[gdal-dev] S57 questions

[ogr user]

Dear all and Frank,

 I sent this patch out a few times before - never got any replies. It's
ok if you are not interested, though I have to say that it works very
well for me and improved handling of data. However, would someone kindly
confirm that you have looked at this patch, acknowledged it and do not
wish to apply it. I'll then go back to maintaining my private copy :)

This following is a fix to a problem I encountered back in the day, to
properly identify exterior vs. interior rings of polygons. This solves
incorrect reading of a number of S57 charts (as an example see US5CA92M,
feature RCID 1167).

Thanks,
--M

===================================================================
--- ogr/ograssemblepolygon.cpp    (revision 17)
+++ ogr/ograssemblepolygon.cpp    (working copy)
@@ -276,6 +276,40 @@
 // perhaps even ordering the direction of rings, though this isn't
 // required by the OGC geometry model.

+    double maxarea, tarea;
+    int maxring = -1, rn, rcount;
+    OGREnvelope tenv;
+    OGRLinearRing *tring;
+
+    tring = poPolygon->getExteriorRing();
+    if (tring) tring->getEnvelope(&tenv);
+    maxarea = (tenv.MaxX - tenv.MinX) * (tenv.MaxY - tenv.MinY);
+
+    rcount = poPolygon->getNumInteriorRings();
+    for (rn = 0; rn < rcount; ++rn) {
+      tring = poPolygon->getInteriorRing(rn);
+      tring->getEnvelope(&tenv);
+      tarea = (tenv.MaxX - tenv.MinX) * (tenv.MaxY - tenv.MinY);
+      if (tarea > maxarea) {
+         maxarea = tarea;
+         maxring = rn;
+      }
+    }
+
+    if (maxring != -1) {
+       OGRPolygon  *poNewPoly = new OGRPolygon();
+
+       poNewPoly->addRing(poPolygon->getInteriorRing(maxring));
+       poNewPoly->addRing(poPolygon->getExteriorRing());
+       for (rn = 0; rn < rcount; ++rn) {
+          if (rn == maxring) continue;
+          poNewPoly->addRing(poPolygon->getInteriorRing(rn));
+       }
+
+       delete poPolygon;
+       poPolygon = poNewPoly;
+    }
+
     if( peErr != NULL )
     {
         if( bSuccess )

Frank Warmerdam wrote:
>
> Mike,
>
> The S-57 driver uses OGRBuildPolygonFromEdges() to form the set of lines
> into a polygon.  No effort is currently made to properly represent
> exterior vs. interior rings as is required by the simple features
> geometry
> specification.
>
> So, I believe this is a bug of the S-57 driver and
> OGRBuildPolygonFromEdges()
> function.  In OGRBuildPolygonFromEdges() I see this comment:
>
> // Eventually we should at least identify the external ring properly,
> // perhaps even ordering the direction of rings, though this isn't
> // required by the OGC geometry model.
>
> You might want to file a ticket on this issue, though I can't say when
> the
> issue will be fixed.
>
> Best regards,

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