10-11-2012, 05:43 PM
Photogrammetry
Photogrammetry.ppt (Size: 167.5 KB / Downloads: 82)
Introduction
Definition of Photogrammetry: the art, science, and technology of obtaining information about physical objects and the environment by photographic and electromagnetic images.
Basic Information
Mapping from aerial photos is the best mapping procedure yet developed for most large projects.
Used successfully for maps varying in scale from 1:1,000,000 1:120 with contour intervals as small as 1 foot.
Topographic mapping is the most common form. – U.S.G.S updated and done this way.
Used to reconstruct a scaled 3-dimensional optical model of the lands surface using a stereoplotter.
Uses: Aerial photos
Aid: geological investigations, soil surveys, land surveys, tax mapping, reconnaissance and military intelligence, urban and regional development, transportation system investigations, quantity estimates, shore erosion, etc.
Mathematical methods have been developed to make precise 3-dimensional measurements from photos.
Phototriangulation: 3-dimensional positioning of survey stations.
Basic Categories
Metrical photogrammetry – obtaining measurements from photos from which ground positions, elevations, distances, areas, and volumes can be computed and topographic or planimetric maps can be made.
Photo interpretation – evaluation of existing features in a qualitative manner.
Types of Photogrammetry
Aerial – series of photographs of an area of terrain in sequence using a precision camera.
Terrestrial – photos taken from a fixed and usually known position on or near the ground with the camera axis horizontal or nearly so.
Close range – camera close to object being observed. Most often used when direct measurement is impractical.
History
Airplanes brought great change to photogrammetry.
1st used in 1913.
Used extensively in WWI – photo interpretation.
Used in WWII – mapping for recon and intelligence.
WWII – 1960 – used often, expensive and accuracy problems for engineering design.
After mid 60’s – advent of computer and plotting has made photogrammetric mapping accurate and affordable.
Photogrammetry for Engineering
Defined: Photogrammetry is the process of measuring images on a photograph.
Modern photogrammetry also uses radar imaging, radiant electromagnetic energy detection and x-ray imaging – called remote sensing.
Geometry of Photographs
Orthographic projection – each point projected normal to reference plane.
Perspective projection – each point projected through a central point, due to points being at different elevations, they look 3 dimensional.
Principal point (center of photo) – located at the intersection of lines joining the Fiducial points.
Elements of Planning
Allowed scale variation.
Variation caused by difference in ground elevation and flying height.
Longer focal length reduces scale variation.
If flying height remains constant and ground elevation increases the area covered by photo becomes less.
Overlap becomes less
Viewfinder needed to control overlap and flightline spacing, thus eliminating possible gaps.
Relief displacement
Affects mosaics most.
Large amount of relief displacement will make it difficult to form continuous picture desired in mosaics.