computing the hat matrix for gwr in spgwr?

Hi,

I'm trying to calculate the hat matrix for gwr by hand using matrix 
manipulation in R.  The hope is that once I get it to match the output 
that gwr gives, I can do the matrix algebra row-/column-wise.  This will 
allow me to estimate the effective number of parameter using the trace 
of the hat matrix on a data set that is too large to do this using the 
computer resources I have using gwr as is.  The first row of the hat 
matrix described on page 55 of fotheringham et als gwr book.  It is the 
hat matrix multiplied by a distance-based weighting function.  The code 
below returns 4.661219 for the trace while gwr returns 3.400793. 
Anything jump out to anyone?  Thanks, Seth Myers

library(spgwr)
data(columbus)
bw=10
crime.gwr<-gwr(crime~income, data=columbus, coords=cbind(columbus$x,columbus$y), bandwidth=bw,gweight=gwr.Gauss,longlat=FALSE,hatmatrix=TRUE)
v1.gwr<-crime.gwr$results$v1 #what I am trying to calculate by hand, the trace of the S (or hat) matrix)
xmat<-mat.or.vec(nrow(columbus),2)
coor<-mat.or.vec(nrow(columbus),2)
xmat[ ,1]<-1
xmat[ ,2]<-columbus$income
coor[ ,1]<-columbus$x
coor[ ,2]<-columbus$y
#creating matrices for use in following loops
xtw<-mat.or.vec(2,nrow(columbus))
w<-mat.or.vec(nrow(columbus),nrow(columbus))
xrow<-mat.or.vec(1,2)
smat<-mat.or.vec(nrow(columbus),nrow(columbus))
for (i in 1:nrow(columbus)){
print(i)
for (j in 1:nrow(columbus)){
dist<-( (coor[i,1]-coor[j,1])^2 + (coor[i,2]-coor[j,2])^2)^.5 #finding distances between observations
w[j,j]<-exp(-((dist/bw)^2))#computing weight matrix for observation i
}
xtw<-t(xmat)%*%w #finding crossproduct of transpose of x matrix and w matrix
xtwx<-xtw%*%xmat #finding crossproduct of above and x matrix

xtwx.inv<-solve(xtwx) #finding inverse of above

xrow<-xmat[i, ] #getting row i of x matrix
x.xtwx.inv<-xrow%*%xtwx.inv # crossproduct of above and inverse of xtwx
temp<-x.xtwx.inv%*%xtw #crossproduct of above and xtw
smat[i, ]<-temp#populate row i of S matrix
}
mytry<-sum(diag(smat))#my try at calculating the trace of the S (or hat) matrix

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