[gdal-dev] problem rasterizing categorical data from shapefile - 0 values at some intersections

[Chaitanya kumar CH]

I suggested the -where option because your shapefile had only two features
that are (almost)inverse of each other. You can select only one of them. You
might want to use the -at option with this.

What do you mean by 'most representative value'?

On Fri, Aug 5, 2011 at 5:40 PM, Etienne <etiennesky at yahoo.com> wrote:

> Hi and thanks!
>
> I set the init value to 255, and the overlapping pixels are set to that
> value (instead of the default of 0).
>
> Unfortunately, using -where resulted in the same gaps not being filled, and
> a combination of -where and -at selects too many pixels (the borders all
> overlap).
>
> Looking more closely, it seems that the problem is almost exclusively where
> there are tiny gaps between the polygons.
>
> It seems that -at doesn't leave any gaps, although it favors the
> higher-index values (as they are processed later, presumably).
> I would prefer to have the most representative value, any hints?
>
> many thanks, Etienne
>
> ------------------------------
> *From:* Chaitanya kumar CH <chaitanya.ch at gmail.com>
> *To:* Etienne <etiennesky at yahoo.com>
> *Cc:* "gdal-dev at lists.osgeo.org" <gdal-dev at lists.osgeo.org>
> *Sent:* Friday, August 5, 2011 4:22:10 AM
> *Subject:* Re: [gdal-dev] problem rasterizing categorical data from
> shapefile - 0 values at some intersections
>
> Etienne,
>
> Rasters of type Byte cannot hold values below 0. So, the nodata value of -1
> is not practical. It's better to choose 0 or 255 depending on your data or
> choose a different data type.
>
> Your problem arises because the two geometries in the shapefile overlap in
> some places and leave some gaps between them in other places.
> Consider selecting only one feature using the -where option in
> gdal_rasterize. You can also use the -at option but it may not work at all
> resolutions.
>
> On Fri, Aug 5, 2011 at 7:40 AM, Etienne <etiennesky at yahoo.com> wrote:
>
> Hi all,
>
> I am trying to create a raster file with categorical data (land
> vegetation/cover positive indices) from a shapefile containing 1 layer of
> polygons.
>
> I use the following command:
> gdal_rasterize  -tr 0.0005 0.0005  -a_nodata -1 -of GTiff  -a_srs EPSG:4618
> -a "CLASSE_ID" -ot Byte -init -1 -l region1 region1.shp region1-3.tif
>
> This creates a  map which looks good, but there are some holes in the data,
> (individual pixels with a value of 0) at the intersection of 2 polygons. It
> looks like the
> intersections pass through the exact center of these pixels. These pixels
> are not set to missing(-1). Depending on the output resolution, different
> pixels get set to a value of 0.
>
> Has anyone ran into this problem?  Any idea how to fix this?
> Those interested will find 2 small shapefiles here: http://db.tt/3X4hI45and
> http://db.tt/phqnGci
>
>
> I tried using gdal_grid, but it doesn't seem to support a shapefile with
> POLYGONs.
>
> many thanks!
> Etienne
>
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>
>
>
>
> --
> Best regards,
> Chaitanya kumar CH.
>
> +91-9494447584
> 17.2416N 80.1426E
>
>
>


-- 
Best regards,
Chaitanya kumar CH.

+91-9494447584
17.2416N 80.1426E
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