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Measure a distance between two points on a plot using locator; the function checks par("plt") and par("usr") to try to ensure that the aspect ratio y/x is 1, that is that the units of measurement in both x and y are equivalent.
locator
par("plt")
par("usr")
airdist(ann=FALSE)
a list with members:
distance measured
coordinates between which distance is measured
annotate the plot with line measured and distance
Roger Bivand Roger.Bivand@nhh.no