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

[Markus Neteler]

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>.
----- snap -----

Please review.

Markus