[R-sig-Geo] rGeo vs. gstat; Question on geostatistical method for specific experimental design

[Schlatter Christian]

Dear Edzer, dear list members

Thank you very much for your comments. It made me investigate quite well. 

The answer to the question "rGeo vs. gstat" is answered by your very helpful article of the DSC 2003 meeting in Vienna (chapter "introduction"): http://www.ci.tuwien.ac.at/Conferences/DSC-2003/Proceedings/Pebesma.pdf

As I'm new and not yet so familiar with the customs of [R-sig-Geo], I did not feel about asking about my personal statistical problems but more about general statistical questions. 

But as you asked for more detail I will gladly describe them (please let me know if this should not be the place for it):

We are looking at parasitism rates in cabbage pests eggs (Lepidoptera: mainly Mamestra brassicae, Pieris rapae) in relation to distance effects from flowering strips. Specifically the following question: In what relation is the parasitism rate to the distance from the flower source (which in the case are sown flower strips). 

Many practical restrictions led to a somehow "compromisical" experimental design consisting of four blocks in one cabbage field (two with flower strips, two without). Each block has two grids of 6x 8 plants (distance in between each plant: 3m) on each side of the flower strip (cp. Image), 96 plants per block. On each plant we collected pest eggs to determine parasitism rate. (Addtionaly we sampled on 4 different days).

The main problem is the low parasitism rate in the field (only about 15-20% of all eggs have been parasitized), consequently many 0 values. 

My idea was to calculate for each of the four blocks variogram parameters and to compare them afterwards (as we had two study sites, we have 8 blocks all in all, making 4 with flowers and 4 without). With so many "0" samples difficult to manage. 

Now I found in Edzers article and the gstat manual the possibility of block kriging. Without knowing exactly what it is, I suppose there is the possibility to keep all four blocks together and defining the blocks with the rectangular block-defining possibility. 

Is this a valid procedure?

Best wishes 

Christian Schlatter (Research institute for organic farming, Switzerland)

-----Ursprüngliche Nachricht-----
Von: Edzer J. Pebesma [mailto:e.pebesma at geo.uu.nl] 
Gesendet: Montag, 17. Oktober 2005 23:43
An: Schlatter Christian
Cc: r-sig-geo at stat.math.ethz.ch
Betreff: Re: [R-sig-Geo] rGeo vs. gstat

Schlatter Christian wrote:

>Dear list members
>
>I'm very new to R but a little informed about geostatistics. 
>
>As I was looking for possibilities of geostatistical analysis in R I encountered at least two very interesting packages: 
>
>Rgeo and gstat
>
>And of course I'm wondering now about the one which fits better my needs which are the comparison of spatial data (insect parasitism rates) for different fields in which we collected data. 
>
> 
>
Christian, I too find these two packages very interesting. Another 
package that you might
find interesting besides geoR is geoRglm. However, you've told us too 
little about what
your problem is to give you advise about which methods to use, and where 
to find them.

You will find even more packages for geostatistical analysis on the 
spatial task view:
R home page -> CRAN -> CRAN mirror -> Task Views -> Spatial.

Best regards,
--
Edzer

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