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@@ -1,14 +1,53 @@
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<!-- meta page description: Image processing in GRASS GIS -->
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<!-- meta page index: imagery -->
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-<h3>Image data in general</h3>
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+<h3>General introduction</h3>
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-In GRASS, image data are identical to <a href="rasterintro.html">raster data</a>.
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-However, a couple of commands are explicitly dedicated to image
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-processing. The geographic boundaries of the raster/imagery file are
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-described by the north, south, east, and west fields. These values
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-describe the lines which bound the map at its edges. These lines do
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-NOT pass through the center of the grid cells at the edge of the map,
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-but along the edge of the map itself.
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+<b>Digital numbers and physical values (reflection/radiance-at-sensor):</b>
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+<p>
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+Satellite imagery is commonly stored in Digital Numbers (DN) for
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+minimizing the storage volume, i.e. the originally sampled analog
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+physical value (color, temperature, etc) is stored a discrete
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+representation in 8-16 bits. For example, Landsat data are stored in
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+8bit values (i.e., ranging from 0 to 255); other satellite data may
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+be stored in 10 or 16 bits. Having data stored in DN, it implies
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+that these data are not yet the observed ground reality. Such data
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+are called "at-satellite", for example the amount of energy sensed
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+by the sensor of the satellite platform is encoded in 8 or more
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+bits. This energy is called radiance-at-sensor. To obtain physical
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+values from DNs, satellite image providers use a linear transform
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+equation <tt>(y = a * x + b)</tt> to encode the radiance-at-sensor
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+in 8 to 16 bits. DNs can be turned back into physical values by
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+applying the reverse formula <tt>(x = (y - b))</tt>.
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+<p>
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+The GRASS GIS module <a href="i.landsat.toar.html">i.landsat.toar</a>
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+easily transforms Landsat DN to radiance-at-sensor. The equivalent
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+module for ASTER data is <a href="i.aster.toar.html">i.aster.toar</a>.
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+For other satellites, <a href="r.mapcalc.html">r.mapcalc</a> can
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+be employed.
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+<p>
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+<b>Reflection/radiance-at-sensor and surface reflectance</b>
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+<p>
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+When radiance-at-sensor has been obtained, still the atmosphere
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+influences the signal as recorded at the sensor. This atmospheric
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+interaction with the sun energy reflected back into space by
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+ground/vegetation/soil needs to be corrected. There are two ways to
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+apply atmospheric correction for satellite imagery. The simple way
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+for Landsat is with <a href="i.landsat.toar.html">i.landsat.toar</a>,
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+using the DOS correction method. The more accurate way is using
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+<a href="i.atcorr.html">i.atcorr</a> (which works for many satellite
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+sensors). The atmospherically corrected sensor data represent
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+surface <a href="http://en.wikipedia.org/wiki/reflectance">reflectance</a>,
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+which ranges theoretically from 0% to 100%. Note that this level of
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+data correction is the proper level of correction to calculate
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+vegetation indices.
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+<p>
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+In GRASS GIS, image data are identical to <a href="rasterintro.html">raster data</a>.
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+However, a couple of commands are explicitly dedicated to image
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+processing. The geographic boundaries of the raster/imagery file are
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+described by the north, south, east, and west fields. These values
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+describe the lines which bound the map at its edges. These lines do
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+NOT pass through the center of the grid cells at the edge of the
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+map, but along the edge of the map itself.
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<p>
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As a general rule in GRASS:
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@@ -21,7 +60,7 @@ As a general rule in GRASS:
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</ol>
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-<h3>Raster import</h3>
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+<h3>Imagery import</h3>
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The module <a href="r.in.gdal.html">r.in.gdal</a> offers a common
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interface for many different raster and satellite image
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Markus Neteler