[Qgis-user] advice needed - merge attributes

[Anne Ghisla]

On Fri, 16 Sep 2011 15:08:45 +0100 (BST)
David <david210701 at yahoo.com> wrote:

> Hi everyone,
> On a shapefile, given to me by the person who created it, I need to
> merge some polygons to make larger units.    For example, select four
> adjacent polygons and use the merge tool and you get one large
> polygon.   Great.   However, on the attributes table what seems to
> happen is that instead of combining the various values (size,
> perimeter etc) three of the four polygons have disappeared leaving
> just one with its original data unchanged.   Would some kind person
> tell me what I'm doing wrong? Thanks in advance. David Asquith UK

Hello David,

the merge tool doesn't update the attributes like size and perimeter
automatically, you can use Geometry Tools > Export/Add geometry columns
to update the values. They will be written in two columns called AREA
and PERIMETER. If those columns exist, the values will be updated,
otherwise they will be created. If you have area and perimeter stored
in columns with another name, they will be ignored by this command.

Another way is to open the shapefile, toggle editing, open Field
Calculator (the little calculator at the right side of the buttons in
attribute table) and update the fields for area and perimeter (see
Manual section 4.7 "Field Calculator"). This second approach is a
bit more complicated.

Hope this helps, feel free to ask for more details if needed!

Anne
-- 
http://gis.cri.fmach.it/ghisla/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 836 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20110916/4609157e/attachment.sig>