[gdal-dev] How can I support polygons with islands in my APPLICATION USINGgdal/ogr library?
sepideh
sepideh.abadpour87 at gmail.com
Thu Sep 12 12:17:28 PDT 2013
well thanks how can I run ogrinfo to get the structures of my geometries in
a layer?
On Thu, Sep 12, 2013 at 11:01 PM, Even Rouault [via OSGeo.org] <
ml-node+s1560n5077587h27 at n6.nabble.com> wrote:
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> Le jeudi 12 septembre 2013 19:56:11, sepideh a écrit :
>
> > MyTest.rar <http://osgeo-org.1560.x6.nabble.com/file/n5077579/MyTest.rar>
>
> > I have uploaded my MyTest.shp created in ArcGIS. If you open it, you
> will
> > see that it has nine polygons and in the attribute table of the layer, I
> > have explained about the features of each polygon. My problem is: I'm
> > developing a Visual C++ MFC application with GDAL/OGR library that will
> > show shapefiles in an OpenGL window. And I have created this shapefile
> to
> > test some parts of my application. *My application will only support
> ESRI
> > shapefiles.* As you know, the OGRPolygon class has only a member
> > function *getNumInteriorRings* and not such function for exterior rings.
> I
> > mean it supports multiple interior rings but not multiple exterior
> rings.
> > So I decided to fetch one of the rings of each polygon with
> > *getExteriorRing()* and the others with *getInteriorRing(int)* and
> decide
> > with *isClockwise()* method whether it is an exterior or interior ring.
> > This approch is best for the polygons with *FIDs : 0,1,2,3,4,5,6 * in my
> > MyTest.shp. When I use this code for them: /int NumberOfInnerRings =
> > poPolygon
> > ->getNumInteriorRings();/ I get these results: FID = 0 -> 5FID = 1 ->
> > 1FID = 2 -> 0FID = 3 -> 1FID = 4 -> 2FID = 5 -> 0FID = 6 -> 2 showing
> > that one of the rings has been accepted as exterior and the others as
> > interior polygons. You see that the polygons with the above FIDs are
> > simple polygons, multipart polygons and polygons with holls inside. But
> > for the two other polygons this is not the case. These are polygons that
> > have islands in as you see in this link:
> >
> http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Creating_new_
> > donut_holes_and_island_polygons/001t0000003p000000/
> > <
> http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Creating_ne
> > w_donut_holes_and_island_polygons/001t0000003p000000/> I get these wrong
> > results for them as the number of Interior rings: FID = 7 -> 1FID = 8
> ->
> > 2 meaning that islands are not known niether as interior nor exterior
> > polygons!!! Can you tell me how can I support islands with GDAL/OGR
> > library or introduce me another open source that does this?
>
> The shapefile displays correctly in QGIS which uses OGR to read Shapefile
> which
> makes me think that the problem is not in OGR.
> Your above description is a bit confusing but my understanding is that
> your
> problem comes from the fact that the shapefile has a mix of 2 distinct
> geometry
> type :
> - POLYGON: they have always one exterior ring, and 0 or more interior ring
> - MULTIPOLYGON: that are maded of 0 or more polygons. On multipolygons,
> getNumInteriorRings() cannot be used. You must iterate first over each
> polygon
> part of the multipolygon
>
> You don't need to check the clockwise orientation to determine if a ring
> is
> exterior or interior. This is already done by the OGR Shapefile driver.
>
> ogrinfo with summary mode will give you the structure of the geometries :
>
> $ ogrinfo MyTest -al -geom=summary
> INFO: Open of `MyTest'
> using driver `ESRI Shapefile' successful.
>
> Layer name: MyTest
> Geometry: Polygon
> Feature Count: 9
> Extent: (-14519050.256578, -31065929.994238) - (26123782.221403,
> 8007498.521952)
> Layer SRS WKT:
> PROJCS["WGS_1984_UTM_Zone_39N",
> GEOGCS["GCS_WGS_1984",
> DATUM["WGS_1984",
> SPHEROID["WGS_84",6378137.0,298.257223563]],
> PRIMEM["Greenwich",0.0],
> UNIT["Degree",0.0174532925199433]],
> PROJECTION["Transverse_Mercator"],
> PARAMETER["False_Easting",500000.0],
> PARAMETER["False_Northing",0.0],
> PARAMETER["Central_Meridian",51.0],
> PARAMETER["Scale_Factor",0.9996],
> PARAMETER["Latitude_Of_Origin",0.0],
> UNIT["Meter",1.0]]
> Id: Integer (6.0)
> type_poly: String (50.0)
> OGRFeature(MyTest):0
> Id (Integer) = 0
> type_poly (String) = polygonWithFiveHolesInside
> POLYGON : 22 points, 5 inner rings (11 points, 11 points, 17 points, 9
> points, 6 points)
>
> OGRFeature(MyTest):1
> Id (Integer) = 0
> type_poly (String) = polygonWithTwoSeperateParts
> MULTIPOLYGON : 2 geometries:
> POLYGON : 11 points
> POLYGON : 11 points
>
> OGRFeature(MyTest):2
> Id (Integer) = 0
> type_poly (String) = singlePolygon
> POLYGON : 10 points
>
> OGRFeature(MyTest):3
> Id (Integer) = 0
> type_poly (String) = polygonWithOneHoleInside
> POLYGON : 11 points, 1 inner rings (17 points)
>
> OGRFeature(MyTest):4
> Id (Integer) = 0
> type_poly (String) = polygonWithTwoHolesInside
> POLYGON : 9 points, 2 inner rings (8 points, 6 points)
>
> OGRFeature(MyTest):5
> Id (Integer) = 0
> type_poly (String) = singlePolygon
> POLYGON : 6 points
>
> OGRFeature(MyTest):6
> Id (Integer) = 0
> type_poly (String) = polygonWithThreeSeperateParts
> MULTIPOLYGON : 3 geometries:
> POLYGON : 7 points
> POLYGON : 9 points
> POLYGON : 11 points
>
> OGRFeature(MyTest):7
> Id (Integer) = 0
> type_poly (String) = PolygonWithAnIslandInside
> MULTIPOLYGON : 2 geometries:
> POLYGON : 9 points, 1 inner rings (9 points)
> POLYGON : 6 points
>
> OGRFeature(MyTest):8
> Id (Integer) = 0
> type_poly (String) = PolygonWithTwoIslandsInsideOneOfTheIslandsHasAHole
> MULTIPOLYGON : 3 geometries:
> POLYGON : 7 points, 1 inner rings (6 points)
> POLYGON : 6 points, 1 inner rings (4 points)
> POLYGON : 6 points
>
> On the later,
> - the "main" polygon is "POLYGON : 7 points, 1 inner rings (6 points)"
> - the island with a hole is the "POLYGON : 6 points, 1 inner rings (4
> points)"
> - the other island is "POLYGON : 6 points"
>
>
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