[GRASS-user] isoareas from DEM data

GRASS GRASS user
1

Hello,

I am absolutely new to GRASS, so please forgive me if my questions are
a bit naive.

I am trying to create isoareas from DEM data.
So far I have managed to create the polygons from DEM data by

r.mapcalc "newElev2 = round(elevation.dem / 100) * 100"

100%

r.to.vect

However, r.to.vect does not support setting z value to polygons.

How should I go on to transfer elevation attribute to the created polygons?
Probably by using v.what.rast in some way, but how?

Is it possible to sort polygons on elevation before writing them to output file?

Thank you in advance,

/Stefan

2

On Fri, November 2, 2007 00:45, Stefan Eriksson wrote:

Hello,

I am absolutely new to GRASS, so please forgive me if my questions are
a bit naive.

I am trying to create isoareas from DEM data.
So far I have managed to create the polygons from DEM data by

r.mapcalc "newElev2 = round(elevation.dem / 100) * 100"

100%

r.to.vect

Have you tried r.contour ?

Moritz

3

On Nov 2, 2007 1:48 AM, Moritz Lennert <mlennert@club.worldonline.be> wrote:

On Fri, November 2, 2007 00:45, Stefan Eriksson wrote:
> Hello,
>
> I am absolutely new to GRASS, so please forgive me if my questions are
> a bit naive.
>
> I am trying to create isoareas from DEM data.
> So far I have managed to create the polygons from DEM data by
>> r.mapcalc "newElev2 = round(elevation.dem / 100) * 100"
> 100%
>> r.to.vect

Have you tried r.contour ?

Yes, but I want polygons rather than lines as output. r.contour only
produce lines, does it not?

// Stefan

4

On 11/2/07, Stefan Eriksson <stefan.piaostefan@gmail.com> wrote:

On Nov 2, 2007 1:48 AM, Moritz Lennert <mlennert@club.worldonline.be> wrote:
> On Fri, November 2, 2007 00:45, Stefan Eriksson wrote:
> > Hello,
> >
> > I am absolutely new to GRASS, so please forgive me if my questions are
> > a bit naive.
> >
> > I am trying to create isoareas from DEM data.
> > So far I have managed to create the polygons from DEM data by
> >> r.mapcalc "newElev2 = round(elevation.dem / 100) * 100"
> > 100%
> >> r.to.vect
>
> Have you tried r.contour ?

Yes, but I want polygons rather than lines as output. r.contour only
produce lines, does it not?

Another approach might be to classify the raster using r.reclass then
convert to polygons using r.to.vect.

--
Matthew T. Perry
http://www.perrygeo.net

"Never ascribe to malice, that which can be adequately explained by
incompetence."

5

after r.contour use v.type to change line to boundary
best
maciej

Maciej Tomaszczyk
Polish Geological Institute
Geological Mapping Department

-----Wiadomość oryginalna-----
Od: grassuser-bounces@grass.itc.it w imieniu Stefan Eriksson
Wysłano: Pt 2007-11-02 19:11
Do: Moritz Lennert
DW: grassuser@grass.itc.it
Temat: Re: [GRASS-user] isoareas from DEM data

On Nov 2, 2007 1:48 AM, Moritz Lennert mlennert@club.worldonline.be wrote:

On Fri, November 2, 2007 00:45, Stefan Eriksson wrote:

Hello,

I am absolutely new to GRASS, so please forgive me if my questions are
a bit naive.

I am trying to create isoareas from DEM data.
So far I have managed to create the polygons from DEM data by

r.mapcalc “newElev2 = round(elevation.dem / 100) * 100”
100%
r.to.vect

Have you tried r.contour ?

Yes, but I want polygons rather than lines as output. r.contour only
produce lines, does it not?

// Stefan

grassuser mailing list
grassuser@grass.itc.it
http://grass.itc.it/mailman/listinfo/grassuser

6

Stefan Eriksson wrote:

> > I am trying to create isoareas from DEM data.
> > So far I have managed to create the polygons from DEM data by
> >> r.mapcalc "newElev2 = round(elevation.dem / 100) * 100"
> > 100%
> >> r.to.vect

Moritz Lennert wrote:

> Have you tried r.contour ?

Stefan Eriksson wrote:

Yes, but I want polygons rather than lines as output. r.contour only
produce lines, does it not?

Convert lines to polygons with 'v.type line,boundary' and v.centroids.

Areas in GRASS are virtual features made from boundaries + centroids.
The centroid will typically hold the attribute data (like elevation range) and
the boundary will not have a category (it is ambiguous as to which area
touching the boundary belongs to the attribute. Note that many GRASS vector
modules only work on features with category numbers, add/remove those with
v.category.

Hamish

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