R help - Jun 2003 - spatial correlation test

At 18:48 30/06/2003, Martin Wegmann wrote:
>hello,
>
>I want to do a test for spatial correlation.
>I tried it with geary.test() but I don't understand the required input.
>x= a numeric vector the same length as the neighbours list in listw (my
>sampled data, I assume)
>listw= a listw object created for example by nb2listw (well when I check
>nb2listw() I get to "neighbours - an object of class nb" - but I
couldn't
>figure out, what nb is or how I create such a class
>
>with sp.mantel.mc {spdep} I have the same problem: listw created by nb2listw
>
>isn't there a more straight forward method ;-)  to check for spatial
>correlation? like x and y coordinates plus my sampled data?
This functions require a list of spatial weights for each geographical 
unit. To construct this list, you must firstly establish the neighborhood 
relationships (who are the neighbors of a point). There are different ways 
to construct this neighborhood: based on distances (dnearneigh), number of 
nearest neighbors (knearneigh)... Then, you must transform this 
neighborhood to weights (see nb2listw for example). Here is a small example 
requiring the tripack library based on tesselation and voronoi mosaic to 
create the neighboorhood:

xy=matrix(rnorm(20),5,2)
z=rnorm(5)
library(spdep)

mynb=tri2nb(xy)
mylw=nb2listw(mynb)

geary.mc(z,mylw,10)

         Monte-Carlo simulation of Geary's C

data:  z
weights: mylw
number of simulations + 1: 11

There is no simplest method.....


statistic = 0.8384, observed rank = 2, p-value = 0.1818
alternative hypothesis: less

hello, 

I want to do a test for spatial correlation.
I tried it with geary.test() but I don't understand the required input. 
x= a numeric vector the same length as the neighbours list in listw (my 
sampled data, I assume)
listw= a listw object created for example by nb2listw (well when I check 
nb2listw() I get to "neighbours - an object of class nb" - but I
couldn't
figure out, what nb is or how I create such a class

with sp.mantel.mc {spdep} I have the same problem: listw created by nb2listw

isn't there a more straight forward method ;-)  to check for spatial 
correlation? like x and y coordinates plus my sampled data?


thanks, Martin


BTW thanks everybody for their help with my last problem -> [R] regression
for
several responses - it worked perfectly

On Monday 30 June 2003 15:23, Barry Rowlingson wrote:
>   Think you may be looking at the wrong sort of spatial correlation! For
> Geary tests you are comparing 'adjacent' objects, where adjacency
is
> defined however you want - N-nearest neighbours, shared border between
> regions etc etc.
sorry, I misunderstood the purpose of geary's I test, thanks for this info. 
>   When you say 'sampled data' it sounds more like you've got
samples
> taken at locations, and you want to investigate spatial correlation as a
> function of distance between samples? Am I guessing right?
Yes you are right. I want to look for spatial correlation of my samples as a 
function of distances between sampling sites (x,y coords). 
>   Take a look at some of the R kriging libraries, which will have
> functions to plot variograms. This is a plot of something like
> E(|Y_i - Y_j|) against distance.
>
> Baz
I found variograms() and correlograms(), but is there a way to get the a 
p-value for spatial correlation? 
additionaly I found sp.correlogram() but again with this mysterious "nb 
class".


thanks, martin