[Gvsig_english] raster limitations
Andre Pais
andre.pais.cvl at gmail.com
Thu Sep 27 12:44:54 CEST 2012
Thanks for the answer Antonio. Unfortunately I didn´t quite well understand
your answer to question 5. My slope classes are: 0 - 10; 10 - 20; 20 - 30;
30 - 40 e >40 so i would do this:
min value max value new value
0 10 1
Sextante - Reclassify - Min < x <= Max
10 20 2
20 30 3
30 40 4
40 101.25 (my max slope) 5
By the way is it normal to have a max slope over 100%? I used a DEM created
in Arcmap and converted it to TIF so that I could use it in gvSIG since I
didn´t know how to use a mask in gvSIG (now I know thanks to you). Then I
went to Sextante > Slope > Percentage, Travis et al. 1975.....the result
varies from 0 to 101.25.
Thanks for the attention once again.
2012/9/27 Antonio Falciano [via OSGeo.org] <
ml-node+s1560n5004907h99 at n6.nabble.com>
> Il 26/09/2012 16.25, Andre Pais ha scritto:
> > Hi there. I´ve downloaded the latest gvSIG version, 1.12 final. I´ve
> > installed it (activated GRASS and SAGA in the Sextante options) and used
> > right away because I have an academic work which consists of developing
> a
> > spacial analysis exercice using raster operations, first using ArcGIS
> and
> > then I must replicate it using an opensource, in my case gvSIG. In the
> > process, here are a few problems I´ve came up to:
>
> Hi Andre,
>
> > 1 - Sextante doesn´t allow to create a DEM-TIN like I did using 3d
> Analyst
> > of Arcmap; SAGA has a function called "Shape to TIN" but it doesn´t
> > generates an output ;
>
> The first one is a known limitation of gvSIG and SEXTANTE, because they
> are not able to manage a 3D TIN at the moment, but this is not the only
> way to create DEM fortunately.
> The second one may depend by SAGA (and GRASS) bindings in SEXTANTE which
> are not stable yet and so not all SAGA modules work fine.
>
> > 2 - I need to create a DEM in order to generate slope and aspect charts
> for
> > my area of study. Since there isn´t an option to define a mask (like
> there
> > is in Arcmap), I did a clip using the shape with altimetric information
> as
> > input and the area of study as clipping layer. The result has the
> altimetric
> > information for my area of study. Then I need to rasterize this result
> > layer. So I went to Sextante and used "Rasterize vector layer" in order
> to
> > convert it to raster format, having defined an appropriate cell size and
> the
> > extent as my area of study ( in the hope that the result would have the
> form
> > of my area of study). Then I did a void filling ( I´ve read this
> somewhere)
> > to generate a proper DEM. But unfortunately the result is a rectangular
> DEM
> > instead of being limited to my area of study (which by the way is the
> > portuguese equivalent to an ayuntamiento) Why?
>
> This behavior is what we should expect. In fact, the result of Void
> filling doesn't take in count your mask because of its definition: it
> "completes the information of one raster layer that has cells without
> data. The information to complete the cells without data is taken from
> the surrounding cells using interpolation methods." (Source: SEXTANTE
> help). So the mask should be applied after (and not before) the Void
> filling by multiplying them.
>
> > 3 - There is an option in Sextante called "Rasterize vector layer (for
> > mask)" but it doesn´t seem to do what i want.
>
> I have tried it and seems to work like a charm. Its result should be a
> mask with ones where there are vector geometries, otherwise zero.
>
> > 4 - The GRASS function v.to.rast is to confusing and doesn´t work
> properly.
> > It gives an error :Failed to import data into GRASS mapset;
>
> See my answer to the point 1. These are SEXTANTE bugs, I'm sorry.
>
> > 5 - The reclassify function of sextante is a bit confusing as well. It
> > requires a min value, a max value and a new value. For example, if i
> need to
> > reclassify a slope chart, e.g, 5 classes (0-10%, 10-20%, 20-30%, 30-40%
> and
> >> 40%). I have put the min and max for the each class. Why do I need to
> write
> > new values? Shouldn´t the program assume that? I´ve tried to write 1, 2,
> 3,
> > 4 ,5 (to match the classes) but the output has 0 value in some pixels
> but
> > there is no class 0. Why?
>
> Because it's not possible to write in the reclassified raster "0-10%",
> "10-20%", "20-30%", "30-40%" and "> 40%", so let's say we need some
> aliases in order to do this task. AFAIK it works everywhere in this way.
>
> > 6 - Why haven´t the programmers of the latest version improved the
> symbology
> > options and visualization in rasters. It´s a pain having to work in
> black,
> > grey and white and not be able to see the classes of a certain raster
> with
> > different colors (with this being shown in the view)
>
> You can apply the color table you like, as Francisco suggested.
>
> Cheers,
> Antonio
>
> --
> Antonio Falciano
> http://www.linkedin.com/in/antoniofalciano
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