Geoscience Reference
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Fig. 7.8 Contour plot of a data set gridded using a biharmonic spline interpolation. At the
location (450,105), very close control points with dif erent z -values have been introduced.
Interpolation causes a paired low and high, which is a common artefact in spline interpolation
of noisy data.
plot(data(:,1),data(:,2),'ko')
text(data(:,1)+1,data(:,2),labels)
hold off
As can be seen from the plot, this method extrapolates the gradients beyond
the area with control points, up to the edge of the map (Fig. 7.9). Such an ef ect
is particular undesirable when gridding closed data, such as percentages, or
data that have only positive values. In such cases, it is recommended that
the estimated z -values be replaced by NaN . For instance, we delete the areas
with z -values larger than 20, which are regarded as unrealistic values. h e
resulting plot now contains a sector with no data.
ZID = ZI;
ZID(find(ZID > 20)) = NaN;
contourf(XI,YI,ZID,v)
caxis([-40 40])
colorbar
hold on
plot(data(:,1),data(:,2),'ko')
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