[GRASS-user] Topology of overlapping intra-site archaeological polygons

[Markus Metz]

On Sat, Oct 22, 2016 at 7:33 PM, Salim Al Razzaz <salim.razzaz at gmail.com> wrote:
> Hi all,
>
> this is not really a problem of GRASS by itself since I know that being
> topological is a choice made by GRASS, and it surely is a needed choice
> in the world of GIS.
>
> However, I happen to work sometimes with material from archaeological
> excavations. Mostly intra-site polygons, created by Arc-users,
> documenting layers (loci, features, and contexts) of soils overlaying
> each others.
>
> The problem here is that these polygons often overlap since the layers
> cover each others to different extents. Not only that, but each polygon
> has attributes in a db table.
> Once these are imported to GRASS and cleaned along the way they change
> considerably.

The overlapping polygons are broken into non-overlapping areas, but
each area inherits the category and thus the attributes of all input
polygons overlapping with that output area. That means that the
attribute table stays the same and you get the equivalent of an
original polygon by selecting all areas with the respective category
or attribute unique to the input polygon.

What do you want to do with the overlapping polygons in GRASS? An
example could clarify how to treat multiple areas with the same
category (the GRASS equivalent of overlapping polygons).

Markus M

> Even when the "Do not clean polygons" (v.in.ogr -c) is
> checked some polygons lose their connection to the table by being
> converted to boundaries without centroids.
>
> I wonder if anyone else have had these issues in some context (whether
> archaeology or some other discipline), because I would like to see if
> there is some workaround. I have been forced to use QGIS to be able to
> check the vectors in these situations, although I would definitely
> prefer to do all my work in GRASS.
>
> Salim Razzaz
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