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I inherited a clipped (500m radius) land cover map containing vector layers of different land cover categories such as buildings, grass, roads, etc from a colleague.

Land Use Map

The empty (white) space in the map is actually paved surfaces in reality but is it possible to fill that "empty space" with another vector layer that creates a new landcover polygon class automatically? I'm under the impression that it is possible to create another layer with the symmetrical difference tool but I think I need two input vector files - the one with all the current layers and another layer. Am I supposed to create another vector layer that covers the exact same area (i.e. 500 m radius about a certain point?) and how do I go about doing that?

--------- EDIT ----------

Ok just to summarize, this is what I did to get the difference layer:

  1. Add a vector layer with center-point to existing maps.
  2. Create a buffer layer using this centerpoint. Vector > Geoprocessing tools > Buffer. Use the new vector layer with center-point as input layer.
  3. With the new buffer layer, you can now use the Difference Tool (Vector > Geoprocessing > Difference). Use the buffer layer as your input layer, and the other layer being your main map.
  4. You will have to edit the attributes of the new layer so it corresponds to the same type of attributes in your base map.
  5. Merge the layers using the mmqgis tool (mmqgis > transfer > merge layers), and create a new shapefile with all the layers merged together.
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up vote 4 down vote accepted

Yes, you need a layer that covers the same area to calculate the difference. The easiest way is to create a buffer around the centerpoint.

If you already have a layer that includes a point feature at the centerpoint, you can select that and then use the Vector/Geprocessing/Buffer tool with the "Use only selected features" check box activated.

If you don't have a point feature at the centerpoint, you can create a new point shapefile and adding a point, probably using the Numerical Digitize plugin to get an accurate position of the point. Then you can run the Buffer tool on that layer.

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This is perfect. Thanks! –  lhmv Jan 30 '13 at 14:23
    
Ok I managed to successfully create a buffer layer like you mentioned. I then tried using both the Symmetrical Difference and Difference tools to get the "white space" I mentioned earlier but I'm not getting anything unfortunately. Am I missing something here? The base map I mentioned earlier comprises several different layers that I combined into one layer using the mmqgis plugin>transfer>merge layers. –  lhmv Jan 30 '13 at 16:46
    
So you created a circular buffer shapefile, say buffer.shp, and you have a polygon layer, say landuse.shp, and they overlap (of course), correct? And then you did Vector/Geoprocessing/Difference, choosing buffer.shp as the Input vector layer, and landuse as the difference layer, and your resulting shapefile is completely empty? –  Jake Jan 30 '13 at 17:04
    
It's not completely empty, there are some fills at the rim of clipped area. I'm supposing there are some topological errors in the base map - I've been getting an error message saying that "One or More Input Features have invalid geometries. Some output geometries may be missing." I'll have to go back and check again. Thanks for confirming the process! –  lhmv Jan 31 '13 at 2:52
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