[GRASS-user] Surface

Hi!

I have a shapefile loaded in grass, as a 3D Vector:

±---------------------------------------------------------------------------+
| Layer: pontos2@PERMANENT |
| Mapset: PERMANENT |
| Location: altimetria_Porcos |
| Database: C:/GRASS GIS DataBase |
| Title: |
| Map scale: 1:1 |
| Map format: native |
| Name of creator: plangeo |
| Organization: |

Source date: Wed Sep 10 10:31:28 2008
Type of Map: vector (level: 2)
Number of points: 6913 Number of areas: 0
Number of lines: 0 Number of islands: 0
Number of boundaries: 0 Number of faces: 0
Number of centroids: 0 Number of kernels: 0
Map is 3D: 1
Number of dblinks: 1
Projection: Universe Transverse Mercator (zone 0)
N: 7834144.073 S: 7833041.84504481
E: 441953.15895164 W: 440981.71795164
B: 0 T: 732.748
Digitization threshold: 0
Comments:
±---------------------------------------------------------------------------+

And a column, named “Elevation”:

column:Elevation
description:
type:DOUBLE PRECISION
len:20
scale:6
precision:18
default:
nullok:yes
select:yes
update:yes

I need to create countour… so, I think, I must interpolate a surface from this points making something like a DEM, and after, interpolate the contours… Is this correct?

Now, I’m trying to interpolate a suface with IDW (very slow…)

v.surf.idw input=pontos2@PERMANENT output=teste npoints=12 layer=1 column=elevation --overwrite

Another question: what is the precision of this surface? What contour interval can I generate?

Regards,

Raphael Saldanha
BRAZIL

Hi,

2008/9/10 Raphael Saldanha <saldanha.plangeo@gmail.com>:

I have a shapefile loaded in grass, as a 3D Vector:

[...]

And a column, named "Elevation":

You don't need that, define layer=0 to use z-coordinates directly.

Martin

--
Martin Landa <landa.martin gmail.com> * http://gama.fsv.cvut.cz/~landa *

On Wed, 10 Sep 2008, Raphael Saldanha wrote:

[...]

I need to create countour... so, I think, I must interpolate a surface from
this points making something like a DEM, and after, interpolate the
contours... Is this correct?

Sounds like a good approach.

Now, I'm trying to interpolate a suface with IDW (very slow...)

v.surf.idw input=pontos2@PERMANENT output=teste npoints=12 layer=1
column=elevation --overwrite

You should use layer=0 if you want to read the elevation co-ordinates as the 3rd dimension of the vector data. The way you have written it is reading these from a database field which will slow it down a bit - you don't need a database connection as you are storing the elevation data as part of the vector map itself.

Another question: what is the precision of this surface? What contour
interval can I generate?

The elevation raster map will be generated according to the extents and resolution of your current region. g.region -p will show this. If your region is too big (i.e. extends beyond the region covered by the points) or the resolution is too high that could be another reason why it runs slowly. In general v.surf.idw should be quite fast.

Perhaps that gives you something to go on.

Paul

I need to create countour... so, I think, I must interpolate a surface from
this points making something like a DEM, and after, interpolate the
contours... Is this correct?

Yes.

In addition to IDW, you can use RST for interpolate points and
generate a raster surface.

First all, set appropiate region and resolution to something
reasonably. For example:

g.region -p -a vect=pontos res=100

Adjust res to fit your needs.

Interpolation can be made with:

v.surf.rst input=pontos scolumn=Elevation elev=dem --overwrite

And generate the contours with:

r.contour input=dem output=contour step=2.5

Now, I'm trying to interpolate a suface with IDW (very slow...)

v.surf.idw input=pontos2@PERMANENT output=teste npoints=12 layer=1
column=elevation --overwrite

Another question: what is the precision of this surface? What contour
interval can I generate?

The accuracy of these models depends mainly on the accuracy of the
original data.
  
The resolution of the grid and step (the contour of separation) also
depend on issues of representation, such as the scale. Contour step
(vertical separation) also depends on topography. For example, in
plane surfaces, step must be small.

I hope this will be helpful.
Saludos
Jose

Regards,

Raphael Saldanha
BRAZIL

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