[R-sig-Geo] Collocated Cokriging of snow height data

[Stefan Zollinger]

Hi

I am trying to spatially interpolate snow height data of about 100 stations 
in a mountain range. In addition, I have a large DEM (SRTM, 90 meters 
resolution, 2.5 million cells) which also serves as an interpolation raster 
(just like meuse.grid). As the snow height and the height above sea level 
correlate strongly, I intend to use collocated cokriging to improve the 
estimation, which is why I studied the example in "Applied Spatial Data 
Analysis with R" by Roger Bivand, Edzer Pebesma and Virgilio Gómez-Rubio.

I have the following questions (especially to the authors):

1. Why and how is the new attribute "distn" being calculated? Would it not 
be sufficient to use the existing attribute "dist" for the collocated 
cokriging (as it shows the same variogram-model properties)?

2. How are the two variogram-models "vd.fit" and "vx.fit" being calculated 
out of "v.fit"? I understand that the range and the type of the three models 
remains the same, but how are the sills and nuggets being changed?

3. How would the calculation of "vd.fit" and " vx.fit" change if a trend 
model was used, like "log(zinc) ~ sqrt(dist)"?

Any advice or help will be highly appreciated

Stefan Zollinger