As I have mentioned I hoped that generalization will help
to solve some problems when interpolating by v.surf.rst from
oversampled contours
(as is the case for example when they are scanned)
and the results were well beyond my expectations
(at least for the small data set I used).
First some test:
Even with very small threshold, large number of points is eliminated
without noticeable change in the contour geometry
http://skagit.meas.ncsu.edu/~helena/grasswork/elcont1m_gen00.png
(from 40,000 to 3,000 points)
result with threshold 5 is more radical
http://skagit.meas.ncsu.edu/~helena/grasswork/elcont1m_gen05.png
(from 40,000 to 500 points)
each line is generalized independently so eventually contours will
cross, so small threshold
is what we want to use (animation from threshold of 0.5 to 12)
http://skagit.meas.ncsu.edu/~helena/grasswork/gencontanim2.gif
Interpolation from given data (40,000 pts) using default parameters
(note that the points along contours are practically continuous,
there are visible segments
on the top of the hill and the computation runs 8 min)
http://skagit.meas.ncsu.edu/~helena/grasswork/rstint_default_pts.png
There are many small segments and the data are not very well
distributed within the segments
http://skagit.meas.ncsu.edu/~helena/grasswork/rstint_default_seg.png
we can improve the results by tuning dmin, npmin, tension and
smoothing as we have done for years,
but there is a better solution now, by running v.generalize first ,
with small threshold we have fewer than 3000 points :
http://skagit.meas.ncsu.edu/~helena/grasswork/rstint_gen_pts.png
Segments are larger and fewer - computation runs under 1 min
http://skagit.meas.ncsu.edu/~helena/grasswork/rstint_gen_seg.png
and the most important - the results are much better
default interpolation with original data (there are visible segments
and waves along contours)
http://skagit.meas.ncsu.edu/~helena/grasswork/elev_rstcontdef3d.jpg
default interpolation with generalized data (no segments and no waves)
http://skagit.meas.ncsu.edu/~helena/grasswork/elev_rstcontgen013d.jpg
BUT I had to set dmax to a large number (1000) - the default value
added 100,000 (!) points right back on the contours.
which brings me to the main question - are there any objections to
make dmax default value
as large as the region to make sure that it does not add any points
(currently it is set to 1.25 resolution value which is obviously not
enough)?
I have been trying to eliminate this parameter for years
as it usually causes more problems than it solves.
Helena
On Aug 4, 2007, at 4:57 PM, Daniel Bundala wrote:
Hello Paul, Wolf and List
On 8/4/07, Paul Kelly <[EMAIL PROTECTED]> wrote:
On Thu, 26 Jul 2007, Wolf Bergenheim wrote:
v.generalize:
~~~~~~~~~~~~~
v.generalize is fully functional complete with manual and
smoothing and
simplification operations. The module works with both areas and
lines.
Attribute tables are also copied and cats are preserved. Please
give the
module a try and send us feedback!
The rest of SoC will be spent in implementing other generalization
operations and getting all the rest of the bugs out.
Hello Wolf and Daniel
Now I've had time to look at v.generalize too and am very
impressed. The
amount of easily-accessible functionality that this module adds to
the
GRASS vector capabilities really seems to be something
significant. At
first glance the amount of options seemed overwhelming but on reading
through the man page and looking at the references there it became
much
more obvious. I think it could still be made clearer, but there is
already
a lot of information and explanation there and also in the source
code,
which is good.
This is true. Actually, the man page does not contain any examples. I
will try to improve this... Moreover, I am planning to write a
tutorial/GSoC Final Report which will demonstrate the capabilities of
this module with a lot of examples and nice pictures...
The main thing I was wondering about is whether the threshold
parameter is
dual-purpose? If I understand correctly, is it used in some
algorithms but
then again also at the end to remove lines left that are shorter
and areas
that are smaller than the threshold? Is that dual purpose use
likely to
cause any problems? Or should these be different parameters?
Yes, you understand it correctly. But this happens only if you
simplify the lines. Just few days ago, I added new flag (-r) to the
module which specifies whether the small/short linear features are
romeved. It is also mentioned somewhere in the newest version of the
man page.
I am curious too as to the spelling of alfa rather than alpha!
Oops. I think that this caused me some problems with TeX as well.... I
will change it.
Compiling with -Wall I see quite a lot of missing function
prototypes - as
for the other Summer of Code module I feel putting in a
local_proto.h for
the functions that can be called from other source files, and marking
the functions local to each file as static, would make things a bit
clearer. Also perhaps Doxygen-style documentation for the
functions? This
one's not a big deal at all. I know it's a bit of work but the
functions
look well organised already, so presumably there is a lot of thought
behind the way they are and it should be easy enough to put that into
words. But in general the code comments are really good and
helpful - only
there where they are needed and left out where it is obvious by
reading
the code, what is going on.
Glad you like me style of comments... You know, *the* most boring part
of the project. And I will check that -Wall stuff.
Was thinking too about all the matrix stuff in the matrix.c file -
sorry
for this lazy question, as if I had more time to look through and
was more
familiar with these things I could answer it myself - but is it
better
than the G_matrix_* functions in lib/gmath, or just an alternative?
It is probably just an alternative, but it was meant to better:) In
the beginnings, it seemed that I will be working with the special type
of sparse matrices only. But this is no more the case.
Would also be interesting to hear if Daniel has any suggestions for
improvements and tidying of the vector API in GRASS. I enjoyed
reading the
code and it seems to utilise the existing API very well, which
makes me
think it's possible suggestions for enhancements and further
development
of the API could even come out of this work.
Hmmm, maybe, I was really missing built-in functions for the work with
the single points/vectors. (Vector from
mathematical/geometrical/physical point of view) Something I have
implemented in point.*
But in summary, I had to search hard to find these few suggestions
for
improvement! It looks like a really excellent piece of work and it
will be
great to have it in GRASS.
Paul
Thanks Paul for your feedback!
I dont know what commit/version did you use, but from the above, it
seems that it was not the very last commit. Well, there were no
changes in the code, but I documented displacement and "network
generalization" operations. Just to keep you informed about the newest
functionalities of v.generalize:) To tell the truth, "displacement"
has very impressive results! (Stay tuned for the tutorial, everything
will be there:)
Daniel
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