Wen ‘n tropiese vakansie

Defining the spatial analysis
methods:
IDW - assumes that each input point
has a local influence that diminishes
with distance.
Spline - an interpolation method that
fits a minimum curvature surface
through the input points.
Kriging - a method of interpolation
that predicts unknown values from
data observed at known locations
using procedures that include
weighting from distant points.
Theissen polygons - constructed
using geometric techniques to create
polygons having the same value as
the original data point, i.e. the measured
point value is converted to a surface
area that the point measurement
represents.
The first point to note is the differences
in the images produced. (Figure 1) Each
analysis method produces different
images! This raises the question: Which
is the correct image? To answer this, a
comparison of the actual measured values
to the method predictions was
done along a line indicated by the
arrows in Figure 1. The results of this
comparison are presented in Figure 2.
In Figure 2 it is once again seen that
there are big differences in the results
from the different spatial analysis tech-
niques. The results obtained using the
Thiessen polygon method match the
actual measured values. The IDW and
Spline methods pass through the measured
values for each measurement point
but show differences for the interpolated
values between measured values. The
IDW method tends to produce individual
camel humps that gives the concentric
circle image in Figure 1a. The
results achieved through Kriging show
very little resemblance to the actual
measured values. Evidently the weightings
from other points are too dominant
resulting in the interpolated values
being severely distorted from the actual
measured values in this case.
Spatial analysis techniques
affect the maps produced
Generally, because one is familiar with
the idea of contour maps, we readily
accept a similar format for a soil survey.
As seen above, this approach can be
very misleading. The polygon method is
actually the more correct representation
of the actual field situation.
One must also remember that a soil
is mainly a solid medium. Thus diffusion,
dispersion or mixing of plant nutrients
or soil features according to a gradient
in relation to distance does not occur
as in the case of solutions or gases. If this
landbou in praktyk
was the case, one could dump fertiliser
at the gate of the land and it would
move to areas of low concentration!
Thus the smoothing of values between
points can introduce inaccuracies. This
is why the interpolated values from certain
techniques are not really a true
reflection of the measured value.
New trend in Senwes farm
mapping service
It is due to these features that the Senwes
Agricultural Services division has decided
to use the Thiessen polygon spatial
analysis method in their farm mapping
service; the emphasis being on correct
representation of field observations. The
division is able to determine the yield
potential of the land, the required plant
nutrient applications and the area (ha) of
each zone. For example, fertiliser application
can be calculated for each area
(ha) providing for variable application
and better management of input costs.
Enquiries
If you require further information on this
aspect, do not hesitate to contact
Senwes Agricultural Services for sound
advice or visit our stand in the Senwes
hall at the Nampo Harvest Day. Or
phone Neil du Sautoy at (018) 464-7391
or Esmond Coen at (018) 464-7524.
Figure 2: Comparison of actual measured values (Blocks) to values generated by four spatial analysis methods (lines for
Kriging, IDW and Spline; dumbbell for Thiessen) along a cross-section between two arrows in Figure 1.
Senwester, Mei 2005
33