[Proj] PROJ4 settings for direct import of Smith & Sandwell topo data, GMT

[Hamish]

[ maybe this wiki page has a clue for Aydin re. HDF5 efforts? it has tips
  on how to set map corner coords and projection metadata for HDF4 using
  gdal_translate, etc. http://grass.osgeo.org/wiki/MODIS ]

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[the following is a re-post from the grass-users list, no answer there.]

Hi,

I have been trying to figure out how to create a GRASS location for the
native projection of Smith and Sandwell's 1-minute global elevation
v9.1b, August 21, 2007.  http://topex.ucsd.edu/marine_topo/mar_topo.html 
(dataset is 712mb uncompressed)

Data is given as simple Mercator on a sphere, the same as GMT's "img2grd
-D". I wish to eventually correlate with or reproject into WGS84 lat/lon,
but to minimize loss of detail I prefer to work with the original data as
long as possible.

What are the magic +proj settings to use? so far nothing I've tried works
well. I can load the original binary file ok with r.in.bin, but the north/
south scaling is rather wrong.

I have a working method using img2grd; g.region; grd2xyz | r.in.xyz.
Although I am still a bit unsure about the north-south resolution, it all
seems to line up well with a world coastline vector overlay.

I can use GMT's img2grd -> grdreformat =bf to convert into an old GMT
native binary (which r.in.bin's -h flag knows about), but that seems to
harm the data values (elevation += ~0.005m ?!). Anyway grd2xyz + r.in.xyz
seems to cover that part of the process cleanly so I ignore the
grdreformat problem for now.


notes and ideas about it here:
  http://grass.osgeo.org/wiki/Marine_Science#Bathymetric_data

shoot down to the "Import Directly" section to see the PROJ.4 settings I
tried.


provided ERMapper .ers file for the dataset is here:
  ftp://topex.ucsd.edu/pub/global_topo_1min/
but I don't understand that much (no joy with MRWORLD projection).


One thing I don't understand is why the GRASS wkt becomes PROJECTION
["Mercator_2SP"], but maybe that is harmless if only 1 std parallel is
actually defined.


has anyone solved this before? eventually I would like the clean up the
above wiki page with a clear mini-howto for using this data with GRASS
and/or GDAL.


thanks,
Hamish