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I have to create a Thiessen polygon for a site in New Jersey and all I have right now is the concentrations from samples we took in the area. What else do I need and how do I input it into QGIS to get my desired outcome? Which is to divide the total excavation into polygons based on concentrations? Any info will be a help.

  • Please describe the desired results and the available input data in more detail. It is not yet clear to me what you are trying to achieve. – underdark Feb 20 '15 at 17:47
  • The desired result is theissen polygons over an excavation area based on contaminate concentrations we found doing soil borings in the excavation. I have the concentrations of said borings and there locations. Do i have to create a shapefile with the point locations or do i with the concentrations? – rkc1316 Feb 20 '15 at 17:57
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    This method will not divide your area up based on concentrations. It will divide it by allocating area to each point; general terms by drawing lines halfway between each pair of points and assigning that polygon the concentration value of the point. But there's not necessarily a correlation between 'halfway between' and whatever measured concentration level you have. Especially depending on the distribution of those points. Interpolation would give you a much better representation of contamination levels across the site. And yes, you need to create a point shapefile for anything. – Chris W Feb 20 '15 at 20:39
  • If it helps the tool in QGIS is called Voronoi : djjr-courses.wikidot.com/soc128:qgis-voronoi-polygons is one tutorial. As you will see by the tag Thiessen and Voronoi as terms are interchangeable as both people came up with the same idea at about the same time without knowing of the others' work. – Michael Stimson Feb 22 '15 at 23:17
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The easiest way to prepare your data for importing it in QGIS usually is to create a CSV similar to

x,y,concentration
1,2,12.5
1,3,20.7
2,2,30.1

Once you have loaded the CSV, you can use the resulting layer as input for triangulation, interpolation, or any other analysis you desire.

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