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Lecture 6 Content

Lecture 6 Content. Errors: Scale Relief Shadow Parallax Other errors. Errors: The geometry of scanned satellite data is more complex than that of aerial photos, and it is very difficult to overcome adequately the effects of geometric errors for mapping purposes

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Lecture 6 Content

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  1. Lecture 6 Content • Errors: • Scale • Relief • Shadow • Parallax • Other errors

  2. Errors: • The geometry of scanned satellite data is more complex than that of aerial photos, and it is very difficult to overcome adequately the effects of geometric errors for mapping purposes • Map projection systems are important considerations • Scanned satellite data sets are typically fitted to an accurate map obtained by other methods

  3. The distortion in scanned satellite data sets results from both the geometry of the scanning device and from the unwanted movements that affect the vehicle that is carrying the device during its collection mission • These movements make it almost impossible to obtain long series of perfectly parallel scan lines • Constant scale along a flight line is unlikely because of changes in vehicle altitude, and because of the nature of the scanning, scale along the scan lines is also not uniform • In addition, the spherical nature of the earth’s surface need also to be considered

  4. The error distortions discussed in this lecture are: • Scale distortion • Relief displacement and shadow • Parallax Ground sampling Increases outwards Objects displaced laterally

  5. Scale distortion • Radar (SLR – Side Looking Radar) uses two types of recording systems: • Slant range – records equal sized features as smaller, closer to the aircraft • Ground range - If the terrain is flat and flying height known, slant range can be converted to ground range

  6. Relief displacement • Relief displacement is one directional and perpendicular to the line of flight, but towards it, not away from it as in a scanned image • That is, tops of buildings are recorded first then the bottom • The reason for this is that a radar pulse meets the top of an object before it meets the bottom • The resulting effect is a “layover”, and it is greater closer to the aircraft

  7. Shadow • This is an effect due to the displacement in relief and the radar pulse not reaching the leeward side of high rise objects • The combined effects of scale distortion, relief displacement and shadow explains why the best results are obtained in a swath of limited width to one side of the aircraft

  8. Parallax • Similar to aerial photos • When images are taken from two different perspectives then stereo viewing is made possible • Different flight lines and relief displacements cause parallax on SLR images • Stereo radar imagery is often acquired from two flight lines at the same altitude on the same side of the terrain feature

  9. Other errors • Image characteristics which are caused by weather conditions, signal distortion, instrument errors, feature distortion due to the spherical nature of the earth, and such like.

  10. … The End …

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