Geoscience Reference
In-Depth Information
for ortho-rectification, but it can also be used as an explanatory variable during
image classification. Furthermore, slope, aspect and angle information can be directly
derived from the DEM.
OTB provides a very useful tool to download the SRTM tiles from the USGS
website, 8 namely otbcli_DownloadSRTMTiles :
-progress
<
boolean
>
Report progress
-il
<
stringlist
>
Input images list (mandatory)
-mode
<
Download/List corresponding SRTM tiles. [download/list] (manda-
tory, default value is download)
-mode.download.outdir
<
string
>
Output directory (mandatory)
-mode.list.indir
<
string
>
string
>
Input directory (mandatory)
-inxml
<
string
>
Load otb application from xml file (itemal, off by default)
We can download the SRTM tiles for the SPOT scene using in Sect. 13.1 by
running the following command:
otbcli_DownloadSRTMTiles -il spot5_scene.tif -mode download
-mode.download.outdir srtm_tiles -mode.list.indir
/tmp/otb_results/
This downloads four zip files containing the SRTMDEM stored as hgt files, which
is a NASA binary format that simply refers to height or elevation. The archives can
be unzipped using 7zip or within a Linux environment with unzip as follows:
for file in $(ls *.zip)
do
unzip $file
done
We then merge the four tiles into one image using the tool, otbcli_Tile
Fusion , which provides the similar functionality to gdal_merge.py presented
in Sect. 8.4 . The following command shows how the TileFusion command can be run,
which requires that each of the input images be listed in the correct order (based on
their position in the output). The number of columns and rows is determined by the
8 http://dds.cr.usgs.gov/srtm/version2_1/SRTM3
 
 
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