[Gdal-dev] gdalwarp -t_srs -s_srs

[Gong, Shawn]

hi list,

Here are a few questions regarding gdalwarp.  Thanks in advance.

Q1: why do I need -s_srs (source spatial reference) ? I know this is
optional.  But can't this be extract from srcfile?

Q2: -t_srs (target spatial reference) for example -t_srs "+proj=utm
+zone=50 +datum=WGS84" What if the zone is in southern hemisphere? 
I used something like:
    gdalwarp -t_srs "+proj=utm +zone=50K +datum=WGS84" -ts 1500 900
src.tif dst.tif
gdalinfo displays that dst.tif's northing as negative numbers (says it
is in northern hemisphere).
How can I specify -t_srs for southern hemisphere?

Q3: is there any detailed documentation for gdalwarp, with more
examples, instead of the 1-2 pages usage intro?

Q4: Where can I get documentation for OGR Well Known Text?

Shawn Gong