Thanks every one who responded to my post. Jeff: it appears that your approach involve smoothing the original grid before generating contours (or polygons from it). Right ? It would be nice if I could have the smoothing either as an after process operation or ideally as an option within the polygons generation process itself.
Daniel: Using 'SIA' technique sounds promising. Does it apply to polygons as well ? or just polylines ? I would be interested to test (and later use if applicable) your approach. If would like an extra tester, please let me know. Graeme: Is your tracing technique a new algorithm different from GDAL_Polygonize ? Is there a C/C++ code I can build or may be an .exe utility I could use to try out ? p.s: i did not have a chance to try out '*'Ramer-Douglas–Peucker' yet.* Thanks Jeff Lacoste On Tue, Feb 19, 2013 at 5:39 PM, Brian Case <r...@winkey.org> wrote: > a few years ago someone had posted some perl code for making polys from > the contour output. > > the idea was to sort the lines into an m-way tree by contains with the > root node being the extent of the data then each node would be outer > ring and a copy of the children being the inner rings > > i have done this myself and it works > > Brian > > > On Tue, 2013-02-19 at 14:28 -0500, Jeff Lacoste 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. > > > > > > Thanks > > > > > > On Tue, Feb 19, 2013 at 1:44 PM, Frank Warmerdam <warmer...@pobox.com> > > wrote: > > On Tue, Feb 19, 2013 at 10:29 AM, Jeff Lacoste > > <jefflacosteg...@gmail.com> wrote: > > > Hi, > > > > > > I'm using gdal_polygonize.py to create polygon from a grid > > and the output > > > polygons seems to follow the edges > > > of the grid pixels. I was wondering if there is any way to > > change/edit the > > > code to instead of following the edges > > > of the pixels, use the center of the pixels as the nodes of > > the created > > > polygons ? > > > > > > So instead of a polygon following the edges of the pixels, > > it will instead > > > be passing through the center of the connected > > > pixels... This is to avoid having a scale effect of > > following the pixels > > > edges. > > > > > > Jeff, > > > > With the current algorithm, this is not really possible. > > > > I am also not clear what it would mean to go from pixel center > > to > > pixel center. The algorithm attempts to identify the borders > > between > > regions of different pixel values and turn them into > > polygons. What > > does it mean to make a polygon boundary that goes through the > > center > > of a pixel with a particular value? > > > > Another algorithm you might want to keep in mind is the > > contour > > generator. It treats the the raster values as a continuous > > field, and > > builds edges based on linear interpolation between pixel > > centers. > > This gives a result that could pass through a pixel center if > > it > > happens to be an exact contour level. > > > > Best regards, > > -- > > > ---------------------------------------+-------------------------------------- > > I set the clouds in motion - turn up | Frank Warmerdam, > > warmer...@pobox.com > > light and sound - activate the windows | > > http://pobox.com/~warmerdam > > and watch the world go round - Rush | Geospatial Software > > Developer > > > > > > _______________________________________________ > > gdal-dev mailing list > > gdal-dev@lists.osgeo.org > > http://lists.osgeo.org/mailman/listinfo/gdal-dev > > -- > Brian Case > KF7WPK > r...@winkey.org > > >
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