT OF A VERTICAL PHOTOGRAPH

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    FLYING HEIGHT OF A VERTICAL PHOTOGRAPH

    FLYING HEIGHT OF A VERTICAL PHOTOGRAPH

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    FLYING HEIGHT OF A VERTICAL PHOTOGRAPH

    Tilted Photograph

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    Tilted Photograph

    Tilted Photograph

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    g p

    Tilt angle (t) :It is the angle

    between the optical axis

    and the vertical axis

    Principal Plane: It is the

    vertical plane through the

    optical axis. Since the plane

    of the photograph is normalto the optical axis, it is

    normal to the principal

    plane.

    Tilted Photograph

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    g

    Principal Line: The line of

    intersection of the principal

    plane and the plane of thephotograph, which is line no

    prolonged, is termed the

    principal line.

    X-Axis and Y-AxisIn the photograph, the x axis is

    placed on the line at which this

    vertical plane intersects the

    plane of the photograph, with x

    values increasing in thedirection of flight. The y axis is

    normal to the x axis, and the

    origin lies at o. The y axis is

    normal to the x axis, and the

    origin lies at o.

    Tilted Photograph

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    Tilt Displacement

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    Vertical Aerial Photographs

    Ground

    Nadir

    Isocenter

    PPoint

    X-axis tilt:

    wings upor down

    Ground

    Nadir

    Isocenter

    PPoint

    Y-axis tilt:

    Nose upor down

    FOR 220 Aerial Photo Interpretation and Forest Measurements

    Definitions

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    Definitions

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    Link

    Link1

    Link2

    Auxiliary Coordinate System

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    Auxiliary Coordinate System

    In tilt-swing-azimuth system, auxiliary coordinate

    system required for some computationsTwo stages:

    Rotate about principal point through an angle

    Translate along principal line from principal point

    to nadir pointOrigin at nadir point, y axis coincides with the

    principal line, x axis is perpendicular to principal

    line at the nadir point, clockwise 90o

    sincos aa yxx

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    = Amount of rotation = s-180o

    s = swing angle cossin aa yxy +

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    Auxiliary Coordinate System

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    Auxiliary Coordinate System

    s180

    Note: if rotation angle defined as

    tfyxy

    yxx

    tancossin

    sincos

    +++

    Then auxiliary coordinate system can be

    expressed as

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    Scale of Tilted Photograph

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    In previous figure

    wk is perpendicular to line Ln and is therefore ahorizontal line

    wp is perpendicular to the principal line and is

    also a horizontal line Plane kwpis horizontal plane

    Plane NWPis horizontal plane

    g p

    Link

    Scale of Tilted Photograph

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    g p

    Scale between planes kwp & NWP found by

    similar triangles

    Because Lk = Ln kn = f sec t y sin t

    hH

    tyt

    f

    hH

    tytfS

    hH

    Lk

    LN

    Lk

    NP

    kp

    sincossinsec

    ''

    Link

    GROUND COORDINATES FROM TILTED PHOTO

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    hH

    tyt

    f

    X

    x

    WP

    wp

    sincos

    x

    tyt

    fhHX

    sin

    cos

    On ground, the Y-axis lies principal plane

    In figure, wp = x and WP = X, then from

    scale:

    Link

    GROUND COORDINATES FROM TILTED PHOTO

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    hH

    tyt

    f

    Y

    ty

    Y

    tnw

    Y

    kw

    NW

    kw

    sincoscos'cos

    ty

    tyt

    f

    hHY cos

    sincos

    Similarly

    Tilt Displacement

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    Tilt Displacement

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    f

    tydor

    yt

    f

    yd

    sin)(

    sin

    )( 2*

    *

    2*

    Parallax and Stereoscopic vision

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    Parallaxis defined as the apparent shift in the position of

    an object, caused by a shift in the position of the viewer.

    Stereoscopic vision is that particular application of

    binocular vision (simultaneous vision with both eyes) that

    enables an observer to view two different perspective

    photographs of an object (such as two photographs takenfrom different camera stations) and get the mental

    impression of three dimensions.

    A stereoscope permits each eye to see as one a pair ofphotographs that shows an area from different exposure

    points and thereby produces a three-dimensional

    (stereoscopic) image (model).

    STEREOSCOPIC DEPTH PERCEPTION

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    STEREOSCOPIC DEPTH PERCEPTION

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    Stereo Model

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    Stereo Model

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    The parallactic angle, also known as the convergence angle, is formed by the

    intersection of the left eye's line of sight with that of the right eye. The closer this

    point of intersection is to the eyes, the larger the convergence angle. The brainperceives the height of an object by associating depth at its top and its base with the

    convergence angles formed by viewing the top and base.

    The X parallax and the parallactic angle are related. As X parallax increases, so too

    does the parallactic angle.

    As the eyes scan overlapping areas between a stereo image pair, the brain receives

    a continuous 3-D impression of the ground. This is caused by the brain constantly

    perceiving the changing parallactic angles of an infinite number of image points

    making up the terrain. The perceived 'virtual' 3-D model is known as a stereomodel

    Stereo Model

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    If the negatives are now printed as

    photographs and viewed

    simultaneously, so that the left eyesees only the left photograph and

    the right eye only the right

    photograph, then a three-

    dimensional image will form in the

    mind. The above condition can bemost easily obtained by viewing the

    photographs under a stereoscope.

    The three-dimensional image

    formed is termed a stereo model.

    stereo pair

    Stereo Model

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    The stereo model is usually exaggerated and this can be useful in the

    heighting process, particularly where the terrain is relatively flat. This

    effect can be increased or reduced when planning the photography.

    If the value of f is fixed, then from the base/height ratio, it can be seen

    that to halve the flying height would double the impression of height.

    Increasing the viewing distance of the stereoscope produces aproportionate increase in the impression of height.

    VIEWING PHOTOS STEREOSCOPICALLY

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    Parallax Measurement

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    parallax ofA (PA).

    Parallax

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    A appears at a1

    when viewed from

    L1; and at a2when

    viewed from L2.

    By overlapping the two

    photographs, the apparentmovement ofA is shown

    as a1a2, (L1a2is parallel to

    L2a2)

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    Parallax Equation

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    Parallax Equation

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    Parallax Equation

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    Parallax Equation

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    Parallax Equation

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    X Parallax

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    X parallax, which is also known as stereoscopic parallax, is

    caused by a shift in the position of observation. To generate a

    stereo pair of aerial photos, a camera on board an aircrafttakes pictures of the Earth at different times and thus from

    different positions.

    Satellite image stereo pairs are generated when a satellitecollects data with two different look angles or two different

    beam positions. The change in observation points causes an

    apparent shift in the position of an object with respect to the

    image frame of reference.

    X Parallax

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    Two fundamental aspects of stereoscopic parallax are:

    the parallax of any point is directly related to the elevation ofthat point; and

    the parallax is greater for higher than lower elevations

    provided the viewing angle is constant.

    These two relationships allow height measurements to be

    made from a stereo pair.

    Stereo photographs

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    Stereo photographs

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    Stereo Model: Vertical Exaggeration

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    Vertical exaggeration is present in all stereo pairs. It exists because of

    disparity between the vertical and horizontal scales of a stereo model.

    Under normal conditions, the vertical scale will appear greater than thehorizontal. Image interpreters must take this effect into consideration

    when estimating heights of objects and rates of slopes.

    Vertical exaggeration is best understood by considering the

    relationship between the imaging geometry and the viewing geometryof a stereo model.

    Vertical exaggeration is the difference between the imaging base to

    height (Bn/Hn) and the stereo-viewing base to height (Bs/Hs) ratios.

    Bn/Hnis the ratio of the air base (distance between the two exposurestations) to the flight altitude above average ground. Bs/Hsis the ratio

    of the eye base to the distance from the eyes at which the stereo-

    model is viewed.

    Stereo Model: Vertical Exaggeration

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    Stereo Model: Vertical Exaggeration

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    Stereo Model: Vertical Exaggeration

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    Stereo Model: Vertical Exaggeration

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    ELEVATION BY PARALLAX DIFFERENCES

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    ELEVATION BY PARALLAX DIFFERENCES

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    ELEVATION BY PARALLAX DIFFERENCES

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    Stereoscopic Parallax

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    Differential Parallax (dp)

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    Pseudoscopic Effect

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    Another illusion we might encounter is accidentally

    reversing the two photos so that the right eye viewsthe left photo and the left eye views the right photo.

    The result is a pseudoscopic view, or reversal of

    relief, a phenomenon illustrated in Figure in nextslide

    Height Calculation

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    STEREOSCOPES

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    Si l i bl i i d

    STEREOSCOPES

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    Simple instruments to enable stereo vision and

    magnification: see each photo separately through a lens

    They can be:

    a) Pocket stereo scopes

    - the simplest

    Note that because of the small base, the photographs must be

    on top of each other.

    Does not enable viewing of the entire stereo coverage

    at the same time.

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    b) Mirror Stereoscope

    The eye base is increased by a system of mirrors

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    The eye base is increased by a system of mirrors

    Thus, the entire stereo model can be viewed.

    In general, the eyebase, the direction of flight, and the line joiningthe centers of the lenses should be parallel.

    Measurement of Parallax

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    Parallax Bar

    Parallax determination using a Parallax Bar

    The parallax bar is a measuring device to measure the x

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    The parallax bar is a measuring device to measure the x

    parallax of points in a fast and precise manner.

    Since x parallax is a function of relief or heights of points

    Parallax determination using a Parallax Bar (Contd..)

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    Since x parallax is a function of relief or heights of points

    above datum, elevations of points can be computed by

    measuring their parallax. Other values can also be derived if unknown such as the

    flying height, or air base.

    To measure parallax of a point you can :

    a) Either measure in mono: measure the value of the xcoordinate of the point on each photograph and subtract

    the left from the right value. For example, if point a

    appears on Left and right photos then:

    pa=xaLxar

    Or measure in stereo using a parallax bar Put the

    Parallax determination using a Parallax Bar (Contd..)

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    Or measure in stereo using a parallax bar. Put the

    photos on a table under a stereoscope, move them right

    and left up and down until you see comfortably in stereo,in this case: air base B as photo base b: o1 o2, and the

    center of the lenses of the device must be in the same

    direction

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    The image of each photo nadir, P.P in a vertical photo (o1and o2)

    appears on the other photo

    Parallax of b = Ddb = xbxbnote that xb is negative)

    L Photo

    base b

    R Photo

    base b

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    pa= xaxa= D (C ra) = (D C) + raAssume that k = (D C)Since D and C are constants, then k is a constant, called the parallax bar

    constant for the setup, then,

    pa = k +r

    To calculate the parallax of a point you measure the value r for it

    Parallax determination using a Parallax Bar (Contd..)

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    To calculate the parallax of a point, you measure the value r for it

    with a parallax bar and multiply it by the constant K, you can do that

    in stereo for a set of points in few minutes. Now, how to compute the constant K??

    It is computed only once by measuring the parallax of two point

    monoscopically by measuring their x coordinate on each photo and

    subtract: p = x- x1

    Now measure the value (r) for each point and apply the equation:

    k = pr

    You get two values for k, take the average.

    You can use photo centers for this process since the x value of eachone on its own photograph is 0, you just need to measure its x value

    on the other photo

    Without the bar using a precise ruler measure the values

    Parallax determination using a Parallax Bar : Summary

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    Without the bar, using a precise ruler measure the values

    of x of two points a and b on both photos, well distinctive

    points

    Calculate the parallax of the two points, where:

    pa = xaxa andpb= ybyb

    Use the parallax bar to measure raand rbfro the samepoints.

    Compute k1= para and k2= pbrb

    K is the average = (k1+ k2)/2 For any point measure r with the bar and multiply by k to

    get the parallax

    ExampleA pair of overlapping vertical photographs were taken from a flying

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    A pair of overlapping vertical photographs were taken from a flying

    height of 4045 feet with a 152.4 mm focal length camera. The air base

    was1280 feet and parallax bar readings of 12.57 and 13.04 mm wereobtained with the floating mark set on principal points (ol) and (o2)

    respectively. If b and b ( left and right photo bases) were measured as

    93.73 and 93.30mm. Parallax bar readings of 10.96 and 15.27 mm were

    taken on points A and B. Also the x and y photo coordinates of points A

    and B were Xa = 53.41 mm, Ya = 50.84mm , Xb= 88.92mm and Yb=-

    46.69mm Calculate the elevations of points A and B

    and the horizontal distance between them.

    Solution:

    Parallax of Point 01=Po1= b'= 93.30mm .

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    1 o1

    Parallax of Point O2= Po2= b = 93.73mm

    Equation of parallax bar: P =K+r K = P - r

    For point 01 : k1=Po1- ro1=93.30 -12.57 = 80.73mm

    For Point 02 : k2=P02-r02=93.73 -13.04 = 80.69mm

    For point a: Pa= k + ra= 80.71+ 10.96= 9l.67mm.

    For point b : Pb= k + rb= 80.71+ 15.27= 95.98mm ,

    Elevations of points:

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    Stereoscopic Plotters

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    Stereoscopic plotters are instruments used to measure

    ground coordinates of points in the overlapped areaby measuring their photo coordinates in stereo mode.

    Two overlapping photos are used, either placed in

    projectors as the case in early models, or displayed indigital format as current technology.

    Such plotters allow for the removal of the Y parallax,

    the effect of different of scale between the photos, tilt

    of photos,

    Stereo plotters may be classified into three categories:

    1- Analogue Optical

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    g p

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    Film or transparent photos are positioned on theprojectors, light is projected through them, their

    l ti iti dj t d t f d l t

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    relative positions are adjusted to form a model to

    scale. Points are measured as they are traced on the tracing

    table (4) in the graph above, a pencil at (11) will draw

    a map as the tracing table is moved.

    Such a device is used to illustrate the idea, but not

    for production today.

    2- Analytical Plotters Still uses photographs, but the model is

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    St uses p otog ap s, but t e ode smathematical,

    Two comparators are used to precisely measurephoto coordinates, which are recorded digitally.

    The stereo-model is seen through optics as acomputer adjusts the photos for stereo viewing andmeasurements as the mouse is moved by drivingservo motors.

    A point is digitized by clicking a mouse when the

    floating mark rests on it to store the coordinates. The digital output is stored and a CAD system canbe used to produce a map, on he fly if needed

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    Zeiss P3 analytical plotter 3- Softcopy Photogrammetric Workstations

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    Softcopy workstations employ digital images, a

    software, a stereovision system, installed on apowerful computer.

    The output is totally digital, and many operations are

    automated. Softcopy is the current technology used for

    photogrammetric measurements.

    Images are captured by a digital camera, or scanningphotographs.

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    Ortho-photos

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    Orthographic projection of photographs.

    Uniform scale, no relief displacement.

    Serve as maps, what is difference between a

    map and a photograph.

    now mainly through digital image processing.

    If tilt displacement is also removed, you get

    ortho-rectified image

    Advantage of orthophoto maps

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    Orthophoto of Washington, DC

    Parallax Summary

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    Interior Orientation

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    146

    Exterior Orientation

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    Y Parallax

    Y parallax is present in many stereo pairs. It is the difference in

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    Y parallax is present in many stereo pairs. It is the difference in

    perpendicular distances between two images of a point from the

    vertical plane which contains the air base. It can be caused by one or

    both images being tiltedwith respect to an exterior coordinate system.

    Tilt can occur as a result of roll, pitch and yaw of an aircraft. Y parallax

    can also be caused by a variation in flying heights, or if images are

    printed at slightly different scales. Finally, Y parallax can occur if the

    viewer lines up the images incorrectly.

    In small amounts, Y parallax can cause eyestrain, however, the brain

    compensates and the 3-D stereo-model remains viewable. In large

    amounts, Y parallax makes stereo viewing of an image pair

    impossible..

    Y Parallax (Contd..)

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    The Y-parallax is caused due to the following reasons:

    Unequal flying height

    Photographic tilt

    Misalignment of flight line

    Misalignment of stereoscope

    Great difference in parallax between adjacent images (in highly

    mountainous/rugged terrain)

    Y Parallax (Contd..)

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    Y Parallax (Contd..)

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    Y Parallax (Contd..)

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    Aerial Photography mission

    When a mapping project requires aerial photographs, one of the first tasks

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    is to select the photo scale factor, type of the lens to used, type of the to be

    used and the desired overlap for stereo viewing. Forward overlap usually

    around 60%, while sideways overlap is around 20%. Furthermore, the date,

    time and season of photography should be considered for light condition

    and shadow effect.

    If the required scale is defined, the following parameters can be

    determined.

    Flying height required above the terrain.

    Ground coverage of a single photograph.

    Number of photo required along a flight line.

    Number of flight lines required.

    Mission Planning:

    Aerial Photography mission

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    Mission Planning:

    Inpu ts for Fl ight Plann ing

    1. Area extents (lat / long)

    2. Scale of photography

    3. Focal length

    4. Format size

    5. Forward and lateral overlaps

    6. Average terrain heights

    Software used for Fl ight Plannin g

    - World Wide Mission Planning

    Navigation

    - Computer Controlled Navigational System with GPS

    Flight Planning

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    Successofphotogrammetricprojectdepends onacquisitionof

    goodqualitypictures

    Due to weather and ground conditions, time frame for

    photographyislimited

    Reflightsareexpensiveandcauseslong delaysonprojectMissionmustbecarefullyplannedand executedaccordingto

    flightplan

    Consists of flight map, (where photos should be taken) and

    specifications

    E h h t ti ll th

    Stereo pair

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    Eachphotocoverspartiallythesamearea

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    Overlaps

    F d l E dl

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    Commonareacoveredbytwosuccessive photosofthesameflightlineorstrip Usually60%5%

    LateraloverlaporSidelap

    Commonareacoveredbytwoadjacentflight Lines.

    About25-30%10% (generally30%)

    Forward overlap or Endlap

    OverlapDirection of

    flight

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    Overlap

    Forward

    overlap/Endlap

    Lateral

    overlap/Sidelap

    Flight lines

    Flight Plan

    What the aircrew has to do as indicated

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    Whattheaircrewhastodoasindicated

    byflightlines

    Thedesignofaerialphotographyflight inorder

    toobtaindesiredphotosata certainscale, i.e.,howtheaircrewwill fly(wheretoputtheflight

    lines,how high,etc.)

    Wh i j t ?

    Flight Planning: Critical Issue

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    Where is project area?

    How many photos needed to cover the area atdesired scale and resolution.

    When is the target in desirable condition?

    Is stereo viewing necessary?Are quantitative reflectance standards

    necessary?

    Rules in determining flight line direction

    Generally follows four cardinal directions East-

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    Generally follows four cardinal directions East

    West(E-W)orNorth-South(N-S) Shouldbealongthelongerdimensionofthe area

    If over mountain ridges or valleys, go along the

    direction of the features to maintain an almost

    constant scale; if a flight line crosses mountains,

    scale will be smaller in the valley than in the

    mountains

    Drift and Crab

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    Drift Crab

    Drift

    Drif t is the lateral shift of the aircraft from the

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    Drif t is the lateral shift of the aircraft from the

    flightline; this may be caused by pilot error or theeffect of wind on the aircraft.

    Crab

    Crab occurs when the aircraft is not oriented with

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    Craboccurs when the aircraft is not oriented with

    the flightline; photo edges are not parallel to theflightline and it usually occurs when the pilot is

    trying to compensate for a cross wind and orients

    the plane into the wind to maintain the flightline.

    wind coming from the South will require an

    Crab (Contd..)

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    airplane flying East to use a crab angle into the

    wind in order to fly due East.

    For example, the crab angle may be determined

    to be 3 degrees, and the airplane will need to fly a

    heading of 93in order to achieve a course along90East.

    This will affect the camera on board the aircraft.

    The crab angle will need to be compensated for in

    order to photograph along straight lines on theground

    Direction of Flight Lines

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    AB

    Required Data for Flight Planning

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    ProjectareaboundaryCamerafocallength3.5,6,or12

    Photoformatsizestandardis9or23cm

    Photoscale

    Overlaprequirements(inpercentage)

    percentageofendlaporsidelapTo be moreeconomical

    Leastnumberofexposures

    Leastnumberofflightlines

    Flight Planning Computations

    Fl i h i ht

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    FlyingheightDistancebetweenexposuresorAirbase(B)Distancebetweenflightlines

    TotalnumberofexposuresFlyingheightabovemeansealevelofeachflightline

    Totaltimeneededforphotography

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    Typical Flight Plan

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    Flying Height

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    Hmgef*sp

    Distance Between Exposures

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    Distance Between Exposures

    Example:

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    Example:

    Given:scale=1:15,000

    f.o.=60%

    s.l.=30%

    s=9=23cm

    Required: De

    Distance Between Exposures

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    Solution:

    De(15,000)(23)(1- 0.60)

    De138,000cm1,380m

    De1.38km

    Distance Between Flight Lines

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    DflDf S(1s.l.)Where:

    S = equivalent ground length of the photoformatsize (s)

    S = (sp)(s)

    s.l. = sidelap (in decimals)

    s = photoformat size

    sp= photoscale factor

    Distance Between Exposures

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    Example:

    Given:

    scale=1:15,000f.o.=60%

    s.l.=30%

    s=9=23cm

    Required: Df

    Solution:

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    Df (15,000)(23)(1 0.30)Df 241,500 cm 2,415 m

    Df 2.42 km

    Total Number of Exposures

    total number of exposures

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    total number of exposures

    (number of exposures per flight line) *

    (number of flight lines)

    Total Number of Exposures

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    Flying Height of Each Flight Line(above Mean Sea Level)

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    HmslHmge+m.g.e

    Total Time of Photography

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    Total Time of Photography

    Example:

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    Given:scale=1:15,000

    f.o.=60%

    s=9=23cm

    averagevelocityofaircraft=300kph

    20exposuresperflightline

    10flightlines

    Required: t

    Total Time of Photography

    Solution:

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    De(15,000)(23)(10.60)

    De138,000cm1,380m1.38km

    No. of Photographs

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    No. of Photographs

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    No. of Photographs

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    No. of Photographs

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    It is an array of overlapping aerial photographssystematically assembled to form a continuous

    AERIAL MOSAICS

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    systematically assembled to form a continuous

    pictorial representation of a terrainPlanning purpose.

    It provides the overview of the terrain- nature and

    distribution of the materials and features occupyingthe terrain.

    Scale variation from photo to photo will be knowncausing gap in the overlap

    A mosaic annotated with local information onrivers,villages etc helps in knowing about thegeographic position of the area interpreted.

    Types of mosaics

    Uncontrolled mosaic: The photographs are

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    oriented in to a position by matching correspondingimages on adjacent photos.

    Semi-controlled mosaic: This is a compilation ofphotographs without using rectified photographs

    but using control for positioning of eachphotograph.

    Controlled mosaic: It is a compilation of scaledand rectified photographs assembled to fit plotted

    controlled points.

    Mosaics

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    Thanks for Listening