I've found a quite puzzling behaviour in gstat's variogram method, with the map=T argument. I believe the result should be a symmetric map because all the semivariances are squared differences. This is indeed the case if I try an example from the Meuse set:
> library(gstat) > data(meuse) > coordinates(meuse) <- ~x + y > class(meuse) [1] "SpatialPointsDataFrame" attr(,"package") [1] "sp" > v <- variogram(log(lead) ~ 1, meuse, cutoff=1000, width=200, map=T) > plot(v) > # symmetric plot as expected, confirm by looking at array > [EMAIL PROTECTED] [1] 0.241586 0.875118 1.329685 0.978228 0.877734 0.854946 0.628017 0.356488 0.323171 0.299103 0.389137 ... [111] 0.387982 0.302654 0.319343 0.356488 0.630622 0.862242 0.877734 0.978228 1.329685 0.875118 0.241586 > # and the number of pairs: > [EMAIL PROTECTED] [1] 2 10 13 36 44 63 86 137 141 122 123 3 7 26 53 60 92 123 150 156 123 87 4 12 41 61 ... [105] 92 61 53 26 7 3 122 122 143 137 87 64 44 36 13 10 2 BUT ... when I try with the Jura dataset I get non-symmetric results: > data(jura) > coordinates(jura.pred) <- ~Xloc + Yloc > class(jura.pred) [1] "SpatialPointsDataFrame" attr(,"package") [1] "sp" > v <- variogram(Ni ~ 1, jura.pred, cutoff=1.2, width=0.18, map=T) > plot(v) # not symmetric! Confirm with array: > [EMAIL PROTECTED] [1] 89.5503 59.3319 88.4276 79.4282 75.1518 147.6931 55.4805 43.6888 70.5159 86.9107 59.5392 ... [188] 105.4008 48.2219 72.3849 35.7244 66.7842 62.4743 72.9742 72.6469 70.9808 > # and the number of pairs > [EMAIL PROTECTED] [1] 42 78 54 97 124 39 159 82 92 62 79 70 93 68 114 78 177 86 92 72 59 82 192 95 192 79 ... [183] 73 135 72 72 97 101 76 137 39 111 105 132 94 31 Clearly both the number of pairs and the resulting average semivariance are not symmetric! Both of the objects are converted to SpatialPointsDataFrames. I can see no obvious reason for this discrepency. D G Rossiter Senior University Lecturer Department of Earth Systems Analysis (DESA) International Institute for Geo-Information Science and Earth Observation (ITC) mailto:[EMAIL PROTECTED], Internet: http://www.itc.nl/personal/rossiter _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo