Chapter 4Chapter 4

PhotogrammetryPhotogrammetry

Introduction to Remote SensingInstructor: Dr. Cheng-Chien Liu

Department of Earth Science

National Cheng-Kung University

Last updated: 23 April 2004

4.14.1 Introduction Introduction

Photogrammetry:Photogrammetry:• Measurements• Maps• Digital elevation models• Other derived products

Photogrammetry Photogrammetry • Where• What areal extent

4.14.1 Introduction (cont.)Introduction (cont.)

SubjectsSubjects• Determining horizontal ground distances and

angles from measurements made on a vertical photograph

• Determination of object height from relief displacement measurement

• Determination of object heights and terrain elevations by measurement of image parallax

• Use of ground control points

4.14.1 Introduction (cont.)Introduction (cont.)

Subjects (cont.)Subjects (cont.)• Generations of maps in stereoplotters

• Generation of orthophotographs and digital elevation models.

• Preparation of a flight plan to acquire aerial photography.

• Application of soft copy or digital photogrammetry.

4.2 Geometric elements of 4.2 Geometric elements of a vertical photographa vertical photograph

Photogrammetry Photogrammetry Vertical photographsVertical photographs• Unintentional tilts: <10 (<30)

Fig4.1Fig4.1• Basic geometric elements of a vertical photo• L: the camera lens exposure station• f: the lens focal length• X-axis: the forward direction of flight• Y-axix: 900 counterclockwise from the positive x-axis• O: the ground principal point• ABCDE abcde a’b’c’d’e’• The x y photocoordinates

4.2 Geometric elements of 4.2 Geometric elements of a vertical photograph (cont.)a vertical photograph (cont.)

Measurement of photocoordinatesMeasurement of photocoordinates• Triangular engineer’s scale rudimentary problem

Metric scale• Glass scale built-in magnifying eyepieces (Fig 4.2)• Coordinate digitizer• Comparator mono (Fig 4.3)

stereoPrecision: 1~5 m

4.2 Geometric elements of 4.2 Geometric elements of a vertical photograph (cont.)a vertical photograph (cont.)

Sources of errorSources of error• Lens distortion• Atmospheric refraction • Earth curvature• Failure of the fiducial axes to intersect at the

principal pt.• Shrinkage or expansion

Usually, correct this errorSlight tilt outweigh other sourcesExample 4.1: treat it as the problem of exchange rate

4.3 Determining horizontal ground 4.3 Determining horizontal ground lengths, directions, and angles from lengths, directions, and angles from

photo coordinatesphoto coordinates Fig 4.4(a). Displacement of terrain pointsFig 4.4(a). Displacement of terrain points

Fig 4.4(b). Distortion of horizontal angles Fig 4.4(b). Distortion of horizontal angles measured on photographmeasured on photograph• Relief displacement

The datum plane: A΄B΄ a΄b΄Terrain points AB ab

• a΄b΄: the accurate scaled horizontal length and orientation of the ground line AB.

• Angle distortion: b΄c a΄ bca.b΄oa΄= boa (no distortion)

4.3 Determining horizontal ground 4.3 Determining horizontal ground lengths, directions, and angles from lengths, directions, and angles from

photo coordinates (cont.)photo coordinates (cont.) Fig 4.5Fig 4.5

• determination of ground coordinates• ∵△ LOAA΄~△ LOAa΄ ∴ XA=(H-hA)xa/f

Likewise: XB=(H-hB)xb/f• ∵△ LA΄A~△ La΄a ∴YA=(H-hA)ya/f

Likewise: YB=(H-hB)yb/f• AB=[(XA-XB)2+(YA-YB)2]1/2

Example 4.2Example 4.2

4.3 Determining horizontal ground 4.3 Determining horizontal ground lengths, directions, and angles from lengths, directions, and angles from

photo coordinates (cont.)photo coordinates (cont.)

Fig 4.6Fig 4.6• determination of line length and direction from

ground coordinates

Example 4.3Example 4.3

4.4 Relief displacement of 4.4 Relief displacement of vertical featuresvertical features

Fig 4.7: the radial nature of relief Fig 4.7: the radial nature of relief displacementdisplacement• Relief displacement radial distance

Fig 4.8Fig 4.8• geometric components of relief displacement.• ∵△ AA΄A΄΄~△ LOA΄΄

∴ D/h = R/H, d/r = D/R∴ h=dH/r

Example 4.4Example 4.4• relief displacement height

4.4 Relief displacement of 4.4 Relief displacement of vertical features (cont.)vertical features (cont.)

Premise:Premise:• Truly vertical photo• Accurate H• Clearly visible objects• Precise location of the principal point• Accurate measurement technique

Correcting the image positions of terrain Correcting the image positions of terrain points appearing in a photographpoints appearing in a photograph• Example 4.5

4.5 Image Parallax4.5 Image Parallax

ParallaxParallax• Principle: moving train viewing window

relative movement distance

• Fig 4.9: Parallax displacements on overlapping vertical photographs.

• Conjugate principal points the flight axis (Fig 4.10)

• Parallax: pa= xa-xa΄

4.5 Image Parallax (cont.)4.5 Image Parallax (cont.)

Fig 4.11Fig 4.11• parallax relationships on overlapping vertical

photos.

• Air base

• Parallax equation

• Example 4.6

• Difference in elevation

4.6 Parallax measurement4.6 Parallax measurement

In example 4.6In example 4.6• parallax 2 measurements required

(cumbersome)

Fig 4.12: single measurement Fig 4.12: single measurement parallaxparallax• Stereopair photographs fasten down with

flight aligned p=x-x΄=D-d single measurement

• a and a΄ are identifiableDifficult to identify if the tone is uniform

4.6 Parallax measurement (cont.)4.6 Parallax measurement (cont.)

EmploymentEmployment• Fig 4.13: Floating mark principle

Half marksLeft one fixed and right one moves along the fight direction fuse

together one mark floating

• Parallax bar:p=r+C where r= the parallax bar reading C=constantDetermination of c: given p, measure r C = p - rC = Ci

Usually use the two principal points

• Example 4.7

4.6 Parallax measurement (cont.)4.6 Parallax measurement (cont.)

Parallax Wedge (Fig 4.16)Parallax Wedge (Fig 4.16)• Constitution: 2 converging lines on a transparent sleet• Can be thought of as a series of parallax bar reading• Fig 4.17 determination of the height of a tree using a

parallax wedge• Example 4.8• Measure absolute parallax

4.7 Ground control for aerial 4.7 Ground control for aerial photographyphotography

Ground control:Ground control:• Horizontal

• Vertical

• GPS promising

• Accuracy is essential

• Cultural features, e.g. road intersection

• Ground survey artificial target premarked

4.8 Use of ground control in 4.8 Use of ground control in determining the flying height and air determining the flying height and air

base of aerial photographsbase of aerial photographs Flying height determinationFlying height determination

• Altimeter approximate H.• S= f /(H-h)

Example 4.9

• Ground control HGiven ground length AB

elevations hA, hB

focal length f.

photocoordinates (xa, ya).(xb, yb) eg. (4,1) (4,4) HIteration: H2=AB (H1-hAB) /AB1 +hAB

where hAB: the average elevation of the two end points of ABExample 4.10

4.8 Use of ground control in 4.8 Use of ground control in determining the flying height and air determining the flying height and air

base of aerial photographs (cont.)base of aerial photographs (cont.) Air Base determinationAir Base determination

• Ground control BGiven H & one vertical control point eq(4.10) BExample 4.11Given two control points BExample 4.12

4.9 Stereoscopic plotting instruments4.9 Stereoscopic plotting instruments

Photogrammetry Photogrammetry topographic mapstopographic maps StereoplottersStereoplotters

• Concept:Stereopair photo: terrain ray lens image planeStereoplotter: photos ray terrain model 3D view

• Three components1. A projection system2. A viewing system3. A measuring and tracing system

• Fig 4.18: a direct optical projection plotterImage tracing table stereoview of terrain modelRelative orientation absolute orientation

4.9 Stereoscopic plotting instruments 4.9 Stereoscopic plotting instruments (cont.)(cont.)

Stereoplotters (cont.)Stereoplotters (cont.)• Fig 4.19: three projectors 2 adjacement

stereopairs to be oriented at once

• Anaglyphic viewing system.Color filter red, cyanOnly for panchromatic photo

• Polarized platen viewer (PPV)Polarizing filter

• Stereo image alternator (SIA)Rapidly alternate the projection of the two photos.

4.9 Stereoscopic plotting instruments 4.9 Stereoscopic plotting instruments (cont.)(cont.)

Tracing table platenTracing table platen• Floating mark raise and low

• Platen table height terrain elevations

• Mapping features pencil

• Compile contours

4.9 Stereoscopic plotting instruments 4.9 Stereoscopic plotting instruments (cont.)(cont.)

Viewing the photographs in stereo Viewing the photographs in stereo through a binocular systemthrough a binocular system• Mechanical or optical-mechanical projection

plotters.

• Fig 4.20

• Coordinatiograph

• Electronic image correlator

• Fig 4.21: analytical stereoplotter

4.10 Orthophotos4.10 Orthophotos

OrthophotosOrthophotos• No scale, tile relief distortions Photomaps• Best of both worlds• Input to GIS• Digital format

Generation Generation analog orthophotosanalog orthophotos• Differential rectification (Fig 4.22)• Orthophotoscopes• Orthophoto negative

4.10 Orthophotos (cont.)4.10 Orthophotos (cont.)

Fig 4.23Fig 4.23• an early version of a direct optical projection

orthophotoscope

• Principle of operation

4.10 Orthophotos (cont.)4.10 Orthophotos (cont.)

Topographic orthophotomapTopographic orthophotomap• Fig 4.24: operating principle of direct optical

projection• Fig 4.25:contour line overlay orthophoto

orthophotoscope• Fig 4.26a: contour map• Fig 4.26b: 3-D perspective view of the terrain• Stereomates

Fig 4.27: an orthophoto and a corresponding stereomate that may be viewed stereoscopically.

4.11 Flight planning4.11 Flight planning

Why need new photographs?Why need new photographs?• Outdated• Wrong season• Inappropriate scale• Unsuitable film type

Planning the flightPlanning the flight• Weather clear weather beyond control

Multi-task in a single clear day

• Time 10am~2pm illumination max shadow min.

4.11 Flight planning (cont.)4.11 Flight planning (cont.)

Planning the flight (cont.)Planning the flight (cont.)• Geometric aspects

• f• Format size• S• Area size• havg

• Overlap• Side lap• Ground speed

Example 4.13Example 4.13

• H΄• Location, direction,

number of flight lines• Time interval• Number of exposures• Total number of

exposures

4.12 Soft copy photogrammetry4.12 Soft copy photogrammetry

Distinctions between traditional analog Distinctions between traditional analog systems and digital systemssystems and digital systems• Photographs digital raster images

• Mathematical modeling (computer-based environment)

Sources: digitized photos, digital Sources: digitized photos, digital cameras, electro-optical scanners, …cameras, electro-optical scanners, …

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