OGR Python Binding – Get the Outer Ring Inner Ring for a Given Polygon

Question

I am trying to read the geometry of some shapefiles, now I meet some problem when I try to identify the polygon with holes, since a polygon may(or not) have one hole(or multiple holes).

This is the code I am using:

def _get_features(coord,datalayer):
    bbox=_get_bbox(coord)
    datalayer.SetSpatialFilter(bbox)

    layer_def=datalayer.GetLayerDefn()
    geom_type=layer_def.GetGeomType()

    for feature in datalayer:
        geometry = feature.geometry().Clone()
        if not geometry.Intersect(bbox):
            continue        
        geometry = geometry.Intersection(bbox)
        fixed_geom=_get_coordinate(geometry)

        #maybe a polygon with hole
        if isinstance(fixed_geom[0], list):
            if geom_type is wktPolygon:
                # now, how to get the rings? outer, inner, multiple inner?





def _get_coordinate(geometry,result=None):
    if result is None:
        result=[]

    if geometry.GetPointCount()>0:
        for i in range(0,geometry.GetPointCount()):
            point=geometry.GetPoint(i)[:2]
            result.extend(map(_fix_coordinate,point))
    elif geometry.GetGeometryCount()>0:
        for i in range(0,geometry.GetGeometryCount()):
            geom=geometry.GetGeometryRef(i)
            _get_coordinate(geom,result)

    return result

Any suggestion?

BTW, is it possible a hole inside a hole of a polygon?

Answer 1

If you have a Polygon you can get the number of rings using

geometry.GetGeometryCount()

The first ring is the outer ring, the following rings are inner rings (aka holes).

Here a small script:

print "Next polygon:"
nbrRings = geometry.GetGeometryCount()
for i in range(nbrRings):
    print geometry.GetGeometryRef(i)

The output for the following polygons is:

Next polygon:
LINEARRING (0.916058394160584 0.333333333333333,1.193430656934307 0.253041362530414,0.947688564476886 0.038929440389294,0.925790754257908 -0.423357664233577,0.409975669099757 -0.381995133819951,0.305352798053528 0.126520681265207,0.916058394160584 0.333333333333333)
LINEARRING (0.577858880778589 0.085158150851582,0.451338199513382 0.004866180048662,0.616788321167884 -0.27007299270073,0.757907542579075 -0.065693430656934,0.577858880778589 0.085158150851582)
LINEARRING (0.774939172749392 0.211678832116788,0.738442822384428 0.160583941605839,0.762773722627737 0.121654501216545,0.791970802919708 0.116788321167883,0.830900243309002 0.131386861313869,0.774939172749392 0.211678832116788)
Next polygon:
LINEARRING (-0.784671532846715 0.669099756690998,-0.495133819951338 0.778588807785888,-0.495133819951338 0.778588807785888,-0.412408759124087 0.708029197080292,-0.156934306569343 0.537712895377129,-0.487834549878345 0.347931873479319,-0.424574209245742 0.114355231143552,-0.706812652068126 0.077858880778589,-1.09610705596107 0.464720194647202,-0.784671532846715 0.669099756690998)