Keeping both attributes would make sense in many cases of overlapping
polygons, which, I would guess, is more often about partly overlapping
polygons due to sloppy digitizing. How is GRASS going to tell what feature
is more important?
But, in case of duplicate rows in your attribute table, except for the cat,
you might be able to remove the duplicate rows in the attribute table using
a simple SQL statement (not sure this works if you use dbf as database
backend), something along the lines of (google if this doesn't work).
DELETE FROM table WHERE cat NOT IN
(SELECT MIN(cat) FROM table GROUP BY XXX);
Whereby XXX would be column with unique values mapping unit (if there is not
such column, you need to GROUP on a set of columns that together uniquely
define each mapping unit). You can do this in the db.execute. Alternatively,
you can use the SELECT statement in the advanced SQL query builder in the
GRASS Attribute Table Manager to select the duplicates and delete them
there.
If you use the dbf as database backend and the above doesn't work, you can
open the dbf file (which you can find in the 'GRASS DB / LOCATION/ MAPSET
/dbf' folder) in Libreoffice and select all duplicates and delete, e.g.,
using a pivot table. Do not use excel, in my experience that may mess up
your dbf file.
In all cases, this is just to remove rows that are identical except for one
column... you'll have to test whether the results make sense in your case
and you are not messing up your polygon layer.
On 04/02/2012 09:09 PM, David J. Bakeman wrote:
Markus Metz wrote:
On Sun, Apr 1, 2012 at 11:45 PM, David J. Bakeman<dbakeman@comcast.net>
wrote:
David J. Bakeman wrote:
Markus Neteler wrote:
On Sun, Apr 1, 2012 at 9:45 PM, David J. Bakeman<dbakeman@comcast.net>
wrote:
I didn't find an answer in the archives so.
I have a shapefile of polygons and some of the polygons are duplicated.
I
thought I could use v.clean tool=rmdupl to get rid of these polygons. I
use
v.in.ogr to read it in and I get the following:
WARNING: 8 areas represent more (overlapping) features, because polygons
overlap in input layer(s). Such areas are linked to more than 1
row in attribute table. The number of features for those areas
is
stored as category in layer 2
That is correct in that there are 8 duplicate polygons but the only
different attribute is the cat which grass added? What am I missing? I
then tried v.clean tool=bpol,rmdupl and nothing changes it still has the
8
duplicates. What am I doing wrong?
I think that you need to add the break tool for v.clean.
Correct that was actually what I was using: v.clean tool=break,rmdupl
Looking closer I see that when I run v.clean it doesn't even report the
duplicates that v.in.ogr did but they are still there. The only thing that
differs in coordinates or attributes is the cat attribute that grass added.
I am using grass 6.3.0 on fedora core 14 linux.
Please note that you can upgrade to grass-6.4.0-4.fc14:
http://koji.fedoraproject.org/koji/buildinfo?buildID=263115
Thanks I'll see if I can upgrade.
I upgraded to the 6.4.0 and the results are exactly the same. The polygons
really are identical in every respect except for they have different values
in the cat column. Is there some other grass tool for removing this kind of
duplicate?
After import with v.in.ogr, there are no duplicate geometries left in
the vector. What you have now is some areas with two categories
assigned to them. Removing the duplicates means in this case removing
one of the two categories, for example with one of the vector
digitizers.
I'm relatively new to grass but that doesn't make sense. I started with a
shapefile with duplicate features. That is polygons with the exact same
attributes and geometry (they are identical). What I thought grass could do
for me was to read it in and delete one of the duplicates without user
intervention. After all it identifies the duplicates so why can't it delete
one?
Are you saying that the duplicate geometry was deleted but it kept both rows
even though they were identical as well? Is there a operation that would
identify and delete rows that differ in only the cat attribute.
Markus M
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