[R-sig-Geo] subdatasets in rgdal/raster

[Roger Bivand]

On Wed, 15 Oct 2014, Michael Sumner wrote:

> Hello,
>
> As far as I know there's nothing in rgdal to determine existing
> subdatasets (SDS) in HDF (or NetCDF for that matter).
>
> In raster, raster(x) will read from the first candidate *variable*,
> and list the others with a warning, which is analogous to an SDS but
> of course it's always providing this via ncdf/ncdf4 rather than via
> rgdal.
>
> Is it true there's no way to find out SDS from HDF or NetCDF in rgdal?

Because these formats are poorly organised (an XML tree describing the 
internal structure would be easy to parse, but self-descriptive metadata 
isn't usual here), GDALinfo() does not provide much.

If you start by identifying the incantation used by command line gdalinfo 
to report the SDS metadata, then look at its source code, it should be 
possible to make some progress. I don't use either if I can help it, and 
do not have the necessary trial set of sample files. You'd need to find a 
robust route covering a wide range of files seen in the wild to allow for 
oddities in the ways things are organised. It is unfortunate to have to 
branch code on the selected driver, but here it is hard to avoid. Once 
GDALinfo is reporting the metadata in a predictable way, the other 
functions can be given appropriate argument values.

Also consider looking at this from the point of view of rgdal2 in addition 
to rgdal; it could possibly be "closer" there and looking at both may 
make it easier to spot:

https://github.com/thk686/rgdal2

Contributions welcome!

Roger

>
> I've not really explored this before much since compiling HDF was
> unavailable to me. gdalUtils fills a gap but only by running the
> installed command line tools, which obviously can be avoided with the
> right features in rgdal/raster. I personally have always run the
> command line tools and then jumped back to R (if I could).
>
> (The vector analogy in rgdal to listing SDS would be ogrListLayers).
>
> This is a missing capability in the raster package, somewhat disguised
> because of the independent support for NetCDF. I guess hardly anyone
> is using HDF? That's a shame since these R tools really help.  It's a
> pretty complicated story all round and I keep re/discovering
> interesting corners. If anyone is writing anything significant about
> all this I'd be keen to be involved.
>
> Cheers, Mike.
>
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no