PhotogrammetryPhotogrammetry

Introduction to Remote SensingInstructor: Dr. Cheng-Chien Liu

Department of Earth Sciences

National Cheng Kung University

Last updated: 4 November 2004

Chapter 6Chapter 6

OutlineOutline

IntroductionIntroduction Basic principlesBasic principles

• Aerial photographs• Photographic scale• Area measurement• Relief displacement（高差位移）• Image parallax （影像視差）• Ground control• Mapping• Flight planning

IntroductionIntroduction

PhotogrammetryPhotogrammetry• Definition

The science and technology of obtaining spatial measurements and other geometrically reliable derived products from photographs

• MeasurementDistance, area, elevations

• ProductDigital elevation modelsOrthophotosThematic GIS dataOther derived products

Approach of this bookApproach of this book• Hardcopy softcopy• Aerial photos spaceborne images

Introduction (cont.)Introduction (cont.)

SubjectsSubjects• Determining horizontal ground distances and angles from

measurements made on a vertical photograph• Using area measurements made on a vertical photograph to

determine the equivalent areas in a ground coordinate system• Quantifying the effects of relief displacement (高差移位 ) on

vertical aerial photographs• 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• Mapping with aerial photographs• Preparation of a flight plan to acquire aerial photography

Aerial photographsAerial photographs

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

Fig 3.6Fig 3.6• 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

Aerial photographs (cont.)Aerial photographs (cont.)

Measurement of photocoordinatesMeasurement of photocoordinates• Hardcopy

Triangular engineer’s scale rudimentary problemMetric scale

Glass scale built-in magnifying eyepiecesCoordinate digitizerComparator mono

• Softcopy

Affine coordinate transformationAffine coordinate transformation Source of errorSource of error

Photographic scalePhotographic scale

Photographic scale = map scalePhotographic scale = map scale• Large scale small scale

Eq. 3.1: Eq. 3.1: SS = = dd / / DD Ex 3.1Ex 3.1 Eq. 3.2: Eq. 3.2: SS = f / = f / HH'' Fig 3.7Fig 3.7 Eq. 3.4: Eq. 3.4: SS = = ff / ( / (HH – – hh)) Ex 3.2Ex 3.2 Eq. 3.5: Eq. 3.5: SSavgavg = = ff / ( / (HH – – hhavgavg))

Photographic scale (cont.)Photographic scale (cont.)

Vertical photo Vertical photo mapmap• Perspective projection (透視投影 )

orthographic projection

• Fig 3.8

Relief displacementRelief displacement

Area measurementArea measurement

AccuracyAccuracy• Measuring device

• Image scale variation due to relief

• Tilt in the photography

Simple waySimple way• Ex 3.4

• Ex 3.5

• Ex 3.6

Relief displacementRelief displacement

CharacteristicsCharacteristics• Lean away from the center of the photograph• Fig 3.12

Correcting for relief displacementCorrecting for relief displacement• Fig 3.14(a). Displacement of terrain points• Fig 3.14(b). Distortion of horizontal angles measured on

photographRelief 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)

• Ex 3.8

Image parallaxImage parallax

CharacteristicsCharacteristics• Principle: moving train viewing window

relative movement distance

• Fig 3.15: Parallax displacements on overlapping vertical photographs.

• Conjugate principal points the flight axis (Fig 3.16)

• Parallax: pa= xa-xa΄

Image parallax (cont.)Image parallax (cont.)

Object height and ground coordinate locationObject height and ground coordinate location• Fig 3.17

Parallax relationships on overlapping vertical photosAir base: B = L - L΄

• Parallax equationExample 3.9Difference in elevation

Parallax measurementParallax measurement• In example 3.9

Parallax 2 measurements required (cumbersome)

• Fig 3.18: single measurement parallaxStereopair photographs fasten down with flight aligned p=x-x΄=D-d single

measurementa and a΄ are identifiable

Difficult to identify if the tone is uniform

Image parallax (cont.)Image parallax (cont.)

Parallax measurement in hardcopy systemParallax measurement in hardcopy system• Fig 3.19: floating-mark principle• Demonstration of stereoscope

Fig 3.21: how to take readings

• Ex 3.10 Parallax measurement in softcopy systemParallax measurement in softcopy system

• Image correlation• Fig 3.22

Reference windowSearch window

• Not constrained to the assumption of parallax equations• Collinearity equations

xyz XYZ (XL, YL, ZL) (, , )Aerotriangulation

Ground controlGround control

Ground controlGround control• Definition

Refers to physical points on the ground whose ground positions are known with respect to some horizontal coordinate system and/or vertical datum

• Horizontal• Vertical• GPS promising• Accuracy is essential• Cultural features, e.g. road intersection• Ground survey artificial target premarked

MappingMapping

Stereoscopic plotting instrumentsStereoscopic plotting instruments• Photogrammetry topographic maps• Stereoplotters

Concept: Stereopair photo: terrain ray lens image plane Stereoplotter: photos ray terrain model 3D view

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

Fig 3.23: a direct optical projection plotter Image tracing table stereoview of terrain model Relative orientation absolute orientation

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

• Polarized platen viewer (PPV) Polarizing filter

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

OrthophotosOrthophotos

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

Generation Generation analog orthophotosanalog orthophotos• Differential rectification• Orthophotoscopes• Orthophoto negative

Generation Generation digital orthophotosdigital orthophotos

Coordinate transformationsCoordinate transformations

2D conformal coordinate transformation2D conformal coordinate transformation• Scale change• Rotation• Translation• Redundancy• Matrix method

3D conformal coordinate transformation3D conformal coordinate transformation 2D projective coordinate transformation2D projective coordinate transformation

• Collinearity equation

Flight planningFlight 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.

Flight planning (cont.)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 3.11Example 3.11

• H΄• Location, direction,

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

exposures

HomeworkHomework

Use your own digital camera to take a Use your own digital camera to take a stereopair. Examine your stereopair stereopair. Examine your stereopair using the stereoscope that we using the stereoscope that we demonstrated in the classroom.demonstrated in the classroom.