[Gdal-dev] gdalwarp documentation/tutorial

[Paul Surgeon]

A have a minor gripe about the gdal tools and it's that I can't find 
sufficient documentation or tutorials on how to use the command line 
parameters.
It may be obvious to a seasoned GIS user but to someone relatively new to the 
GIS scene it's a black box and reading source files and API documentation is 
not helping me at all.

For starters what are all the projections that I can use in gdalwarp?
I understand that it's using proj.4 but I can't find what command line syntax 
to use.

What I'm trying to do is really simple.
I want to warp Landsat 7 ETM+ images into an unprojected format so that I can 
cut them along a lat and lon boundary and stick them through an image 
processing tool.

I have tried :
gdalwarp -s_srs "+proj=utm +zone=35 +datum=WGS84" -t_srs "+proj=latlong 
+datum=WGS84" input.tif  output.tif
as well as :
gdalwarp -s_srs "+proj=utm +zone=35 +datum=WGS84" -t_srs "+proj=ortho 
+datum=WGS84" input.tif  output.tif

For both I get an error message :
ERROR 5: GDALWarpOptions.Validate()
  pfnTransformer is NULL

What does that error mean?
What is the difference between the latlon and ortho projections?
I know that's a general GIS question.

I also have the accompanying "met" file with all the projection info for the 
bands but how do I get gdalwarp to read it or what must I extract out of it 
and stick in the command line?
Is that neccessary or will the GeoTIFF header have enough info in it for 
gdalwarp to work with?

Thanks
Paul