Photogrammetriy & Remote sensing Faculty of Applied Engineering and Urban Planning Photogrammetriy & Remote sensing Civil Engineering Department 2nd Semester 2008/2009

Photogrammetry is the science and technology of obtaining reliable measurements, maps, digital elevation models, and other derived products from photographs. Emphasis: quantification of feature location (X, Y, Z) extent (area & volume) Generation of GIS data base maps orthophotos digital elevation models (DEMs)

Topics to be covered Geometric types of airphotos Taking vertical airphotos Geometry of maps vs. airphotos Photographic scale Ground control Area measurement Object height determination from relief displacement from parallax measurement Mapping from photographs stereoplotters Orthophotos Digital photogrammetry

Geometric Types of Airphotos “Vertical” - taken with the optical axis of the camera in a vertical position “Tilted” - unintentional inclination of the optical axis from vertical “Oblique” - optical axis intentionally inclined from vertical high oblique: contains image of horizon low oblique: horizon not visible convergent obliques: sequential overlapping low obliques

Geometric Types of Airphotos

nadir line : The line traced on the ground directly beneath the aircraft during acquisition of photography Most vertical aerial photographs are taken with frame cameras along flight lines, or flight strips. This line connects the image centers of the vertical photographs. Figure 3.1 illustrates the typical character of the photographic coverage along a flight line.

Successive photographs are generally taken with some degree of endlap Successive photographs are generally taken with some degree of endlap. Not only does this lapping ensure total coverage along a flight line, but an endlap of at least 50 percent is essential for total stereoscopic coverage of a project area. Stereoscopic coverage consists of adjacent pairs of overlapping vertical photographs called stereopails. Stereopairs provide two different perspectives of the ground area in their region of endlap. When images forming a stereopair are viewed through a stereoscope, each eye psychologically occupies the vantage point from which the respective image of the stereopair was taken In flight The result is perception is 3D stereomodel.

Successive photographs along a flight strip are taken at intervals that are controlled by the camera intervalometer, a device that automatically trips the camera shutter at desired times. The area included in the overlap of successive photographs is called the stereoscopic overlap area. Typically, successive photographs contain 55 to 65 percent overlap to ensure at least 50 percent endlap over varying terrain, in spite of unintentional tilt.

Figure 3.2 illustrates the ground coverage relationship of successive photographs forming a stereopair having approximately a 60 percent stereoscopic overlap area.

Most project sites are large enough for multiple- flight-line passes to be made over the area to obtain complete stereoscopic coverage. Figure 3.4 illustrates how adjacent strips are photographed. On successive flights over the area, adjacent strips have a sidelap of approximately 30 percent. Multiple strips comprise what is called a block of photographs. Modem aerial surveys usually employ data from the aircraft's precise GPS navigation system to control flight line direction, flight line spacing, and photo exposure intervals.

A given photographic mission can entail the acquisition of literally hundreds of exposures. Quite often, a flight index mosaic is assembled by piecing together the individual photographs into a single continuous picture. This enables convenient visual reference to the area included in each image. See Figure 3.5 in text book , which illustrates such a mosaic.

Photocoordinate Measurement Measurements of photocoordinates may be obtained using any one of many measurement devices. These devices vary in their accuracy, cost, and availability Photocoordinates can also be measured using a coordinate digitizer Such devices continuously display the xy positions of a spatial reference mark as it is positioned anywhere on the photograph. Another option for photocoordinate measurement is the use of a precision instrument called a comparator. A monocomparator can be used to measure very accurate coordinates on one photograph at a time; a stereocomparator can be used for making measurements on stereopairs.

In softcopy photogrammetric operations, individual points in a photograph are referenced by their row and column coordinates in the digital raster representation of the image. The relationship between the row and column coordinate system and the camera's fiducial axis coordinate system is determined through the development of a mathematical coordinate transformation between the two systems. This process requires that some points have their coordinates known in both systems. The fiducial marks are used for this purpose in that their positions in the focal plane are determined during the calibration of the camera, and they can be readily measured in the row and column coordinate system

errors These errors stem from sources such as camera lens distortions, atmospheric refraction, earth curvature, failure of the fiducial axes to intersect at the principal point, and shrinkage or expansion of the photographic material on which measurements are made. Sophisticated photogrammetric analyses include corrections for all these errors.

PHOTOGRAPHIC SCALE

Scale Expressing Scale Engineer’s scale (unit equivalents) 1” = 400’ Representative fraction 1/4800 Ratio 1:4800

Which is the smaller scale: 1:10,000 or 1:120,000 ? Small vs. Large Scale Which is the smaller scale: 1:10,000 or 1:120,000 ?

Photo Scale Ranges Small scale 1:50,000 or smaller Medium scale 1:12,000-1:50,000 Large scale 1:12,000 or larger

Scale Over Flat Terrain

Scale

Scale Over Variable Terrain

Sample Scale Computations

Example Assume a vertical photograph was taken at a flying height of 5000 m above sea level using a camera with a 152-mm-focal-length lens. (a) Determine the photo scale at points A and B, which lie at elevations of 1200 and 1960 m. (b) What ground distance corresponds to a 20.1-mm photo distance measured at each of these elevations? Solution The ground distance corresponding to a 20.1-mm photo distance is

Often it is convenient to compute an average scab for an entire photograph. This scale is calculated using the average terrain elevation for the area imaged. Consequently, it is exact for distances occurring at the average elevation and is approximate at all other elevations. Average scale may be expressed as

A photograph results from projecting converging rays through a common point within the camera lens. Because of the nature of this projection, any variations in terrain elevation will result in scale variation a d displaced image positions. On a map we see a top view of objects in their true relative horizontal positions. On a photograph, areas of terrain at the higher elevations lie closer to the camera at the time of exposure and therefore appear larger than corresponding areas lying at lower elevations. Furthermore, the tops of objects are always displaced from their bases (Figure 3.8). This distortion is called relief displacement and causes any object standing above the terrain to "lean" away from the principal point of a photograph radially.

A photograph results from projecting converging rays through a common point within the camera lens. Because of the nature of this projection, any variations in terrain elevation will result in scale variation a d displaced image positions. On a map we see a top view of objects in their true relative horizontal positions. On a photograph, areas of terrain at the higher elevations lie closer to the camera at the time of exposure and therefore appear larger than corresponding areas lying at lower elevations. Furthermore, the tops of objects are always displaced from their bases (Figure 3.8). This distortion is called relief displacement and causes any object standing above the terrain to "lean" away from the principal point of a photograph radially.

Determining Ground Areas From Photo Measurements Simple scales may be used to measure the area of simply shaped features. For example, the area of a rectangular field can be determined by simply measuring its length and width. Similarly, the area of a circular feature can be computed after measuring its radius or diameter.

Example A rectangular agricultural field measures 8.65 cm long and 5.13 cm wide on a vertical photograph having a scale of 1 :20,000. Find the area of the field at ground level. Solution Ground length = photolength x 1/S = O.0861m x 20,000 = 1730 m Ground width = photo width X 1/S = 0.0513 X 20,000 = 1026 m Ground area = 1730 x 1026 = 1,774,880 ha The ground area d an irrugulary shaped feature is usually determined by measuring the area of the feature on the photograph. The photos, area is then converted to a ground area from the following relationship:

Determining Ground Areas From Photo Measurements Assumptions: (1) Flat terrain (2) Rectangular object Approach: ground area = ground length x ground width

Simple scales may be used to measure the area of simply shaped features. For example, the area of a rectangular field can be determined by simply measuring its length and width. Similarly, the area of a circular feature can be computed after measuring its radius or diameter.

Dot Grid for Area Measurement One of the simplest techniques employs a transparent grid overlay consisting of lines forming rectangles or squares of known area. The grid is placed over the photograph and the area of a pound unit is estimated by counting grid units that fall within the unit to be measured. Perhaps the most widely used grid overlay is a dot grid This grid, composed of uniformly spaced dots, is superimposed over the photo, and the dots falling within the region to be measured are counted. From knowledge of the dot density of the grid, then photo area of the region can be computed.

Dot Grid Computations Example: Flooded area covered by 43 dots Dot density = 10 dot/cm2 grid Photo scale = 1:25,000

The dot grid is an inexpensive tool and its use requires little training. When numerous regions are to be measured, however, the counting procedure becomes quite tedious. An alternative technique is to use either a coordinate digitizer or digitizing tablet These devices are typically interfaced with a computer such that area determination simply involves tracing around the boundary of the region of interest and the area determination simply involves Read out direct y. When photographs are available in soft copy formant , measurement often involves digitizing from a computer monitor using a mouse or other form of cursor control.

Relief Displacement الازاحة الناشئة عن التضاريس

an increase in the elevation of a feature causes its position on the photograph to be displaced radially outward from the principal point. Hence, when a vertical feature is photographed, relief displacement causes the top of the feature to lie farther from the photo center than its base. As a result, vertical features appear to lean away from the center of the photograph.

Relief Displacement Equation Where d = relief displacement r = radial distance on the photograph from the principal point to the displaced image point h = height above datum of the displaced image point H = flying height above the same datum chosen to reference h

Geometric Characteristics of Relief Displacement d increases with distance from principal point (r) d increases with height of object (h) d decreases with increase in flying height (H) at the principal point, r = 0, d = 0 can be outward (for points above datum) or inward (for points below datum) causes objects to appear to lean causes straight lines to appear crooked

Sample Relief Displacement Calculation Given: The top of a 100’ tall tower is located a radial distance of 3.60” from the principal point of a vertical photo taken at a flying height of 3000’ above the ground. Find: The distance (d) the top of the tower is displaced relative to the base of the tower. Solution:

Relief Displacement Equation Where d = relief displacement r = radial distance on the photograph from the principal point to the displaced image point h = height above datum of the displaced image point H = flying height above the same datum chosen toreference h

Anaglyphic stereo image Red lens on right eye Blue lens on left eye

measure parallel to flight line. Image Parallax Definition: The change in position of an image from one photograph to the next, caused by the aircraft’s motion between exposures. Image Parallax Characteristics: • Measured parallel to the flight line • Directly related to the elevation of a ground point • Basis for viewing and measuring in 3-D • Minimum 50% overlap for stereoscopic coverage Measurement: pp = xp - xp’ measure parallel to flight line.

Image Parallax

Using Parallax Measurements to Obtain Elevations Where hP = elevation of any point P H = flying height B = air base (between exposures) f = focal length PP = parallax of point P

Given: A stereopair with the following characteristics: Parallax Example Given: A stereopair with the following characteristics: H = 1200 m B = 600 m f = 152.4 mm Point A has a parallax of 114.06 mm Point B has a parallax of 126.06 mm Find: Elevation of points A and B

Parallax Example Solution:

Measurement of Parallax Monoscopic measurement Stereoscopic measurement Digital image matching

Anaglyphic stereo image Red lens on right eye Blue lens on left eye

Major Sources of Error in Parallax Methods (1) Locating and marking the flight line on photos (2) Errors in coordinate and parallax measurement (3) Print shrinkage (4) Unequal flying heights (5) Tilt (6) Lens distortion Overcome all these through the use of a stereoplotter

Stereoplotters Instruments designed to calculate the 3-D position of objects based on measurements from overlapping photographs Primary use: generation of topographic maps General concept: establish a model in which the orientation of the photographs within the stereoplotter directly corresponds to the geometry of the camera when the photos were taken Components: • Projection system • Viewing system • Tracing/measurement system

Stereoplotter Components Projection System (1) Direct optical projection (2) Mechanical or optical-mechanical projection (3) Projection of a mathematical model based on precise coordinate measurements of images on both the left and right photos forming the stereopair Analytical plotters - use stereocomparator measurements at discrete points (b) Softcopy systems - use digital raster images of all points in overlap area

Stereoplotter Components Viewing System (1) Anaglyphic (blue-green and red filters) (2) Stereo-image alternator (SIA) (3) Polarized-platen viewing (PPV) (4) Binocular eyepieces as part of an optical train of lenses, mirrors, and prisms (5) Computer monitor with provision for stereo

Stereoplotter Components Tracing and Measuring System (1) Platen and analog measurement (2) Hand wheel / foot disk system with digital encoders (3) Electronic image correlation (4) Digital image correlation

Compensates for camera tilt & relief displacement Orthophotography Orthophoto = A photograph showing images of objects in their true orthographic projections Compensates for camera tilt & relief displacement “Best of both worlds”: Properties of maps & photos • Map: uniform scale (even in varying terrain) • Photo: show actual objects (not just symbols)

Digital Elevation Models (DEMs) DEM: A raster grid in which pixel values represent elevations for each grid cell Many possible visual representations of basic DEM Easy to use in a geographic information system Can use to calculate slope, aspect Numerous applications Existing DEMs available for most of the world (at varying resolutions) from USGS & other sources

Digital Photogrammetry • “Softcopy photogrammetry” is another name for “digital photogrammetry” Uses digital images rather than hardcopy images Based on “analytical photogrammetry” (analysis of 3-D models created mathematically rather than through optical or mechanical projection)

Photogrammetric Workstation • Stereoplotting using various viewing systems (e.g., liquid crystal shutters with IR link) DEM preparation Orthophoto production (digital and hardcopy) Direct capture (in 3-D) of GIS data

Photogrammetry: Steps Involved Interior orientation: Reconstruct geometry of camera/film system Determine calibrated focal length of lens Determine exact location (on film) of fiducial marks Determine exact location (on film) of principal point Determine distortion pattern of lens Relative orientation: Reconstruct relative geometry of two images Produces a relative model Not concerned about absolute scale or ground coordinates Hold one photo fixed, adjust other to fit 3 rotations: ω (omega), Φ (phi), Κ (kappa) 3 translations: X, Y, Z (but X doesn’t matter at this step)

Photogrammetry: Steps Involved Strip Formation/Block Formation: Combine multiple models into a strip or block Hold photo 1 fixed Do relative orientation of photo 2 to photo 1 Do relative orientation of photo 3 to photo 2… Absolute orientation: Reconstruct actual photo/ground relationship “Scaling” & “leveling” Coordinate transformation from block coordinates to ground coordinates Need to have control points (both sets of coordinates known) Based on collinearity equations

xP, yP = image coordinates of any point p f = focal length Collinearity xP, yP = image coordinates of any point p f = focal length XP, YP, ZP = ground coordinates of point P XL, YL, ZL = ground coordinates of exposure station L m11...m33 = coefficients of a 3x3 rotation matrix defined by the angles ω, φ, and κ that transforms the ground coordinate system to the image coordinate system

Applications of Collinearity: Space Resection For a single photograph… Given: X, Y, Z (on ground) for 3 control points x, y (on photo) for same 3 points Interior orientation (e.g., focal length) Find: Position of exposure station (XL, YL, ZL) and rotation of camera (ω, Φ, Κ) Using the collinearity equations

Applications of Collinearity: Space Intersection For a pair of photographs… Given: xp, yp (on photo 1) for point P xp, yp (on photo 2) for the same point P Absolute orientation parameters for both photos (XL, YL, ZL and ω, Φ, Κ) Find: Ground coordinates (Xp, Yp, Zp) for point P Using the collinearity equations

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