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wiki:weld_visual_fire

Data visualization using openev

The goal of this exercise is to demonstrate how to use Openev to visualize WELD .tif products.

The data are stored at /home/user/ost4sem/exercise/welddataprocessing/x6JIatZlSUMMER2008

Set your working directory:

DIR=/home/user/ost4sem/exercise/weld_data_processing/x6JIatZlSUMMER2008
cd $DIR


We can check that this Linux variable is correct by typing: echo $DIR

Create a stack image

In this exercise we will visualize the WELD red, green and blue wavelength bands as a true color image by displaying them on the red, green and blue monitor channels. Openev requires multiple bands to be stored together as a stack image, the WELD FILL values (no data values) are designated with a pixel value of -32768 so we need to ignore them when we generate the stack image as:

gdal_merge.py  -separate -n -32768 -co COMPRESS=LZW $DIR/Band3_TOA_REF.TIF $DIR/Band2_TOA_REF.TIF $DIR/Band1_TOA_REF.TIF  -o $DIR/Band321_TOA_REF.tif

We will now visualize the stack image as:

openev $DIR/Band321_TOA_REF.tif &

We can change the minimum and maximum values that are displayed for each band (this is called contrast stretching) to better visualize the data by right clicking on the layers window:


We can change the band order (Red Band:3, Green Band:2, Blue Band:1) or change the minimum and maximum values as below. Remember to press the keyboard “enter” every time that you insert a value.


Create a log-stretch stack image

For better visualization we can display the natural logarithm of the 3 WELD bands. We can use gdal_calc.py to calculate the log of each band as:

gdal_calc.py -A $DIR/Band3_TOA_REF.TIF --calc "log(A)" --NoDataValue -32768 --type Float32   --outfile  $DIR/Band3_TOA_REF_ln.tif
gdal_calc.py -A $DIR/Band2_TOA_REF.TIF --calc "log(A)" --NoDataValue -32768 --type Float32   --outfile  $DIR/Band2_TOA_REF_ln.tif
gdal_calc.py -A $DIR/Band1_TOA_REF.TIF --calc "log(A)" --NoDataValue -32768 --type Float32   --outfile  $DIR/Band1_TOA_REF_ln.tif

Then we can stack the log bands together as:

gdal_merge.py  -separate -n -32768 -co COMPRESS=LZW $DIR/Band3_TOA_REF_ln.tif $DIR/Band2_TOA_REF_ln.tif Band1_TOA_REF_ln.tif  -o $DIR/Band321_TOA_REF_ln.tif

We will now visualize the stack image as:

openev $DIR/Band321_TOA_REF_ln.tif &

We can change the minimum and maximum values that are displayed for each log band by right clicking on the layers window:

wiki/weld_visual_fire.txt · Last modified: 2017/12/05 22:53 (external edit)