Chapter 6 Photogrammetry Introduction to Remote Sensing Instructor

Chapter 6 Photogrammetry Introduction to Remote Sensing Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 4 November 2004

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

Introduction § Photogrammetry • Definition v. The science and technology of obtaining spatial measurements and other geometrically reliable derived products from photographs • Measurement v. Distance, area, elevations • Product v. Digital elevation models v. Orthophotos v. Thematic GIS data v. Other derived products § Approach of this book • Hardcopy softcopy • Aerial photos spaceborne images

Introduction (cont. ) § Subjects • 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 photographs § Photogrammetry Vertical photographs • Unintentional tilts: <10 (<30) § Fig 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. ) § Measurement of photocoordinates • Hardcopy v. Triangular engineer’s scale rudimentary problem Metric scale v. Glass scale built-in magnifying eyepieces v. Coordinate digitizer v. Comparator mono • Softcopy § Affine coordinate transformation § Source of error

Photographic scale § Photographic scale = map scale • Large scale small scale § Eq. 3. 1: S = d / D § Ex 3. 1 § Eq. 3. 2: S = f / H' § Fig 3. 7 § Eq. 3. 4: S = f / (H – h) § Ex 3. 2 § Eq. 3. 5: Savg = f / (H – havg)

Photographic scale (cont. ) § Vertical photo map • Perspective projection (透視投影) orthographic projection • Fig 3. 8 § Relief displacement

Area measurement § Accuracy • Measuring device • Image scale variation due to relief • Tilt in the photography § Simple way • Ex 3. 4 • Ex 3. 5 • Ex 3. 6

Relief displacement § Characteristics • Lean away from the center of the photograph • Fig 3. 12 § Correcting for relief displacement • Fig 3. 14(a). Displacement of terrain points • Fig 3. 14(b). Distortion of horizontal angles measured on photograph v Relief displacement Ø The datum plane: A΄B΄ a΄b΄ Ø Terrain points AB ab v a΄b΄: the accurate scaled horizontal length and orientation of the ground line AB. v Angle distortion: b΄c a΄ bca. Ø b΄oa΄= boa (no distortion) • Ex 3. 8

Image parallax § Characteristics • 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. ) § Object height and ground coordinate location • Fig 3. 17 v Parallax relationships on overlapping vertical photos v Air base: B = L - L΄ • Parallax equation v Example 3. 9 v Difference in elevation § Parallax measurement • In example 3. 9 v Parallax 2 measurements required (cumbersome) • Fig 3. 18: single measurement parallax v Stereopair photographs fasten down with flight aligned p=x-x΄=D-d single measurement v a and a΄ are identifiable Ø Difficult to identify if the tone is uniform

Image parallax (cont. ) § Parallax measurement in hardcopy system • Fig 3. 19: floating-mark principle • Demonstration of stereoscope v Fig 3. 21: how to take readings • Ex 3. 10 § Parallax measurement in softcopy system • Image correlation • Fig 3. 22 v Reference window v Search window • Not constrained to the assumption of parallax equations • Collinearity equations v xyz XYZ (XL, YL, ZL) (w, f, k) v Aerotriangulation

Ground control § Ground control • Definition v. 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

Mapping § Stereoscopic plotting instruments • Photogrammetry topographic maps • Stereoplotters v Concept: Ø Stereopair photo: terrain ray lens image plane Ø Stereoplotter: photos ray terrain model 3 D view v Three components 1. A projection system 2. A viewing system 3. A measuring and tracing system v Fig 3. 23: a direct optical projection plotter Ø Image tracing table stereoview of terrain model Ø Relative orientation absolute orientation • Anaglyphic viewing system. v Color filter red, cyan v Only for panchromatic photo • Polarized platen viewer (PPV) v Polarizing filter • Stereo image alternator (SIA) v Rapidly alternate the projection of the two photos

Orthophotos § Orthophotos • • No scale, tile relief distortions Photomaps Best of both worlds Input to GIS Digital format § Generation analog orthophotos • Differential rectification • Orthophotoscopes • Orthophoto negative § Generation digital orthophotos

Coordinate transformations § 2 D conformal coordinate transformation • • • Scale change Rotation Translation Redundancy Matrix method § 3 D conformal coordinate transformation § 2 D projective coordinate transformation • Collinearity equation

Flight planning § Why need new photographs? • • Outdated Wrong season Inappropriate scale Unsuitable film type § Planning the flight • Weather clear weather beyond control v. Multi-task in a single clear day • Time 10 am~2 pm illumination max shadow min.

Flight planning (cont. ) § Planning the flight (cont. ) • Geometric aspects • • f Format size S Area size havg Overlap Side lap Ground speed § Example 3. 11 • • • H΄ Location, direction, number of flight lines Time interval Number of exposures Total number of exposures

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