[Qgis-user] Problems with Union tool in QGIS 2.6

[Nicolas Cadieux]

Hi, 
You can use the point sampling tool plugin for that.  Extract your grid centroid, use the new point  layers with the point sampling tools. You can also sample raster and vector values with the tool. 
You can rejoin both layer later in a number of ways.    If you want to eliminate points, (or grid squares if both have been joined) you can use a query.  If you have millions features, this may be slow. In that case, use the "split vector file" using a column. 
For more tools for your work, look at the plugins for "stats".  You will find a number of good plugins for extracting statistics from vector or raster data.  There are also many tools in the " processing" tools. 
Send me a mail if you need more help.  I Like what you are doing... 
Nicolas Cadieux M.Sc. 
Les Entreprises Archéotec inc.  
8548, rue Saint-Denis Montréal H2P 2H2 
Téléphone: 514.381.5112  Fax: 514.381.4995 
Le 2014-12-05 09:37, "Colin D. MacLeod [via OSGeo.org]	" <ml-node+s1560n5176645h56 at n6.nabble.com> a écrit : 

	






Hi All, 
  
I have been trying to translate a series of practical exercises to teach
marine biologists from ArcGIS into QGIS to help promote use QGIS in marine 
biology as an alternative to commercial GIS sofftware.  However, I have run
into a problem with one particular step in my standard training exercises. 

  
This step involves creating a vector grid with the vector grid tool, and
then using the union tool to divide the vector grid cells into sections which
are on land and on water, so the land portions can be removed (leaving just the
portions which are in the sea – this allows for the calculation of the area of
each vector grid cell which is covered in water and use it to correct for
various biases this can introduce in analyses). 
  
When working with QGIS 2.2 and 2.4, this process worked more or less
perfectly, but in 2.6, I am having a intermittent problem where it will work on
some occasions but not on others, using what should be more or less identical
data layers and identical settings. When it doesn’t work, the Union tool hangs
at a specific percentage (in this case 23%), and there is an error messsage
which says: Feature Geometry Not Imported (OGR Error:). 
  
I don’t know if it makes any difference, but I am using a user-defined
coordinate reference sytem that is based on a transverse mercator projecton fr
both the project and the input data layers for the union tool (this seemed to
cause some problems in earlier versoins of QGIS, but work arounds were
possible). 
  
Can anyone offer advice as to what might be going on here and how to get
round this issue? I can provide sample data layers on request. 
  
The system which this problem is occurring on is a laptop running Windows
8.1 and QGIS 2.6. 
  
I am keen to try to find some sort of solution to this as this is a
standard process for marine biologists, and being unable to do it would limit
the ability to work with QGIS as an alternative to commercial GiS
software. 
  
Thanks for any advice which can be provided. 
  
All the best, 
  
Colin 
  
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