Hi Stuart,
Stuart Buchanan wrote:
> Your announcement of having a Topologically clean dataset suggests
> that we're getting significantly closer to being able to build the
> entire world using new data such as CORINE and custom data sets. Could
> you outline what work is still required, and how much effort it is
> likely to require?
Whereas CLC00 and CLC06 on the MapServer are probably the best you can
get (one day I'll might also add the metadata to the DB, but that's
irrelevant to building FlightGear Scenery), v15 even include
Switzerland (hint, hint), I'm still not satisfied with the result of
VMap0.
Just as a little background for your pleasure (hopefully ;-):
Whereas CORINE and quite likely all the other datasets, which have
mostly been aquired from remote sensing, just contain the coverage you
can see from above, VMap is significantly different in it's structure.
CORINE shows just the vegetation, let's say evergreen forest _or_ the
bare soil, where _no_ vegetation is present. VMap instead may feature
the forest _and_ the soil, on which the forest grows, in the same
place. Even in really big areas, not just in small, accidential
snippets.
Thus, in order to transform VMap0 into a topology of the CORINE style,
you have to cut feature A out of B and store the remains of B as M.
Then add A plus M as, let's say, X, cut X out of C and store the
remains of C as N, add X and N into Y, .... and so on.
That's rather time consuming, even in GRASS.
Unfortunately there's no reliable progress meter. Well, there is one,
but in certain cases it rises up to 98 % in just two hours and remains
there for an undetermined period. Thus you never know wether your job
is going to take just a few days, a few weeks or a few months.
I don't like jobs running several months. I had a few of these but the
most promising ones were killed when the machine had to be rebooted due
to a planned power outage - or an unplanned power outage - or some
other reason. Therefore I've recently been trying to re-run the VMap0
cleaning using a slightly different approach.
After VMap0 has proven to be really clean, the next step would be to
either cut the CLC06 coverage out of CLC00 and then cut the result out
of VMap0, or first cut CLC00 out of VMap0 and later replace CLC00 by
CLC06 where available.
Then a decision has to be made over the various custom landcover
pieces. I think all of John Holden's work in North America has to be
preserved, but I'm not sure about what to do with the custom land cover
of London, Madrid and other places, where CORINE is available.
>From my perspective the benefit of CORINE is the fact that the land
cover classes they chose are verified. Thus, when they declare an area
as deciduous forest, then I'd trust them that there is no significant
amount of evergreen forest. This is quite difficult to tell when you
classify Landsat7 imagery. On the other hand there's a lot more detail
in John Holden's London work, Gijs Frisian islands and all the other
custom stuff (Helgoland comes into my mind as well, I can't remember
all the individual places). Detail which I consider as highly valuable
and which therefore deserves to be preserved.
Adjusting these detailed custom land cover areas against the classes in
CORINE might be a worthwhile operation to get the best of both.
Cheers,
Martin.
--
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