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My goal is to make a shapefile with polygons for variable rate fertilizer application within a farm field.

I have a point shapefile with soil sample values (pH, P, K and so on) and a line/polygon file with field borders. IDW is recognizes as a suitable interpolation method for soil samples. I have gotten as far as having the interpolated points as a raster file and I have experimented with the "Contour" function to make a suitable classification.

I'm looking for a solution in QGIS to convert/make/bake the classified raster in to polygons, and clip the results to the field border. Does any one have a suitable solution for this using QGIS?

Edit:

I don't know if my question is clear enough. To illustrate my goal the attached picture shows how I need my final shapefile to be.

Picture illustrating the final result I'm after. The points are interpolated, clipped to the field border and then (some how) the classified raster is converted to a polygon shapefile.

The picture illustrates the final result I'm after. The points are interpolated, clipped to the field border and then (some how) the classified raster is converted to a polygon shapefile.

  • Have you tried Raster > Conversion > Polygonize? – Joseph Jan 29 '15 at 11:22
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@Joseph is right. Use the polygonize tool to make the raster into a polygon. Choose the option to write the raster values to an attribute column (ie: dn). This will allow you to dissolve the polygon on dn values that are the same, which will result in a cleaner looking file. With your input polygon that is a field boundary, use the clip tool (vectors/geoprocessing) to clip your rasterized polygon.

  • Thank's! This is rely helpful. The polygonizer tool classifies the raster values from 0 to 26 in intervals of 1. Is there a way to change the interval? – user29934 Jan 29 '15 at 14:16
  • Can you comment more on what you mean, or what you would like to have as an outcome. – Ryan Garnett Jan 30 '15 at 14:11
  • My rasterfile has values from 0 to 26. I'd like the final polygonized file to have polygons with intervals of 0,5. The Raster to Vector function only makes intervals between the polygons of 1,0. – user29934 Feb 1 '15 at 10:48
  • You could add a new column to your polygon and do a reclassification based on your needs – Ryan Garnett Feb 1 '15 at 12:51

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