Thanks Even and Jukka for the clarification. Due to historical reasons, MapWinGIS is not using GDAL/OGR to read/write shapefiles. It is using its own implementation. For now, I will clip with OGR first and then fix with MapWinGIS, this will produce valid shapes. It does take a bit longer.
Should I create a test dataset to reproduce this issue. I'm not capable of debugging GDAL myself. Paul *Paul Meems * Release manager, configuration manager and forum moderator of MapWindow GIS. www.mapwindow.org Owner of MapWindow.nl - Support for Dutch speaking users. www.mapwindow.nl *The MapWindow GIS project has moved to GitHub <https://github.com/MapWindow>!* Download the latest MapWinGIS mapping engine. <https://github.com/MapWindow/MapWinGIS/releases> Download the latest MapWindow 5 open source desktop application. <https://github.com/MapWindow/MapWindow5/releases> 2017-10-13 13:01 GMT+02:00 Even Rouault <[email protected]>: > On vendredi 13 octobre 2017 09:06:46 CEST Paul Meems wrote: > > > Thanks Even for your quick reply. > > > > > > I was using MapWinGIS to perform the validation and I assumed it was just > > > using GEOS. > > > I did a closer look at the code and it is first doing some internal > checks > > > like the minimum number of points and the order of the points. > > > The code is checking if the polygon shape only has 1 part. If so it > should > > > be clockwise. > > > After that, it is using the GEOS:IsValid method. > > > As I understand you correctly I should interpret this message as a > warning, > > > not as an error. > > > > Different software, standard, formats have differen rules on if there must > be a winding order and which one it is... > > > > A return of false by GEOS:IsValid() should be examined a bit more closely > (but apparently you get this error before calling IsValid()) > > > > > > > > I also tried your suggestion: > > > ogr2ogr -sql "select ST_ForceLHR(geometry) from ogr_clipped" -dialect > > > sqlite ogr_clipped_lhr.shp ogr_clipped.shp > > > > Jukka answered on the LHR vs RHR confusion, but I completely missed the > fact that you were outputting to shapefile. In that case, whatever the > original orientation, or if you modified it with a SQL statement, the OGR > shapefile writer will force the polygons to follow the shapefile > specification, which requires the outer ring to be in clockwise order. So > you shouldn't get this error... Which means that either there's an issue in > some case in the shapefile driver (numerical precision issue), or in the > check in MapWinGIS. > > > > Even > > > > -- > > Spatialys - Geospatial professional services > > http://www.spatialys.com >
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