[GRASS-dev] Which v.* modules are working with v.external'ed data

[Markus Metz]

On Wed, Nov 15, 2017 at 4:35 PM, Markus Neteler <neteler at osgeo.org> wrote:
>
> On Wed, Nov 15, 2017 at 3:49 PM, Markus Metz
> <markus.metz.giswork at gmail.com> wrote:
> > On Wed, Nov 15, 2017 at 1:36 PM, Helmut Kudrnovsky <hellik at web.de>
wrote:
> >>
> >> Background of the question in subject: the GRASS algs in upcoming QGIS3
> >> are working with v.external linked data.
> >>
> >> I've seen some modules doesn't work with such kind of data.
> >>
> >> Is there maybe a list already available?
> >
> > It depends also on the format that is linked in with v.external. For all
> > formats that do not have a key column (e.g. shapefile), attributes are
not
> > accessible, and attributes would get lost when modifying the
> > geometries.Therefore it is generally not safe to link vector data with
> > v.external. In many cases it does not make sense to use v.external
linked
> > data with simple features, instead vector data should be imported to get
> > true topology. If upcoming QGIS3 is working with v.external linked
data, the
> > GRASS interface of QGIS is effectively broken.
>
> I have turned this into a potential new snippet for the v.external manual:
>
> ----- snip -----
> Due to these data model differences <em>v.external</em> does not work
> with all data formats. In general, for all formats that do not have a
> key column (e.g. SHAPE file), attributes are not accessible, and
attributes
> would get lost when modifying the geometries. Therefore it is generally
> not safe to link vector data with <em>v.external</em>. In many cases it
> does not make sense to use <em>v.external</em> linked data with simple
> features, instead vector data should be imported with <em>v.import</em>
> or <em>v.in.ogr</em> to get true topology support. Importantly, point
> cloud data which do not have topology, can be linked with
<em>v.external</em>

as long as there are no attributes attached to these point cloud data, or
if the format of the point cloud data has a key column that allows to link
vector geometries to attributes.

> ----- snap -----
>
> Please review.

Importing point cloud data is fast, therefore there is little gain in using
v.external instead of v.in.ogr.

Markus M
>
> Markus
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