[S] Pie Diagrams

Garrett, Robert (garrett@nrcan.gc.ca)
Fri, 13 Nov 1998 09:09:26 -0500

I agree that Pie Diagrams have their problems, but I would like to give one
example where they did help visualization of a complex spatial data set.

Heavy minerals were separated from glacial tills in the Canadian Prairies
and semi-quantitatively analysed. As a result the data set consisted of a
spatial location, the total weight of heavy minerals separated, mg/kg, and
the weights, mg, of the individual types of heavy minerals for some 816
sites. An initial investigation showed that six of the heavy mineral types
provided useful information for identifying the regions which were the
sources of the glacial tills, but that absolute weight of heavy minerals was
important. The solution to vizualizing this data set was to plot pies at
their spatial locations on a map where the diameter of the pie was
proportional to the sum of the six heavy mineral weights, and the pie was
divided and coloured in proprtion to the contributions of the six heavy
minerals. This provided a vizualition where the seven important variates
were displayed in a form that allowed recognition of individual glacial
tills with characteristic heavy mineral assemblages in the Canadian Prairie
region, and provided geologists with information they could use to ascertain
the sources areas of the tills.

I realize that this is a very specific case, but it demonstarates two
things. Firstly, a very wide range of scientists/economist/others are
interested in data vizualiztion (few geoscientists are heard from on S-News,
but I learn an enormous amount from the participants to S-News); and
secondly, despite drawbacks there are cases where a procedure, despite its
drawbacks, can provide a useful visualization. The case above demonstrates
this point, where seven variates were of simultaneous relevance and interest
to a problem.

Robert G. (Bob) Garrett
Applied Geochemistry and Geophysics
Geological Survey of Canada
601 Booth St., Ottawa, Ontario K1A 0E8
Internet: garrett@gsc.NRCan.gc.ca
Tel.: 613+995-4517 FAX: 613+996-3726

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