[gdal-dev] Questions about polygonization

[John Twilley]

The program is not shifting the pixels, but the program does assume
the pixels to be points and not rectangular areas so maybe I will have
to shift the pixels.  Do you know what transformation I would need to
use?

Jack.
--
mathuin at gmail dot com


On Tue, Aug 19, 2014 at 7:17 PM, Chaitanya kumar CH
<chaitanya.ch at gmail.com> wrote:
> Jack,
> The GDALPolygonize algorithm draws the polygon edges along the pixel edges.
> It assumes the pixels to be rectangular areas instead of points.
> In the example you described, the first polygon should contain four vertices
> with the pixel in the centre.
> Make sure that the program you are using to display them is not shifting the
> pixels.
>
> On 20 Aug 2014 07:12, "John Twilley" <mathuin at gmail.com> wrote:
>>
>> I am trying to use GDAL's polygonize algorithms to help me identify
>> regions in landcover data.  I made a trivial example, but I am having
>> trouble understanding the results that I get, nor can I determine how
>> to get the results I want.
>>
>> Given the following raster:
>>
>> 11 11 11 11 11
>> 11 11 11 11 11
>> 11 11 12 11 11
>> 11 11 11 11 11
>> 11 11 11 11 11
>> 11 11 11 11 11
>>
>> If I were to polygonize this raster, I would expect two polygons:  one
>> which contains the single point 12, and another with an outer ring was
>> the perimeter of the raster and an inner ring circling the single
>> point 12.  I would also expect that the first polygon would contain
>> the single point 12 and that the second polygon would contain all the
>> points 11.
>>
>> Instead, what I got was a polygon starting from the single point 12
>> and extending down and to the right one pixel, and another polygon
>> with an outer ring that went off the down and right edges and an inner
>> ring equal to the first polygon's outer ring.  The first polygon
>> doesn't actually contain any points and the second one doesn't contain
>> any points on either ring but does contain all the other points.
>>
>> What do I need to do to get the results I am expecting?
>>
>> Jack.
>> --
>> mathuin at gmail dot com
>> _______________________________________________
>> gdal-dev mailing list
>> gdal-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/gdal-dev