Data reduction: 16 to 8 bit, blockaverage vs incrementing
figure to the left of the screen was obtained from the full resolution image
by means of a block averaging of 16x16 pixels moving window. The average value
of each window is written to a new image file which dimension is reduced by
16 times in lines and columns. This is a conventional way to obtain a good quality
Quick-Look images starting from a full-resolution image.
On the right part of the screen an 8 bit version of the full resolution image
is shown. It has been obtained merely sampling the full image using a step increment
of 16 pixels. The process results in a final image that has the same size of
the image displayed on the left.
For practical reasons also a 16 to 8 bit conversion is done. Original statistics
are conserved best when applying a square root to the original 16 bit data.
Especially the information from point targets (strong scattering) are retained.
If mainly distributed targets (e.g. agriculture) are evaluated, a simple division
(e.g. by four) is applied. This procedure clips all values greater than 1020,
in the presented cases a division by four has been performed
The dimension of the 16 bit full-resolution image which was initially about
128 Mbytes has now been reduced by a factor 256 for the processing and by a
2 for the reduction from 16 to 8 bit. This results in a final size for the image
of about 250 Kbytes.
The main difference characterizing the two images is in the amount of noise.
In the right one it is possible to note that the increase of noise due to the
resampling process masks the smaller features of the image and part of the SAR
information is lost. In this image, even if the effect of the rain cells is
still visible in the central part of the figure, most of the small features
such as the presence of ships or the local effects of the wind on the sea surface
are lost. Note also that the urban environment (bright part), in the upper part
of the left figure, appears, in the right image, defocused because
of the noise.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,