Contouring Structural Data: Kalsbeek Net

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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If we divide a circle into ten zones of equal width, the innermost circle will contain 1% of the area. The next circle is twice as large and will contain 4%, but 1% is in the inner circle, so the annulus will contain 3% of the area, and so on.

If we stack triangles, each row will contain 1, 3, 5... triangles. A stack ten rows high will contain 100 triangles.



If we divide a 60 degree sector of the circle into triangles of equal area, each sector will contain 100 triangles, each with 1% of the area of the sector.
The Kalsbeek counting net is based on this principle. It consists of ten equally spaced circles. Each annulus is divided into triangles. Altogether there are 600 triangles. At each vertex, six triangles meet. The hexagon of triangles around each vertex contains 1% of the area of the net.
Plot the data on an equal area net then transfer the overlay to the counting net. Of course, the two nets must be the same diameter!
At each vertex, count the number of points in the surrounding six triangles and plot the number at the vertex. You may want to do this on a second overlay above the data overlay.

Each triangle is common to three hexagons so every point is counted three times. (No, this does not mean the densities have to be divided by three.) Be certain to check every vertex close to the data points to be sure of not missing any.
Remove the numbered overlay and contour the data.

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Created 16 June 2005, Last Update 30 January 2012
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