SRTM Correction

Check our map's basic information using, Imagery > Report and Statistics > Report Basic File Information
Open which map ("srtm_mentawai1") will be viewed it's information, press "Run"
Check the output window
There were some weird value, the negative could means below sea level. Maybe some radar error or many factors, but anyway we'll only use the higher than zero.
Use Raster > Map Calculator
Select the source map ("srtm_mentawai1") as A and create some formulation that can remoce the unwanted data values ("if(A<0,null(),A)"). Don't forget to enter the name of output map ("srtm_mentawai2").
Wait for a while, GRASS takes some times to processing (check the progress from output window)
Select the raster layer again, browse and select the generated output map ("srtm_mentawai2")
Switch to "Map Display" window, press the "Display Active Layer" icon on the upper icon bar
Check again with Imagery > Report and Statistics > Report Basic File Information (but now we're using "srtm_mentawai2", the new output map)
Now our map's data range is normal

On to Next Step: Cropping Map