Photogrammetry
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This document defines photogrammetry as obtaining reliable measurements through photography. It discusses how aerial photographs provide single-point perspective views of the Earth's surface rather than orthogonal maps. Key aspects of photogrammetry covered include preparing maps from overlapping aerial photos, quantitative and qualitative data extraction, aerial camera geometry, scales, distortions, determining object heights from shadows and relief displacement.
Photogrammetry
- 2. DefinitionsDefinitions The science or art of obtaining reliable measurements by means of photography (American Society of Photogrammetry, 1956) The art, science and technology of obtaining reliable information about physical objects and the environment by recording, measuring and interpreting photographic images (American Society for Photogrammetry and Remote Sensing, 1987) The use of photography for accurate measurement is called photogrammetry (simple definition)
- 3. Aerial photographs areAerial photographs are notnot mapsmaps Aerial Photographs are single-point perspective views of the Earth’s surface, whereas maps are orthogonal representations of the surface Sizes shapes and positions of objects are distorted in aerial photographs However, aerial photographs can be used to construct maps and to accurately measure distances, heights and elevations.
- 4. Orthogonal representations of theOrthogonal representations of the surface (MAP)surface (MAP) Figure - 1
- 5. Single-point perspective views of theSingle-point perspective views of the Earth’s surfaceEarth’s surface (AERIAL PHOTOGRAPH)(AERIAL PHOTOGRAPH) Figure - 2
- 6. If the terrain is completely flat and horizontal, the vertical photograph is same as map. If difference in height occur, then relief displacement becomes appearance. Pole AB appears as a point on map and as line on aerial photo. So aerial photo cannot be used as a map.
- 7. Figure - 3
- 8. Then how to prepare maps from aerialThen how to prepare maps from aerial photographs?photographs? To overcome relief displacement, which is shown above, two photographs are taken of the same area at different positions. These photographs are overlapped and viewed stereo viewing devices and using this principle a map can be prepared. Figure – 4
- 9. There are two kinds of photographs used in photogrammetry, aerial and terrestrial. ◦ Aerial photographs are usually acquired from aircraft but can also come from satellites, hot air balloons or even kites. ◦ Terrestrial photographs come from cameras based on the ground (Photo Theodolites), and generally are used in different applications from aerial
- 10. There are two main data extraction methods used for analyzing these photographs: ◦ Quantitative: that is size, length, shape, height, area etc. ◦ Qualitative : geology, vegetation, drainage, land use etc. This section is primarily concerned with the quantitative evaluation of survey photographs.
- 11. The Aerial CameraThe Aerial Camera
- 12. The rays of light pass through one central point in the lens of a survey camera, and the distance between this point and the film is calibrated to decimals of a millimeter. This distance is known as the focal length. The film plane is as flat as possible, often using flattening devices to achieve this. The camera body also carries reference marks known as fiducial marks that define a coordinate measurement axis and allow film stretch to be determined. The camera is also very large, the size of the negatives is 23cm by 23cm which is somewhat bigger than the average snapshot. The cameras are constructed in this manner so that measurements of a high accuracy and precision can be obtained.
- 13. Air-photo terminology Fiducial marks: Marks on the photograph margins used to locate principal point in photo. Nadir point:-The vertical line passing through the perspective centre (camera lens) intersects the negative plane of at nadir point (n). Principal point: The orthogonal projection of the perspective centre on the negative (photo) is the principal point. In other words it is the geometric centre of photograph. Principal distance:- The distance from projection centre to negative plane (c). Iso centre: The angle α is called the angle of the maximum tilt. The point where the line bisecting this angle intersects the negative plane is the isocentre (i). Principal axis:- The perpendicular line from the perspective centre on the negative plane.
- 14. Principal line:- The line joining the nadir point and principal point. Principal plane:- The plane containing perspective centre , nadir point, iso centre, principal point, principal axis and principal line. Iso line:- A line representing the intersection of the plane of a vertical photograph with the plane of an over lapping oblique photograph. Axis of tilt :- A line through the perspective centre perpendicular to principal plane. Conjugate principal point: Point in overlapping photo that is equivalent to principal point of adjacent photograph. Photo base: Distance between principal point and conjugate principal point measured on a single photograph. Ground (air) base: Ground (air) distance between principal points of overlapping photographs. Parallax: Apparent shift in relative positions of objects when viewed (photographed) from different vantage points.
- 15. Exercises: 1. On photo, the distance between nadir point and principal point is 0.5cm. Focal length is 200mm. Determine the tilt. Tan a = 5/200 a = 1° 25’ 55.6” 2. The distance between principal point & nadir point 20mm. C=210mm. Where is the iso center?
- 16. Comparison of aerial photographs vertical, oblique and high oblique Photographs High oblique Low oblique Vertical Characteristic Horizon on photo No horizon photo Tilt smaller than 4 degrees Coverage Greater (wide) Less (normal) Least (narrow) Area (shape) Trapezoidal Trapezoidal Rectangular Scale Decrease from for ground to back ground Like as high oblique but lesser extent Uniform for one level Comparison with map Greatest difference Less Least Advantage Economic Easiest to map
- 17. The Geometry of the Aerial Photograph The average scale of the aerial photograph can be computed either by taking the ratio of the flying height above the ground and the focal length (f/H), or by taking the ratio of a known distance in the photograph and the distance on the ground. As the flying height above the ground is not usually known accurately, the second method is employed where a more reliable scale is needed.
- 18. Exercises: 1. Focal length = 6” Flying height = 12000ft Scale = 6”/ 12000’ = 1:20000 2. c = 210mm and Z = 4200 m Scale = 210mm / 4200m = 1:2000 3. The measured distance between points A & B is 215mm. The given co-ordinates are XA =205346.39 m, YA = 10793.16m ; XB =205100.11m, YB = 11587.98 m. Find the scale
- 19. Neither of the two scale calculation methods give an accurate scale as there are two distortions that affect measurements made on a single aerial photograph.
- 20. a. Height distortion: Because a lens and a photograph give a perspective or central projection, objects that are above or below the plane will be shifted by an amount approximately b. Tilt distortion: Although a lot of care is taken in the flying of aerial photography the photographs are rarely taken exactly horizontal. The tilts that occur in the aircraft, although kept to a minimum by the levelling of the camera system, do affect the position of objects on the photograph. There are other distortions such as film shrinkage, earth curvature, refraction effects and so on but these are only significant when dealing with precise photogrammetry.
- 21. Determining the height of an object from shadow length h = L tanα where H = flying height h = height of the vertical object L = length of the shadow α = sun elevation angle f = focal length Conditions:- The shadow on which the ground falls is level. The object is vertical
- 22. Sun elevation angle:Sun elevation angle: The sun angle above local horizon can be derived using a solar ephemeris table, given the latitude and logitude of the site and photograph acquisition date and time.
- 23. Relief displacementRelief displacement Any objects that are higher or lower than the principal point are displaced from its true planimetric (x,y) location on a vertical aerial photograph. This displacement is referred to as relief displacement or terrain distortion. The relief displacement is outward from the principal point for objects whose elevations are above the local datum, and toward the principal point for objects whose elevation are below the local datum. The direction of relief displacement is radial from the principal point.
- 24. Height from Relief DisplacementHeight from Relief Displacement Figure
- 25. Simple definitionSimple definition:: Relief displacement is the distance between the position of a point on the photograph if it was on the reference plane and its actual position due to relief.
- 26. Conditions for Relief DisplacementConditions for Relief Displacement Method to find object heightMethod to find object height Aerial photographs must be vertical or near vertical (<3° of tilt). Namely, the principle point is the photo nadir. The top and the bottom of the object are visible The object is on level base and vertical.
- 27. Exercises: 1. On flat horizontal terrain there is erected a flag pole of 25 m. The distance from the nadir point of the vertical photograph to the pole is 300 m. The flying height is 1000 m. Principal distance is 200 mm. Compute the relief displacement 2. On a photo of flat terrain there is an isolated hill 200m high. Flying height is 2000m. What is the greatest value of relief displacement which we may find ?