Hi Roger,
thanks for clarification. I understand why it happens but I still think
this is somewhat unexpected behaviour.
However, to wrap it up, I guess it's always best to have a 'cat' column
in the attribute table as then this won't happen.
Tobias
Am 21.10.21 um 09:55 schrieb Roger Bivand:
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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