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    Introduction toPhotogrammetry

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    What is Photogrammetry?

    The science, art and technology ofobtaining reliable measurements, maps,

    DEMs and other derived products fromphotographs emphasis on quantitative measurements


    area height


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    Background ofPhotogrammetry

    1856 - Toumachon ascends in a balloon (nearParis) and takes the first aerial photo

    Aerial photos are interpreted for mappingpurposes

    has become a profession some features are clearly visible, while others are

    more difficult to recognize

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    Background ofPhotogrammetry

    Interpretation is based on extractinginformation from shape, size, patterns,

    shadow, grey tones, colour, texture andthrough context and comparison withcontiguous areas

    black and white (panchromatic)colour (Infrared)

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    Advantages of AerialPhotographs

    Synoptic views of large areas

    stop action capabilities

    permanent storage

    allows spatial relationships to be

    determined which are not possible fromthe ground

    cost effective

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    Tone/Colour- distinguishable variationsin shades of black to white or colour

    can distinguish many more colours thanshades of grey

    Resolution- ability of the entire system

    to create a sharply defined image may be discussed in terms of camera lens,

    ground resolution

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    OrderGeometric Arrangement

    Size- can be used for judging thesignificance of objects and features

    both relative and absolute sizes important

    Shape- aids in identification

    man made - tend to have straight edges natural - tend to have irregular shapes

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    OrderSpatial Arrangement

    Texture- frequency of change andarrangement of tones visual inspection of smoothness or


    e.g. Water typically smooth, grass ismedium texture, and forest is rough

    there are always exceptions

    Pattern- spatial arrangement of objects

    grid network of street, drainage patterns

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    OrderLocational or Postional

    Site - how objects are arranged withrespect to one another or to terrain

    features aspect, topography, geology, soil,

    vegetation and cultural features

    Association - some objects arecommonly identified with other featuresand tend to confirm the existence ofanother

    helpful for manmade installations

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    Height- provides detail about many featuresand is useful for analytical studies

    height of trees depth of an excavation

    Shadow- may enhance or inhibitinterpretation

    tree identification can be enhanced by theshadows that are recorded

    geologist prefer a low sun angle

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    Aids and Techniques forViewing

    collateral material

    image analysis keys

    field verification

    handling of imagery

    stereoscopic viewing

    use of multiple images

    convergence of evidence

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    How Aerial Photos areCollected

    A study area is identified and the airplaneis sent to record a series of successive

    photosflight lines - are the straight lines ofplanned flight

    nadir line - actual path of the plane tracedon the ground

    end lap - over lap between successivephotos along the flight line ensures complete coverage

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    Flight Lines

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    Example of Vertical Air Photo

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    Low and High Oblique

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    High Oblique Aerial Photo

    Petaluma River, San Francisco

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    x w u rPhoto

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    Advantages of Vertical OverOblique Aerial Photographs

    Uniform scale

    directions (bearing or azimuth) can be

    performed in the same manner as a mapeasier to interpret

    tall buildings less likely to mask other objects

    minimum of mathematical correction req.stereoscopic study is more effective

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    Advantages of Oblique OverVertical Aerial Photographs

    Covers more ground at the same altitude andfocal length

    may be more effective if clouds are presenthave a more natural view because silhouettesare visible

    objects not visible on vertical may be visiblefeature elevations are more accurate

    may use inexpensive cameras

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    Photographic CoordinateSystem

    x-y axis defined by connecting lines ofopposite fiducial marks

    the point where the x and y axes cross isthe origin

    x-axis assigned to fiducial axis most

    coincident with the line of flightprogresses in a positive fashion from theorigin in the direction of flight

    negative in opposite direction

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    Photographic CoordinateSystem

    y-axis is perpendicular to the x-axis

    intersection of axes is photo-coordinate

    originorigin coincides with the principle point(ideally)

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    Photo Centres

    Most aerial photos are not perfectly vertical

    three photo centres

    principle point nadir


    each is important in terms of different types of

    geometric distortions

    if perfectly vertical, all three centres wouldcoincide at one point (principle point)

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    Photo Centres

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    Principle Point

    Geometric centre of the the photo

    occurs at the intersection between theprojection of the optical axis and theground

    defined by the intersection of lines

    drawn between opposite fiducial marks

    defines the origin of the x and y axes inthe photo coordinate system

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    also known as the vertical or plumb point

    intersection between the plumb line directly

    beneath the camera centre and the ground atthe time of exposure

    unlike the principle point, there are no markson the photo to locate the nadir

    locating nadir requires stereoscopic plottingtechniques and expensive instruments andground control information

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    Relief displacement is radial from nadir so tallbuildings can be used as an indicator of nadir

    by projecting lines along the displacementedges of those features

    if photo not perfectly vertical, the nadir andprinciple point have different positions

    nadir is always on the down side of the tiltedphotograph

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    The point halfway between the principlepoint and the nadir on a line joining

    those two pointsis the focus of tilt displacement

    on a truly vertical photo, there would be

    no tilt displacement only reliefdisplacement and radial displacement

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    Radial Displacement

    On vertical photos, is the apparent leaningout or displacement of the top of any objecthaving height in relation to its base

    direction of displacement is radial from theprinciple point on a true vertical photo orfrom the isocentre on a vertical photograph

    distorted by tilt of the platform

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    Relief Displacement

    Displacement due to variation in terrainelevation

    an increase in elevation causes features in thephoto to be displaced radially outward from theprinciple point and nadir

    increases as distance from pp increases

    increases as elevation increases

    decreases with increases of flying height

    no relief displacement at principle point

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    Relief Displacement

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    Tilt Displacement

    Displacement due to tilt of the platformrelative to the ground

    degree of displacement increases awayfrom the isocentre

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    Tilt Displacement

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    States that one unit of distance on thephoto represents a specific number ofunits on the ground

    unit equivalent scale e.g. 1mm on photo represents 25m on


    representative fraction e.g. 1:50 000 (unitless)

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    Large vs small scale: Large scale = small area with lots of detail

    Small scale = larger area with less detailMethod for determining scale measure distance on photo and compare

    to actual ground distance

    must convert to same units photo distance/ground distance

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    Example of Large Scale

    This is a 1:5000 air photo

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    Example of Small Scale Photo

    This is a 1:50000

    air photo

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    Scale Variation

    Within the same photo, there isgeometric distortion

    all points on a map are depicted in theirtrue planimetric relationships

    all points on a photo are not depicted in

    their true planimetric relationshipphoto results from projecting convergingrays through a common point in a camera


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    Calculating Scale

    S = d/D


    S = photo scale

    d = photo distance

    D = ground distance

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    Example Calculation

    If you are given an aerial photographwith a scale of 1:5000 and a building is

    measured with a width of 15.3 mm,what would the width of the building beif measured on the ground?

    S = d/DAnswer: 76.5 m

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    Calculating Scale

    S = f/H


    S = photo scale (RF)f = camera focal length

    H = flying height above terrain

    H = H - hh = terrain elevation

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    Example Calculation

    If the scale of a photo is 1:15000 and avertical photograph is taken from a

    flying height of 2780m above mean sealevel, what is the focal length of thecamera, if the terrain is flat and locatedat an elevation of 500m?

    S = f/H

    Answer: 152 mm

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    Ground Coverage

    Function of:

    camera format size

    focal length

    flying height above terrain

    format size is the actual size of the imagerecorded on film

    larger format = larger ground coverageshorter focal length = larger ground cov.

    Lower flying height = less ground coverage