[Qgis-user] Updating DBF with Field calculator

[Philippe de FRANCLIEU]

Hi all,
 
I 've created an Intersection layer with two shapefile layers (polygons).
I now want to update the new database (DBF) and feed 4 new fields, say
IN_AREA, IN_PERIM, IN_X and IN_Y :
1) I wish I could specify the units. How could I?
2) The length button (Operators / 1.7.3) remains greyed-out though
$perimeter as Field calculator expression seems to work.
3) $x and $y return NULL's : how do I extract X's and Y's of centroïds and
feed IN_X and IN_Y?
 
Using 1.7.3 and 1.8 on Windows 7x64.
Thanks in advance for your help.
 
Regards.
 
Philippe de FRANCLIEU
 
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