Stereoscopic Parallax
57 likes28,449 views
This document discusses parallax and stereo viewing in aerial photography, including concepts like stereo-pairs, relief displacement, and radial displacement. It explains methods for determining object heights using differential parallax and introduces tools like parallax wedges, parallax bars, and stereoplotters for precision measurement. Additionally, it highlights advancements in digital photogrammetry and the significance of orthophotos in presenting accurate spatial information.
Stereoscopic Parallax
- 1. Siva Subramanian M Centre for Land Resource Management
- 2. The displacement of an object caused by a change in the point of observation is called Parallax Stereoscopic parallax is caused by taking photograph of the same object but from different point of observation
- 3. Adjacent but overlapping aerial photos are called stereo-pairs and are needed to determine parallax and stereo/3D viewing http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/stereosc/chap4/
- 4. Overlapping photography End lap - ~60% Side lap - ~20-30%
- 5. - Locate the principal point (PP, optical center or nadir) of each photo by drawing a line between the corner fiducial marks -Locate the conjugate principal point (CPP) which is the PP of the adjacent photo -Draw the line between the PP and CPP - this is the flight line - Align the photos so that all 4 points lie on a straight line
- 6. A photo’s central projection leads to image displacement where objects are shifted or displaced from their correct positions Relief displacement is due to differences in the relative elevations of objects. All objects that extend above or below a specified ground datum plane will have their images displaced. The taller the object, the greater the relief displacement
- 7. Even from great flying heights, tall objects can exhibit image displacement. In this example from a Quick bird satellite image, the Washington Monument appears to lean outwards http://www.mfb-geo.ch/text_d/news_old_d8.html
- 8. Objects will tend to lean outward, i.e. be radially displaced. The greater the object is from the principal point, the greater the radial displacement. Example: storage tanks towards the edge of photo show greater radial displacement. Center of photo Edge of photo
- 9. Maps: Scale is constant No relief displacement Photos: Scale varies with elevation Relief displacement
- 10. Note the displacement between the top and base of the storage towers in this photo stereo-pair Line of Flight top bottom
- 11. PP = Principal point = Center of photo CPP = Conjugate principal point = Adjacent photo’s PP Absolute stereoscopic parallax the average photo base length = average distance between PP and CPP Photo base PP CPP PP
- 12. Differential parallax - the difference between the stereoscopic parallax at the top and base of the object. 15.2 mm 13.5 mm dP = 15.2mm – 13.5mm = 1.7 mm
- 13. Photo 1 Photo 2 dP = 2.06-1.46 = 0.6 in 1.46 2.06 Following example taken from: T.E. Avery & G.L. Berlin. 1992, Fundamentals of Remote Sensing and Air Photo Interpretation, MacMillan P Calculating the height of the Washington Monument via stereo parallax
- 14. h = (H’) * dP / (P + dP) where h = object height H’ = flying height dP = differential parallax P = average photo base length
- 15. h = (H’) * d / (r) Where, h = object height H’ = flying height = 4,600ft d = relief displacement from base to top = 0.6in same as dP r = distance from PP to top of object same as (P + dP)
- 16. Object heights can be determined as follows: ◦ calculate flight altitude (H’) by multiplying the RF denominator by the focal length of the camera ◦ h = d * H’ / r where: h = Object height d = length of object from base to top r = distance from P.P. to top of object r d r
- 17. Parallax wedge - simplest device for determining differential parallax Parallax bar - movable floating mark can placed at base and tops of objects to measure differential parallax
- 18. Stereoplotters - precision instruments designed to duplicate the exact relative position and orientation of the aerial camera at the time of photo acquisition to recreate the stereo-model. A floating mark can be used trace specific elevations. Relief displacement is removed creating a planimetric map. Photo from http://www.wsdot.wa.gov/mapsdata/Photo grammetry/PhotogImages/earlyStation.gif
- 19. •Soft-copy photogrammetry workstations - computer software recreates the stereomodel and allows for digital mapping •Soft-copy photogrammtery has largely replaced optical-mechanical systems Photos from: http://www.wsdot.wa.gov/mapsdata/ Photogrammetry/About.htm Digital scanner Soft copy workstation
- 20. One view displayed in red; the other perspective view in blue spatially shifted The spatial shift is a function of the differential parallax To visualize, use red-blue glasses NASA Mars Lander
- 21. Orthophoto - reconstructed airphoto showing objects in their true planimetric position Geometric distortions and relief displacements are removed Orthophotoquad - orthophotos prepared in a standard quadrangle format with same positional and scale accuracy as USGS topographic maps
- 22. Digital ortho-photography/ortho-imagery is increasingly the imagery of choice for many applications Sometimes referred to as DOQ - digital orthophoto quad NJ has DOQ imagery for 1995 and 2002 Digital orthophoto on computer screen Photo from: http://www.wsdot.wa.gov/mapsdata/Photogrammetry/About.htm