Shapefiles – Determine if Polygon Type is Multipolygon

polygonshapefile

I'm wondering about the ESRI shapefile with shape type 5.

When reading the spec I see that "A polygon may contain multiple
outer rings.". Does this mean that these outer rings must be combined to form a polygon or does it mean that it is in fact a "MultiPolygon" type? So that multiple polygons can be in one entry?

EDIT: Background => I'm writing a reader for shapefiles.

Best Answer

From the GDAL page for the Shapefile driver:

SHPT_POLYGON shapefiles, reported as layers of type wkbPolygon, but depending on the number of parts of each geometry, the actual type can be either OGRPolygon or OGRMultiPolygon.

So the answer to your question is YES ;-)

Related Solutions

Shapely – Translating Between Polygons and MultiPolygons Using Shapely

Further to relet's answer on how to get individual polygons, you can then run an intersection on all the polygons to create the holes. If your dataset contains overlapping polygons though you're out of luck.

Explain again what is wrong with existing shapefile readers?

Would it not be easier to export feature IDs and M values from the shapefile and then join them back to the polygons after using an existing shapefile reader?

For multipatches you can use the same technique of assigning polygon IDs to a "patch ID" and then adding this attribute back to the features.

Edit: Whilst you say you don't want to use OGR, just in case you change your mind..

import ogr
# Get the driver
driver = ogr.GetDriverByName('ESRI Shapefile')
# Open a shapefile
shapefileName = "D:/temp/myshapefile.shp"
dataset = driver.Open(shapefileName, 0)

layer = dataset.GetLayer()
for index in xrange(layer.GetFeatureCount()):
    feature = layer.GetFeature(index)
    geometry = feature.GetGeometryRef()
    #geometry for polygon as WKT, inner rings, outer rings etc. 
    print geometry

The geometry should be output as follows:

POLYGON ((79285 57742,78741 54273...),(76087 55694,78511 55088,..))

The first bracket contains the coords of the exterior ring, subsequent brackets the coords of interior rings. If you have Z values points should be in the format 79285 57742 10 (where the last coord is a height).

Otherwise you could use the Shapely Contains and Within functions to assess every polygon with each other and apply a spatial index beforehand - http://pypi.python.org/pypi/Rtree/ to speed up processing.

[GIS] intersection between two multipolygons yielding anomalous GeometryCollection object full of LineString’s & Polygon’s (trying to get intersect area)

Solution for now: ignore the LineStrings, these are probably produced by an "st_touches" type spatial join where only the perimeters of the multipolygons are touching

RGeo allows you to break out the pieces of a Geometry Collection so I wrote some code to catch the intersect shape if its a GeometryCollection type, sending it to a different area to pluck out the polygons, sum up their size, mark that as the intersect size, and move on

    x = shape.intersection(p.proj_shape_2264)
    if x.geometry_type.to_s == "GeometryCollection"
      area = 0
      (0..(x.count - 1)).to_a.each do |k| 
        collection_shape = x.geometry_n(k)
        next if collection_shape.geometry_type.to_s == "LineString"
        area  += collection_shape.area            
      end
      z.intersect_size = area
    else

Best Answer

Related Solutions

Shapely – Translating Between Polygons and MultiPolygons Using Shapely

[GIS] intersection between two multipolygons yielding anomalous GeometryCollection object full of LineString’s & Polygon’s (trying to get intersect area)

Related Question