Photogrammetry Chapter 6 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 Approach of this book Definition The science and technology of obtaining spatial measurements and other geometrically reliable derived products from photographs Measurement Distance, area, elevations Product Digital elevation models Orthophotos Thematic GIS data 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 Fig 3.6 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 Triangular engineer’s scale  rudimentary problem Metric scale Glass scale  built-in magnifying eyepieces Coordinate digitizer Comparator mono Softcopy Affine coordinate transformation Source of error

Photographic scale Photographic scale = map scale Eq. 3.1: S = d / D 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 Simple way 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 Correcting for relief displacement 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 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) 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 Parallax relationships on overlapping vertical photos Air base: B = L - L΄ Parallax equation Example 3.9 Difference in elevation Parallax measurement In example 3.9 Parallax  2 measurements required (cumbersome) Fig 3.18: single measurement  parallax Stereopair  photographs fasten down with flight aligned p=x-x΄=D-d  single measurement a and a΄ are identifiable 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 Fig 3.21: how to take readings Ex 3.10 Parallax measurement in softcopy system Image correlation Fig 3.22 Reference window Search window Not constrained to the assumption of parallax equations Collinearity equations xyz  XYZ  (XL, YL, ZL)  (w, f, k) Aerotriangulation

Ground control Ground control Definition Horizontal Vertical 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 Concept: Stereopair photo: terrain  ray  lens  image plane Stereoplotter: photos  ray  terrain model  3D view Three components A projection system A viewing system 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

Orthophotos Orthophotos Generation  analog 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 2D conformal coordinate transformation Scale change Rotation Translation Redundancy Matrix method 3D conformal coordinate transformation 2D projective coordinate transformation Collinearity equation

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 Planning the flight Outdated Wrong season Inappropriate scale Unsuitable film type Planning the flight Weather  clear weather  beyond control Multi-task in a single clear day Time 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