fields (version 5.02)

vgram: Finds a traditional or robust variogram for spatial data.

Description

Computes pairwise squared differences as a function of distance. Returns either raw values or statistics from binning.

Usage

vgram(loc, y, id=NULL, d=NULL, lon.lat=FALSE, dmax=NULL, N=NULL, breaks=NULL)

Value

A list with these components.vgramVariogram valuesdPairwise distancescallCalling stringstatsMatrix of statistics for values in each bin. Rows are the summaries returned by the stats function or describe. If not either breaks or N arguments are not supplied then this component is not computed.centersBin centers.

References

See any standard reference on spatial statistics. For example Cressie, Spatial Statistics

See Also

vgram.matrix bplot.xy, vgram.matrix

Examples

#
# compute variogram for the midwest ozone field  day 16
# (BTW this looks a bit strange!)
#
data( ozone2)
good<- !is.na(ozone2$y[16,])
x<- ozone2$lon.lat[good,] 
y<- ozone2$y[16,good]

look<-vgram( x,y, N=15, lon.lat=TRUE) # locations are in lon/lat so use right
#distance
# take a look:
#plot( look$d, look$vgram)
#lines(look$centers, look$stats["mean",], col=4)

brk<- seq( 0, 250,,25)
 
## or some boxplot bin summaries

bplot.xy( look$d, sqrt(look$vgram), breaks=brk,ylab="sqrt(VG)")
lines(look$centers, look$stats["mean",], col=4)