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ASAR Image Orthorectification

April 3rd, 2009 Category: Earth Observation

Figure 1 - 3D Model

Figure 1 - 3D Model

These images of Reunion Island provide a clear example of the improvements made through the use of the orthorectification technique.
The original product has been processed, orthorectified and projected using the Chelys SRRS (Satellite Rapid Response System). The orthorectification process is now fully automatic and transparent. The Digital Elevation Model used is the Nasa SRTM v4 (Shuttle Radar Topography Mission).

The proper DEM is selected according to the image resolution: from 15 arcseconds up to 1 arcsecond (only available for USA). The performance of the system are kept in real-time. The entire correction process takes less than 20 seconds per product.

Once the image has been geometrically corrected, its projection can be mapped with absolute precision using the DEM to simulate the third dimension. The result is a new, more interactive way to navigate a SAR image. An example of a 3D model is shown in Figure 1 (the Xvid codec, if missing, can be downloaded here).

Synthetic Aperture Radar and Orthorectification

Reunion Island - Original and Orthorectified Image

Reunion Island - Original and Orthorectified Image

Figure 2 - Synthetic Aperture Radar

Figure 2 - Synthetic Aperture Radar

SAR (Synthetic Aperture Radar) instruments transmit radar signals and then measure how strongly those signals are scattered back. An analogy with photography can be made: when it’s dark, a camera’s flash sends out light and then the film records objects that the flash illuminates. In both cases the SAR and the camera are not dependent upon the sun because they provide their own illumination.

The time it takes for a transmitted signal to travel to an object and back tells you how far away the object is. If you transmit a signal and receive two separate “echoes,” you can use the time difference between when you record the first and second responses to determine the distance between the two sensed objects (dependent on where you stand). In this way the spaceborne SAR measures how far objects are from the spacecraft and the distance between the two objects, along the direction the spacecraft is looking (Figure 2). These distances are said to be recorded in slant range, since they are measured in a direction which is at an angle/slant to the ground (Figure 3).

However, SAR uses a side-looking imaging mode whose angle is much greater than that of the optical image. This mode lends a great influence to geometric distortion of SAR image.

Figure 3 - Slant Range vs Ground Range

Figure 3 - Slant Range vs Ground Range

If you have information about a region’s topography, like a Digital Elevation Model (DEM), you can make the slant to ground range conversion more sophisticated. In effect this terrain correction can compensate for foreshortening by spreading data representing the mountain’s facing side into more pixels and compacting returns from the back face into fewer pixels. It’s nearly impossible, though, to reliably extract the separate returns from data values representing the facing slope. Sometimes people try to compensate for shadowing as well. Knowing the mountain’s slopes, they can approximate how the strength of backscattered signals were affected by the changed incidence angle and adjust results accordingly. These procedures, though inexact, can greatly improve SAR image analysis.

SAR’s ability to pass relatively unaffected through clouds, illuminate the Earth’s surface with its own signals, and precisely measure distances makes it especially useful for the following applications:

  • Sea ice monitoring
  • Cartography
  • Surface deformation detection
  • Glacier monitoring
  • Crop production forecasting
  • Forest cover mapping
  • Ocean wave spectra
  • Urban planning
  • Coastal surveillance (erosion)
  • Monitoring disasters such as forest fires, floods, volcanic eruptions, and oil spills

Some of the larger current research projects include: mapping the Antarctic continent; mapping the Amazon rainforest; using interferometric analysis for predicting or analyzing earthquakes and volcanic activity; and generating “Arctic Snapshots” of the Arctic ice extent.

14 Responses to “ASAR Image Orthorectification”

  1. 1
    Earth Snapshot • Greater Caucasus Mountain Range and Tbilisi, Georgia :

    [...] ASAR image has been orthrectified, removing geometric distortion. This gives a more precise view of the geographic features, [...]

  2. 2
    Earth Snapshot • Lake Geneva and Surrounding Mountains - May 27th, 2009 :

    [...] image has been orthorectified, giving a truer view of the contours of the two mountain chains surrounding the city, the Alps and [...]

  3. 3
    Earth Snapshot • Mountains Around Strait of Messina, Italy :

    [...] Etna volcano (left). The mountains in this image can be observed with great clarity, thanks to the orthorectification technique used to eliminate geometric distortion. var addthis_pub = ‘eosnap’; var [...]

  4. 4
    Earth Snapshot • Ragusa and Water Currents off Southern Sicilian Coast, Italy :

    [...] the surrounding terrain are visible without geometric distortion thanks to correction with an the orthorectification technique. var addthis_pub = ‘eosnap’; var addthis_language = ‘en’;var addthis_options = ‘digg, [...]

  5. 5
    Earth Snapshot • Isle of Anglesey and Caernarfon Bay, Wales :

    [...] This orthorectified image shows Anglesey (above), also known by its Welsh name Ynys Môn, an island and, as Isle of Anglesey, a county off the north west coast of Wales. With an area of 720 square kilometres (278 sq mi), Anglesey is the largest Welsh island, the sixth largest surrounding the island of Great Britain, and the largest island in the Irish Sea ahead of the Isle of Man. [...]

  6. 6
    Earth Snapshot • Edinburgh by Firth of Forth, Scotland – November 21st, 2011 :

    [...] This orthorectified image shows the Firth of Forth (lower half of image), the estuary or firth of Scotland’s River Forth, where it flows into the North Sea. It is located between Fife to the north, and West Lothian, the City of Edinburgh and East Lothian to the south. Populated areas appear white in this image. [...]

  7. 7
    Earth Snapshot • Coastline of Cornwall County, England :

    [...] This orthorectified image shows the county of Cornwall, England, which forms the tip of the south-west peninsula of the island of Great Britain. The coastline is composed mainly of resistant rocks that give rise in many places to impressive cliffs. [...]

  8. 8
    Earth Snapshot • Offshore Turbines of Robin Rigg Wind Farm in Solway Firth, United Kingdom – November 25th, 2011 :

    [...] This orthorectified image shows the  Solway Firth, a firth that forms part of the border between England and Scotland, between Cumbria (including the Solway Plain) and Dumfries and Galloway. It stretches from St Bees Head, just south of Whitehaven in Cumbria, to the Mull of Galloway, on the western end of Dumfries and Galloway. The Isle of Man (visible at the lower left of the full image) is also very near to the firth. [...]

  9. 9
    Earth Snapshot • Milford Haven Waterway in Pembrokeshire, Wales :

    [...] This orthorectified image shows the Milford Haven Waterway (Welsh: Aberdaugleddyf), lower left quadrant, a natural harbour in Pembrokeshire, West Wales. As one of the deepest natural harbours in the world, it is a busy shipping channel; some ships and docks can be observed inside it upon opening the full image. [...]

  10. 10
    Earth Snapshot • Kirkwall in the Orkney Islands, Scotland :

    [...] This orthorectified image shows some of the Orkney Islands, an archipelago in northern Scotland, situated 16 kilometres (10 mi) north of the coast of Caithness. Orkney comprises approximately 70 islands of which 20 are inhabited. [...]

  11. 11
    Earth Snapshot • San Francisco and Bridges Crossing San Francisco Bay, USA – February 24th, 2012 :

    [...] bright white parts of this orthorectified image mainly belong to the city of San Francisco (lower left quadrant) and the San Francisco Bay [...]

  12. 12
    Earth Snapshot • Agriculture Around Lower Mississippi River, USA :

    [...] orthorectified image shows part of the section of the Mississippi River called the Lower Mississippi River. This [...]

  13. 13
    Earth Snapshot • Ottawa River Separating Ottawa and Gatineau, Canada :

    [...] image is  orthorectified, showing hillier terrain dotted with lakes to the north of the river, and mostly flat terrain to [...]

  14. 14
    Earth Snapshot • Agriculture in Great Central Valley, California, USA :

    [...] Valley is flat, more mountainous terrain is visible to the north in the full version of this orthorectified image. Here, agricultural fields appear as dark and medium grey rectangles, while more populated [...]

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