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Gstat & declustering & others
Dear all Gstat users,
I use Gstat and I've some questions for you.
- I decided to use Gstat for declustering my data set with the polygonal
method (Goovaerts, Geostatistics for Natural
Resources Evaluation, 1997, pp.79-81; Isaaks and Srivastava, An
Introduction to Applied Geostatistics, 1989, pp.238-239).
Well, though this method could be improved according to Dubois (How
representative are samples in a sampling
network?, Journal of Geographic Information and Decision Analysis,
vol.4, no.1, pp.1-10, on-line), I decided to apply it just as weights to
apply to my data depending on polygonal area.
I added an index column to my data set, so that every sample could be
seen as unique.
Using Gstat I executed the following command:
#col 1 contains unique identifier -index- I put 'average' because maybe
I've values coming from different times on the
same sample point or laying in the same cell (see following discussion).
>data(clus): 'zinc.eas', x=2, y=3, v=1, average, max=1;
Using PCRaster I executed the following pcrcalc script:
>#! --clone mask.map --unitcell --nondiagonal
># transform distarea to ordinal/nominal class: is 'ordinal' the same in
>distcls=nominal(distarea*1000); # *1000 is to create different classes
for <> decimal values (if the case) until 3rd
significant decimal digit
>idealarea=maptotal(area)/maparea(area); #idealarea stands for
declustered mean area
>report declarea=area/idealarea; #calculate weights for samples
Finally using Gstat again I created a new file containing my data with a
column weight added:
>data(): 'zinc.eas', x=2, y=3, v=1, average; # prediction locations
>set output = 'decl_data.eas';
Now I could multiply all my column data by weights with the software I
want and use them.
Well my questions for you are:
- do you think my method works well or there's something I did wrong? If
I've missing values in a column I should repeat
the procedure for that column, given that weights change because I've
sample points that are missing (with their own
area) in my study area.
- Does anybody know how Gstat treats data which have been sampled on
different points, but that lie on the same map cell (for example
xa=181072 ya=333611, xb=181062 yb=333601 and cell size 20m (->cellarea
400m2)? Does it average them?
- another problem in converting *.eas file->PCRaster map, is that
coordinates are treated as the center of the cell; when I
do the back conversion I don't obtain original coordinates, but the
center of the cell. So, if I want to be nearest to my
original coordinates I've to choose a very little cell size. Anyway this
is not always useful due to huge map created if
points lie far away. Besides, kriging in geostatistics uses relative
distances for weights: if they are changed, weights
change. Is there a solution to this problem?
Thanks in advance for help.
Giovanni De Ferrari