[GRASS5] Re: v.clean rmsa option - what does it do ?

[Radim Blazek]

On 4/28/06, Moritz Lennert <mlennert at club.worldonline.be> wrote:
> >>>>The man page says "remove small angles between lines at nodes", but I
> >>>>don't really understand what this means and the French translators of
> >>>>the manuals are trying to understand...
> >>>
> >>>
> >>>If angle between 2 lines is smaller than threshold the lines
> >>>are broken and the partialy merged.
> >>
> >>Thanks for the explanation.
> >>
> >>So if I understand correctly it does what is shown here:
> >>
> >>http://geog-pc40.ulb.ac.be/grass/rmsa.pdf
> >
> >
> > No, this can be don with 'prune' but tolerance is distance.
> >
> > I have attached picture of what rmsa does.
>
>
> Thanks, that makes it much clearer. However, one more thing. In the
> source code you write:
>
>
> "It may happen that even if the angle between 2 boundaries at node is
> very small, the calculated angle is 0 because of representation error."
>
> And when one tries to set a threshold, the module says "WARNING:
> Threshold for tool 1 may not be > 0, set to 0".
>
> So, does this mean that this tool only works automatically for those
> nodes where the calculation ends up with an angle of 0 ? The user cannot
> change this threshold.
>
> Should the description of the tool thus read something like this:
>
> rmsa: remove small angles between lines at nodes (Only concerns angles
> which are so small that the calculated angle is 0. The user cannot
> change this threshold.)

Yes.
Radim

> Moritz
>
> >
> > Radim
> >
> >
> >>i.e. converting the black line with a small angle into the new red line.
> >>
> >>However, when I try to apply this to this line:
> >>
> >>http://geog-pc40.ulb.ac.be/grass/rmsa_example.jpg
> >>
> >>defined as follows in v.build:
> >>
> >> > v.build brol
> >>Building topology ...
> >>3 primitives registered
> >>Building areas:  100%
> >>0 areas built
> >>0 isles built
> >>Attaching islands:
> >>Attaching centroids:  100%
> >>Topology was built.
> >>Number of nodes     :   4
> >>Number of primitives:   3
> >>Number of points    :   0
> >>Number of lines     :   3
> >>Number of boundaries:   0
> >>Number of centroids :   0
> >>Number of areas     :   0
> >>Number of isles     :   0
> >>
> >>
> >>
> >>in the following projection:
> >>
> >>  > g.proj -p
> >>-PROJ_INFO-------------------------------------------------
> >>name       : Lambert Conformal Conic
> >>proj       : lcc
> >>datum      : bel72
> >>a          : 6378388
> >>es         : 0.0067226700
> >>lat_1      : 49.83333333333334
> >>lat_2      : 51.16666666666666
> >>lat_0      : 90
> >>lon_0      : 4.356939722222222
> >>x_0        : 150000.01256
> >>y_0        : 5400088.4378
> >>no_defs    : defined
> >>-PROJ_UNITS------------------------------------------------
> >>unit       : metre
> >>units      : metres
> >>meters     : 1
> >>
> >>Then whatever the threshhold value I set (going up to ridiculous values,
> >>I continue to get 4 nodes, with  3 lines which look identical to the
> >>original.
> >>
> >>Am I still wrong in my understanding of rmsa, or is it something else ?
> >>
> >>Moritz
> >>
> >>
> >>
> >> ------------------------------------------------------------------------
> >>
>
>