Chapter 4 Photogrammetry Introduction to Remote Sensing Instructor: Dr. Cheng-Chien Liu Department of Earth Science National Cheng-Kung University Last updated: 23 April 2004

4.1 Introduction Photogrammetry: Photogrammetry  Measurements Maps Digital elevation models Other derived products Photogrammetry  Where What areal extent

4.1 Introduction (cont.) Subjects 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

4.1 Introduction (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 a vertical photograph Photogrammetry  Vertical photographs Unintentional tilts: <10 (<30) Fig4.1 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 a vertical photograph (cont.) Measurement 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) stereo Precision: 1~5 mm

4.2 Geometric elements of a vertical photograph (cont.) Sources 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 error Slight tilt  outweigh other sources Example 4.1: treat it as the problem of exchange rate

4.3 Determining horizontal ground lengths, directions, and angles from photo coordinates Fig 4.4(a). Displacement of terrain points Fig 4.4(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)

4.3 Determining horizontal ground lengths, directions, and angles from photo coordinates (cont.) Fig 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 4.2

4.3 Determining horizontal ground lengths, directions, and angles from photo coordinates (cont.) Fig 4.6 determination of line length and direction from ground coordinates Example 4.3

4.4 Relief displacement of vertical features Fig 4.7: the radial nature of relief displacement Relief displacement  radial distance Fig 4.8 geometric components of relief displacement. ∵△AA΄A΄΄~△LOA΄΄ ∴D/h = R/H, d/r = D/R ∴h=dH/r Example 4.4 relief displacement  height

4.4 Relief displacement of vertical features (cont.) Premise: Truly vertical photo Accurate H Clearly visible objects Precise location of the principal point Accurate measurement technique Correcting the image positions of terrain points appearing in a photograph Example 4.5

4.5 Image Parallax Parallax 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.) Fig 4.11 parallax relationships on overlapping vertical photos. Air base Parallax equation Example 4.6 Difference in elevation

4.6 Parallax measurement In example 4.6 parallax  2 measurements required (cumbersome) Fig 4.12: 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

4.6 Parallax measurement (cont.) Employment Fig 4.13: Floating mark principle Half marks Left 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=constant Determination of c: given p, measure r  C = p - r C = S Ci Usually use the two principal points Example 4.7

4.6 Parallax measurement (cont.) 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 photography 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 determining the flying height and air base of aerial photographs Flying height determination Altimeter  approximate H. S= f /(H-h) Example 4.9 Ground control  H Given ground length AB elevations hA, hB focal length f. photocoordinates (xa, ya).(xb, yb) eg. (4,1)  (4,4)  H Iteration: H2=AB (H1-hAB) /AB1 +hAB where hAB: the average elevation of the two end points of AB Example 4.10

4.8 Use of ground control in determining the flying height and air base of aerial photographs (cont.) Air Base determination Ground control  B Given H & one vertical control point eq(4.10)  B Example 4.11 Given two control points  B Example 4.12

4.9 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 4.18: a direct optical projection plotter Image  tracing table  stereoview of terrain model Relative orientation  absolute orientation

4.9 Stereoscopic plotting instruments (cont.) Stereoplotters (cont.) Fig 4.19: three projectors  2 adjacement stereopairs to be oriented at once 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.

4.9 Stereoscopic plotting instruments (cont.) Tracing table platen Floating mark  raise and low Platen table height  terrain elevations Mapping features  pencil Compile contours

4.9 Stereoscopic plotting instruments (cont.) Viewing the photographs in stereo through a binocular system Mechanical or optical-mechanical projection plotters. Fig 4.20 Coordinatiograph Electronic image correlator Fig 4.21: analytical stereoplotter

4.10 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 (Fig 4.22) Orthophotoscopes Orthophoto negative

4.10 Orthophotos (cont.) Fig 4.23 an early version of a direct optical projection orthophotoscope Principle of operation

4.10 Orthophotos (cont.) Topographic 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 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.

4.11 Flight planning (cont.) Planning the flight (cont.) Geometric aspects f Format size S Area size havg Overlap Side lap Ground speed Example 4.13 H΄ Location, direction, number of flight lines Time interval Number of exposures Total number of exposures

4.12 Soft copy photogrammetry Distinctions between traditional analog systems and digital systems Photographs  digital raster images Mathematical modeling (computer-based environment) Sources: digitized photos, digital cameras, electro-optical scanners, … Trend from now to future