[gdal-dev] get different result when use different parameters

[weixj2003ld]

I download a elevation data (SRTM,GTOPO30) from http://glcfapp.glcf.umd.edu:8080/esdi/index.jsp
 
 use 
gdal_translate -b 1 -of PNG -ot Byte source.tif des.png
and 
gdal_translate -b 1 -of PNG -ot UInt16 source.tif des.png
differently,
with parameter 'UInt16',I get a black picture,with Byte,get a different one,why?
BTW,I want to get a png file from elevation data(.tif),must I use command gdal_translate?
Thk u in advance.
 
 
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