[gdal-dev] gdal_polygonize and polygon edges

[Jeff Lake]

It all depends on the data your having gdal_polygonize contour ...

here is a snippet of what I'm doing with a csv I'm creating of current 
weather

/usr/local/bin/gdal_grid -zfield "temp" -a 
invdist:power=2.0:smoothing=1:nodata=-9999 -outsize 800 600 -l temp 
/var/www/html/metar/temp.vrt /var/www/html/metar/temp.tif
python /usr/local/bin/gdal_polygonize.py /var/www/html/metar/temp.tif -f 
"ESRI Shapefile" /var/www/html/metar/temp/temp.shp temp temp

and then the resulting shapefile is used to create the following
http://www.michiganwxsystem.com/maps/curfore/

I have messed around with the 'power' and 'smoothing' settings these 
seem the remove the stair steps quite well


-----------------------------------------------------
-Jeff Lake
MichiganWxSystem.com
AllisonHouse.com
TheWeatherCenter.net
GRLevelXStuff.com

On 2/19/2013 15:12, Jeff Lacoste wrote:
> Yes visually attractive or smooth polygons is the goal. Thanks again 
> Frank.
>
> Doing a web search about simplification algorithm i found one named 
> '*Ramer-Douglas--Peucker' 
> (*http://en.wikipedia.org/wiki/Ramer-Douglas-Peucker_algorithm).
> It appears that 'Geos'' library implement this algorithm. Is this 
> algorithm exposed through OGR ?
>
> Could this algorithm help smoothing a polygon without necessary make 
> the new nodes too far from the original one ? Or may be there
> are other *more* recommended algorithms ?
>
> If any one could suggest a simplification algorithm or had some 
> experience with smoothing polygons, I appreciate their input.
>
> Thanks
>
> Jeff
>
>
>
>
> On Tue, Feb 19, 2013 at 2:42 PM, Frank Warmerdam <warmerdam at pobox.com 
> <mailto:warmerdam at pobox.com>> wrote:
>
>     On Tue, Feb 19, 2013 at 11:28 AM, Jeff Lacoste
>     <jefflacostegdal at gmail.com <mailto:jefflacostegdal at gmail.com>> wrote:
>     > Hi Frank,
>     >
>     > Thanks for your quick response. Following the edges of the
>     pixels seems a
>     > perfect solution for non continuous grid (ex. land use, etc.) as
>     > the boundary between the class is important to keep when
>     constructing the
>     > polygon. However for continuous grid (.ex elevations), the
>     boundaries are
>     > a bit not clear and not clear cut. When following the pixels
>     edges, the
>     > created polygons appear to have the stairs effect and are less
>     visually
>     > attractive.
>     >
>     > I thought of a smoothing the polygons to not have *rough* edges
>     using the
>     > current gdal_polygonize by trying to not follow the pixels edges
>     and use
>     > instead of the
>     > pixel centers. Basically do something similar to what contour
>     generator does
>     > by treating the raster values as continuous.
>
>     Jeff,
>
>     Ah, I see, you are looking for visually attractive polygons from
>     continuous fields.
>
>     I have wondered if it would be reasonable to produce a version of the
>     contour generator that actually produces polygon regions.  If we had
>     that then applying appropriate simplification to the resulting very
>     detailed edges should give something attractive and with reasonable
>     information density.  An appropriate simplification algorithm might do
>     this in a reasonable way for the existing polygonize output but I
>     don't know enough about the simplification algorithms to suggest one.
>
>     I don't think aiming for pixel centers in gdal_polygonize would really
>     solve the problem.
>
>     Best regards,
>     --
>     ---------------------------------------+--------------------------------------
>     I set the clouds in motion - turn up   | Frank Warmerdam,
>     warmerdam at pobox.com <mailto:warmerdam at pobox.com>
>     light and sound - activate the windows |
>     http://pobox.com/~warmerdam <http://pobox.com/%7Ewarmerdam>
>     and watch the world go round - Rush    | Geospatial Software Developer
>
>
>
>
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