[Qgis-user] Shape file - Change "Polygon" to "Multipolygon"

[Andrea Giudiceandrea]

> Riccardo riccardopani at hotmail.com
> Thu Apr 27 03:06:54 PDT 2023

> I have shape file already created for a standard delivery.
> The geometry type of this shape file is “Polygon” and I have to change it into “Multipolygon”.
> Is it possible? And how can I change this property in a shape file already created without creating a new one?

Hi Riccardo,
if with "shape file" you are referring to the ESRI Shapefile format, 
then, as you can read in the "ESRI Shapefile Technical Description" [1], 
there is only one type of polygon layer (apart from the M and Z+M 
variants) in the ESRI Shapefile format: it is called Polygon in the 
format nomenclature and it can stores natively both multi-part and 
single-part (which are actually multipart with only 1 part) geometries.

An ESRI Shapefile Polygon layer is imported in QGIS (at least, for 
recent versions) as a "Polygon (MultiPolygon)" layer, i.e. a 
MultiPolygon layer, even if it does not contains any feature or if only 
contains geometries with only 1 part.

In QGIS, any geometry contained in an ESRI Shapefile Polygon layer is 
considered a MultiPolygon geometry: if it has only 1 part, it will be a 
MultiPolygon geometry with only one part; if it has more then 1 part, it 
will be a MultiPolygon with more than 1 part.

Is your "shape layer" an ESRI Shapefile layer? If yes, could you better 
explain why do you think the geometry type of this "shape file" is (non 
Multi) "Polygon". If not, could you please name the format of the layer?

Best regards.

Andrea Giudiceandrea

[1] 
https://www.esri.com/content/dam/esrisites/sitecore-archive/Files/Pdfs/library/whitepapers/pdfs/shapefile.pdf#page=12