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I have a vector layer with incidence data (i.e. frequencies) and I made a graded color map from this using the above frequencies. Now I am looking forward to creating an interpolated (kriging) map from the above frequencies. Is there a method to create such interpolated map from my incidence data? (or else is it possible to create a interpolated map with out a point layer?) Thanks.

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    Please clarify: You say you want to interpolate without a point layer ... What layer do you have then? – underdark Nov 17 '13 at 10:41
  • This is a polygon layer (eg. country) with a attribute table containing incidence data for some of randomly selected sub unit (eg. selected districts) and I need to interpolate these incidence data to the whole country. – user24066 Nov 18 '13 at 7:44
  • You want to create a raster table by interpolation of polygons? Why not convert the vector directly to raster with menu Raster > Conversion > Rasterize. You could also convert the polygon layer to centroid points and then do the interpolation. – Jakob Nov 18 '13 at 17:14
  • @Jake Thank you, Your second suggestion worked for me. – user24066 Nov 19 '13 at 16:13
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Based on the comments, it sounds like @Jakob found an appropriate solution. The polygon layer needs to first be converted to centroid points. This is done using:

Vector -> Geometry Tools -> Polygon Centroid

and then creating the output point layer. The interpolation may then be performed using one of the various methods available. This post has a good discussion of interpolation in QGIS.

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