net.correlogram {stream.net}R Documentation

Calculate Correlogram for Stream Segment Attribute

Description

Compute the autocorrelation coefficients of an attribute for a sequence of separations in distance (lags) between stream segments.

Usage

  net.correlogram (net, dist, segatt, nlags=10)

Arguments

net A net.object.
dist an upstream/downstream distance matrix from net.dist, probably using ends=0.5.
segatt attribute associated with each segment. If segatt is a character vector of length one, then get attribute from net$segs$segatt, else assume segatt is a numeric vector of length equal to number of segments and in correct order.
nlags number of lags in the output function.

Details

Uses total distances, ignoring upstream/downstream. The algorithm for autocorrelation coefficients, from the reference below, is

r(x) = sum(2 * z[i] * z[j])/sum(z[i]^2 + z[j]^2)

for all segment pairs i,j at lag x. Values are in [-1, 1].

Lag distances are in method units used in net.dist, and are in range [0, max(dist)].

Value

A list with the following elements:
r vector of autocorrelation coefficients at each lag
lag vector of lag distances
num vector of number of pairs at each lag

Author(s)

Denis White, white.denis@epa.gov

References

Henley S. 1975. Autocorrelation coefficients from irregularly spaced areal data. Computers and Geosciences 2(4):437-438.

See Also

net.object net.dist net.autocorr.att net.autocorr.onelag

Examples

  # Q model random net
  net <- net.qmodel (5)
  net <- net.addsegs (net)
  dmat <- net.dist (net, ends=0.5, method="segment")

  # make rnorm and autocorrelated attributes
  att.rnorm <- net.addatt (net, dist="Normal",
    boundscaling=TRUE, vector=TRUE)
  att.autoc <- net.autocorr.att (net, dist=dmat, vector=TRUE,
    outscaling=TRUE) * 10

  # plot correlograms
  cg1 <- net.correlogram (net, dist=dmat, segatt=att.rnorm)
  cg2 <- net.correlogram (net, dist=dmat, segatt=att.autoc)
  plot (cg1$lag, cg1$r, pch=19, type="b", ylim=c(-1,1), 
    main="Random Normal")
  plot (cg2$lag, cg2$r, pch=19, type="b", ylim=c(-1,1),
    main="Autocorrelated")

[Package stream.net version 1.0.6 Index]