[GRASS-user] Import of multi-part polygons using v.in.ogr?

[Markus Metz]

On Sun, Dec 2, 2012 at 3:33 PM, Benjamin Ducke <benducke at fastmail.fm> wrote:
> Hi List --
>
> Looking at function centroid() in geom.c
> of v.in.ogr, it seems to me that the code
> for import of polygons always assumes that
> the first ring is the exterior boundary and
> the following ones are all interior boundaries
> (see comment on lines 70 and 71).
>
> Is this a limitation of GRASS' vector model
> or would it be possible to import multi-part
> polygons with more than one outer boundary by
> amending the code in v.in.ogr?

Import of multi-features such as wkbGeometryCollection,
wkbMultiPolygon, wkbMultiLineString, wkbMultiPoint is supported by
v.in.ogr. For centroid(), see lines 160ff, for geom(), see lines
432ff.

>
> I understand that it is possible to import
> multi-part geometries into GRASS and assign
> each part the same "cat" number. However,
> when exporting with v.out.ogr, they will be
> exported as separate parts, each with its
> own attribute table row. Is there any
> way to preserve multi-part features using
> v.in.ogr and v.out.ogr?

Multi-part features are preserved by v.in.ogr, all included features
get the same category.

For v.out.ogr, I think this is not so easy because not every
OGR-supported output format supports multi-features (may be wrong).

>
> If this is currently not possible: Would it
> be a viable solution to add a "-j" (join)
> flag to v.out.ogr so that it will merge
> geometries with the same "cat" into multi-
> part features on export?

I guess technically yes, if multi-features are supported by the
selected output format.

Markus M