[OSM-talk] The long tail - lowest common denominator

[Christopher Schmidt]

On Mon, Jul 10, 2006 at 08:16:53AM -0400, Christopher Schmidt wrote:
> On Mon, Jul 10, 2006 at 09:17:29AM +0100, Nick Black wrote:
> > On 7/9/06, Lars Aronsson <lars at aronsson.se> wrote:
> > >Nick Black wrote:
> > >
> > >> I'm not sure that I am  following you Lars - do you mean that we could
> > >> use automatic feature detection to detect the edges of water features?
> > >
> > >Yes, that's what I was philosophizing about.  But I haven't tried
> > >it or even planned to try it.  I'm far too lazy for that.
> > 
> > One of the most significant features of the SRTM as a global dataset
> > is that it is self-consistent and internal errors fall within
> > specified and documneted thresholds.  This was acheived by a lot of
> > post-processing work.  Part of the work involved created a global
> > water bodies mask at the same scale (1 arc-second/3 arc-sec) as the
> > complete topographic dataset.
> > 
> > The mask is called the SWBD.  It is supplied in ESRI shapefile format
> > and is downloadable from the same ftp site as the SRTM data:
> > 
> > ftp://e0srp01u.ecs.nasa.gov/srtm/version2/
> > 
> > So thats cool - Free 3-arc-sec water body data.  Well, its only as
> > cool as the one degree tiles that it is supplied in.  If someone could
> > process this data to derive a complete water-bodies mask it would be
> > invaluable to OSM and to other projects.  A derived version of the UK
> > SWBD is already up on Chris's OSM freemap site, where is really shows
> > up the VMAP data (shows how coarse it is).
> 
> Oh, you didn't mention this data was so easily available -- I don't know
> about processing it into a single shapefile (which seems like it should
> be almost trivial), but I can absolutely set it up so that it's rendered
> -- should this be in addition to the water layer you offered me, or
> should I just drop that one and use the worldwide data instead?

I have been foiled. As the attached screenshot shows, the dataset didn't
end up behaving quite as I expected. We're going to do some work in
GRASS to try and fix up the data, but this is not going to be as easy as
I thought it would be.

-- 
Christopher Schmidt
Web Developer