On Wed, Nov 15, 2017 at 4:35 PM, Markus Neteler <nete...@osgeo.org> wrote: > > On Wed, Nov 15, 2017 at 3:49 PM, Markus Metz > <markus.metz.gisw...@gmail.com> wrote: > > On Wed, Nov 15, 2017 at 1:36 PM, Helmut Kudrnovsky <hel...@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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