I am trying to create a soil-type map of an area from point data using QGIS. I tried interpolating the data, since soil is continuous. I did this by allocating a number to each point type and then using the interpolation function. I later created contours from the output file to delineate the areas. The problem that I am having is that the results from the interpolation and contours do not line up with the point data. The point data and contours are in close proximity to each other but do not overlay accurately.

I would like the output to be as if though I manually drew lines that connected and delineate the different types of soils/point data. Now the points lie outside of the delineated area. Is there another technique to do this or what am I doing wrong?

I used the Raster Interpolation Plugin in QGIS Wien. The interpolation method was IDW, Number of columns =300, Number of rows =300, Cellsize X = 0.00004, Cellsize Y =0.00006


1 Answer 1


Contours are intended to be generalization of a surface rather than a representation of a contact. Therefore, they may not intersect your points exactly.

While soils are continuous in nature, your goal is to create a map with hard contacts rather than a continuous surface. Your data is not really continuous, it's discrete and arbitrarily assigned.

Another approach you could take is to divide your interpolated surface into zones. You come up with your breaks in the interpolated values, and assign an integer for each soil class. For example, values on the interpolated surface between 3.00 and 3.49 get a value of 3 in the new raster. Your output is a raster with discrete values (zones).

You could then convert raster to polygon if you need vector data.

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