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

Etienne etiennesky at yahoo.com
Fri Aug 5 08:10:06 EDT 2011

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


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/3X4hI45 and http://db.tt/phqnGci
>I tried using gdal_grid, but it doesn't seem to support a shapefile with POLYGONs.
>many thanks!
>gdal-dev mailing list
>gdal-dev at lists.osgeo.org

Best regards,
Chaitanya kumar CH.

17.2416N 80.1426E
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