On Thu, 21 Oct 2021, Tobias Pilz wrote:
Hi Vero,
as I understand it, vector files in GRASS need a key column if they have an
attribute table. But I don't see that this has to be a column named 'cat'.
However, GRASS is quite inconsistent here. Some functions ensure the
existence of a 'cat' column, such as v.db.dropcolumn (just gives an error if
you try to remove the 'cat' column), whereas other functions let one define
other columns as key column (e.g. manually in the GUI or v.db.addtable).
The latter happened with my vector map, i.e. a different column was defined
as key column. Therefore it is confusing why rgrass7::writeVECT() just adds
the cat column and without any notification. This makes no sense to me and is
annoying as my subsequent workflow produced unexpected results and I had to
trace back the error which took a while.
In sf vector objects, there is no concept of key column, I think. In sp
vector objects (used before, should be avoided from now on as sf is
actively maintained), the geometry ID is keyed to the row.names of the
data slot data.frame object, but is also not seen as a user-facing key
column.
So you are adding (as a user) a concept not present in the R-side
objects.
In practice, cat can be useful where cleaning has split line or
polygon objects imported into GRASS into multiple features, which can then
be unioned back after exporting from GRASS to R, but this isn't a concept,
it is a pragmatic fix.
Hope this clarifies,
Roger
From my perspective as user it would be great if a more consistent solution
regarding the treatment of key columns could be found, i.e. either always
ensure the existence of a column named 'cat' or just accept if the user
specifies a different key column.
Regards,
Tobias
Am 20.10.21 um 20:41 schrieb Veronica Andreo:
Hello Tobias,
The cat colum is the ID that GRASS uses for vectors, hence this is
expected
and right. The same happens if you import a gpgk or shape or any other
vector file into GRASS. See
https://grass.osgeo.org/grass78/manuals/vectorintro.html for further info
HTH,
Vero
El mié, 20 oct 2021 a las 9:34, Tobias Pilz (<top...@pik-potsdam.de>)
escribió:
Dear list,
when I use in R the writeVECT() command the resulting map in GRASS got
the 'cat' column silently added to its attribute table if it didn't
exist.
The vector object in question was previously imported to R via
readVECT() and had a different column defined as key column.
Is this behaviour of writeVECT() intentional? If yes, why so and
wouldn't it make sense to give a warning?
Note: I used rgrass7 version 0.2-6 together with GRASS 7.8.5 under Linux
(opensuse).
Regards,
Tobias
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Roger Bivand
Emeritus Professor
Department of Economics, Norwegian School of Economics,
Postboks 3490 Ytre Sandviken, 5045 Bergen, Norway.
e-mail: roger.biv...@nhh.no
https://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
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