[R-sig-Geo] Kriging with GRASS and R: Automatic trend detection

[Ebrahim Jahanshiri]

It has been long that I wanted to suggets this for automatic trend
detection based on our previous conversations with Edzer and Anne. I
found two ways that seem to be reasonable and have potential for
automizing the trend detection ( I got these from my conversations
with Margaret Oliver and Dick Bruc both prominent soil scientists):

1-If we fit some 2D surfaces and look at the percentage variance
accounted for by the surface  . If it is much more than 25% then you
have trend that needs to be dealt with for geostatistics.

[By "fitting a surface" I think means we can use the default
parameters for just creating a surface. i dont know if we could use
other methods lik IDW for just checking]

this can be done pretty well in ArcGIS but I am sure that we could
come up with something to do it automatically and give the user the
result...

2- For detecting the order, If we fit the trend by Ordinary Least
Squares (which implies that  we assume the residuals are independent)
we can test whether the regression coefficients differ significantly
from 0.

 linear trend:

model z
fit  x, y


second order polynomial:

model z
fit x, y, xy, x2, y2

or any other order.



-- 
Ebrahim Jahanshiri