Geoscience Reference
In-Depth Information
One or more no-data values can be defined for the input images with the option
-srcnodata
. For multi-band images, the option
-bndnodata
indicates to which
band the no-data values correspond.
For instance, suppose we want to create a cloud free composite from overlapping
remote sensing images. As input, we have a list of MODIS raster files covering the
same area, acquired during different months in 2010. Each raster file is a multi-
band image with eight bands. The first seven bands (0-6) correspond to the spectral
information of the MODIS sensor. The last band (band 7) contains cloud information
and will define if a pixel is valid (set with the option
-bndnodata 7
). The cloud
information is coded as follows:
1. cloudy with high confidence
2. cloudy with low confidence
3. cloud free.
For a cloud free composite rule, we will consider only those pixels as valid that
have a value of 3 (cloud free) in band 7. Values in the input (source) images equal
to 0, 1 and 2 are therefore set as no-data. A value 0 (option
-dstnodata 0
)is
assigned to those pixels for which not a single cloud free observation was found.
pkcomposite -i modis_201006.tif -i modis_201007.tif -i
→
modis_201008.tif -o modis_summer.tif -srcnodata 0
→
-srcnodata 1 -srcnodata 2 -bndnodata 7 -dstnodata 0
As an alternative method to obtain a cloud free composite, we apply the maximum
composite rule (
-cr maxband
). Based on the cloud coding scheme of band 7,
pixelswith a higher value aremore likely to be cloud free.We therefore select the pixel
with the maximum value in band 7 by selecting the maximum composite rule (
-cr
maxband
) acting on band number 7 (
-cb 7
). The algorithm will first select pixels
that are cloud free (value 3), then probably cloudy (value 2) and only then cloudy
(value 1). The multi-band composite image still contains the cloud information for
the retained pixel in band 7. Invalid pixels in the input image have been coded with
a value of
−
32768.
pkcomposite -i modis_201006.tif -i modis_201007.tif -i
→
modis_201008.tif -o modis_summer.tif -srcnodata -32768
→
-bndnodata 0 -dstnodata 0 -cr maxband -cb 7
The respective outputs for the two composite variants are shown in Fig.
12.1
.
Notice that in the first case, there are still some black pixels indicating no-data in the
output. In case of the maximum composite, these pixels obtained the value of the
input image with the maximum value in band 7. In this particular example, none of
the input images actually contains cloud pixels. The cloud detection algorithm was
confused by the snow that covers the glacier mountains in the Alps.
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