[Gvsig_english] raster limitations

[Antonio Falciano]

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