GRASS algorithm in QGIS: v.to.lines

Question

I'm using the v.to.lines algorithm (GRASS algorithm) to convert a layer from polygons to lines in QGIS. The algorithm works without problems but the output layer only has an attribute called "cat", I would like to know if it is possible to inherit the attributes of the input layer.

Answer 1

The vector data model in GRASS is an arc-node type, so there's a 2-to-1 relationship between areas and outlines. To carry area data over, the algorithm would have to either choose data from one 'side' of the line, or create a doubled set of attribute columns with data from each side. It looks like doing this is left to the user as a subsequent task.

Having just run v.to.lines on a polygon dataset in GRASS 7.4.1, I see my lines output has two layers. Layer 1 just has 'cat', but layer 2 has three columns - cat, left, and right. The ID's in left and right correspond to the cat of the polygons on either side of the line. This means you can use those fields in a join to pull attribute data into the lines dataset. (e.g. join by lines.left=polygons.cat to get 'left side' polygon attributes).

You can do this via the QGIS layer properties dialog box (assuming you've got the GRASS plugin running etc). In GRASS itself, use v.db.join e.g.

v.db.join map=lines layer=2 column=left other_table=polygons other_column=cat

Repeat as many times as you need for your attributes of interest.

Note 1: I'm not sure what determines which polygon is considered 'left'.

Note 2: I'm noticing that if joins are made with v.db.join in GRASS and then that joined layer is opened in QGIS 3.2.2 (using the GRASS plugin), QGIS gets pretty crashy. It may be more stable to do your joins in QGIS and export to a new dataset, as they aren't persistent.

As jac mentions, QGIS's polygons-to-lines does transfer attributes, but it uses a simple-features data model, so there's a 1-1 parent-child relationship between polygon and line. When polygons are converted to lines, the result is two lines directly on top of each other wherever polygons abut, each with the attributes of their parent area.

Answer 2

If you are not restricted to using the GRASS tools, then try the QGIS geoalgorithm, "Polygons to lines". It pulls in the attributes from the original polygons.