knitr::opts_chunk$set(echo = TRUE,eval=TRUE,fig.align="center",fig.width = 7,fig.height = 6)
The R package
ClustGeo implements a Ward-like hierarchical clustering algorithm including spatial/geographical constraints. Two dissimilarity matrices
D1 are inputted, along with a mixing parameter
alpha in $[0,1]$. The dissimilarities can be non-Euclidean and the weights of the observations can be non-uniform. The first matrix gives the dissimilarities in the "feature space"" and the second matrix gives the dissimilarities in the "constraint space". The criterion minimized at each stage is a convex combination of the homogeneity criterion calculated with
D0 and the homogeneity criterion calculated with
D1. The idea is to determine a value of
alpha which increases the spatial contiguity without deteriorating too much the quality of the solution based on the variables of interest i.e. those of the feature space. This procedure is illustrated on the
estuary dataset available in the package.
estuary refers to $n=303$ French municipalities of Gironde estuary (a south-ouest French region). This dataset contains:
datwith the description of the 303 municipalities on 4 socio-economic variables,
D.geowith the distances between the town halls of the 303 municipalities,
SpatialPolygonsDataFramewith the map of the municipalities.
library(ClustGeo) data(estuary) dat <- estuary$dat head(dat) D.geo <- estuary$D.geo map <- estuary$map
map is an object of class
SpatialPolygonsDataFrame (of the package
sp) and the method
plot can be used to visualize the municipalities on the map.
# description of 5 municipalities in the map head(map@data[,4:8]) # plot of the municipalities library(sp) ?"SpatialPolygonsDataFrame-class" sp::plot(map, border="grey") # plot method sel <- map$NOM_COMM%in% c("BORDEAUX", "ARCACHON", "ROYAN") # label of 3 municipalities text(sp::coordinates(map)[sel,], labels = map$NOM_COMM[sel]) # we check that the municipalities in map are the same than those in X identical(as.vector(map$INSEE_COM),rownames(dat))
hclustgeo implements a Ward-like hierarchical clustering algorithm with soft contiguity constraint. The main arguments of the function are:
D0with the dissimilarities in the "feature space" (here socio-economic variables for instance).
D1with the dissimilarities in the "constraint" space (here a matrix of geographical dissimilarities).
alphabetween 0 an 1. The mixing parameter sets the importance of the constraint in the clustering procedure.
scalewith a logical value. If
TRUEthe dissimilarity matrices
D1are scaled between 0 and 1 (that is divided by their maximum value).
choicealpha implements a procedure to help the user in the choice of a suitable value of the mixing parameter
choicealpha can be combined to find a partition of the $n=303$ French municipalities including geographical contiguity constraint. The two steps of the procedure are :
D0. The clusters of this partition are homogeneous on the socio-economic variables and no contiguity constraint is used.
alphain order to increases the geographical cohesion of the clusters (using the dissimilarity matrix
D1) without deteriorating too much the homogeneity on the socio-economic variables.
Ward hierarchical clustering of the 303 municipalities is performed using the dissimilarity matrix
D0 (calculated with the socio-economic variables). The partition in $K=5$ clusters is chosen from the Ward dendrogram.
D0 <- dist(dat) # the socio-economic distances tree <- hclustgeo(D0) plot(tree,hang = -1, label = FALSE, xlab = "", sub = "", main = "Ward dendrogram with D0 only") rect.hclust(tree ,k = 5, border = c(4,5,3,2,1)) legend("topright", legend = paste("cluster",1:5), fill=1:5,bty= "n", border = "white")
This partition is plotted on the
# cut the dendrogram to get the partition in 5 clusters P5 <- cutree(tree,5) city_label <- as.vector(map$"NOM_COMM") names(P5) <- city_label plot(map, border = "grey", col = P5, main = "Partition P5 obtained with D0 only") legend("topleft", legend = paste("cluster",1:5), fill = 1:5, bty = "n", border = "white")
This map shows that municipalities in the same cluster of the partition
P5 are not necessary contiguous. For instance municipalities in
cluster 5 are contiguous wehereas municipalities in
cluster 3 are separated.
# list of the municipalities in cluster 5 city_label[which(P5==5)]
In order to get more spatially compact clusters, the matrix
D1 of the distances between the town halls of the municipalities, is included in the clustering process along with the mixing parameter
alpha used to set the importance of
D1 <- as.dist(D.geo) # the geographic distances between the municipalities
The mixing parameter
alpha is chosen in such a way that the geographical cohesion of the clusters is improved without deteriorating too much the socio-economic cohesion.
The mixing parameter
alpha in $[0,1]$ sets the importance of
D1 in the clustering process. When
alpha=0 the geographical dissimilarities are not taken into account and when
alpha=1 it is the socio-economic distances which are not taken into account and the clusters are obtained with the geographical distances only.
The idea is then to calculate separately the socio-economic homogeneity (denoted
Q0) and the geographic homogneity (denoted
Q1) of the partitions obtained for a range of different values of
alpha and a given number of clusters $K$. The homogeneity
Q1) is the proportion of explained inertia calculated with
range.alpha <- seq(0,1,0.1) K <- 5 cr <- choicealpha(D0, D1, range.alpha, K, graph = FALSE) cr$Q # proportion of explained inertia
The plot of the curves of
Q1 is a tool to choose a value of
alpha that is a compromise between the lost in socio-economic homogeneity and the gain in geographic cohesion.
#postscript("plotQ-1.eps",width = 950,height = 489) plot(cr) #dev.off() #postscript("plotQnorm-1.eps") plot(cr,norm=TRUE) #dev.off()
We see that the proportion of explained inertia calculated with
D0 (the socio-economic distances) is equal to 0.81 when
alpha=0 and decreases when
alpha inceases (black line). On the contrary the proportion of explained inertia calculated with
D1 (the geographical distances) is equal to 0.87 when
alpha=1 and decreases when
alpha decreases (red line).
Here the plot suggest to choose
alpha=0.2 which correponds to a lost of socio-economic homogeneity of only 7 \% and a gain of geographic homogeneity of about 17 \%.
The dendrogram obtained with the function
alpha=0.2 of is cut to get the new partition in 5 clusters.
tree <- hclustgeo(D0,D1,alpha=0.2) P5bis <- cutree(tree,5)
The modified partition
P5bis is visualized on the map.
sp::plot(map, border = "grey", col = P5bis, main = "Partition P5bis obtained with alpha=0.2 and geographical distances") legend("topleft", legend=paste("cluster",1:5), fill=1:5, bty="n",border="white")
We see that the geographical cohesion of the partition
P5bis is increased compared to partition
Here a different matrix of dissimilarities
D1 is considered to take the neighborhood between the municipalities into account rather than the geographical distance. Two municipalities with contiguous boundaries (sharing one or more boundary point) are considered as neighbours. The adjacency matrix
A is the binary matrix of the neighbourhoods between the municipalities.
library(spdep) ?poly2nb list.nb <- poly2nb(map, row.names = rownames(dat)) #list of neighbours of each city ?nb2mat A <- nb2mat(list.nb,style="B") diag(A) <- 1 colnames(A) <- rownames(A) <- city_label A[1:5,1:5]
The dissimilarity matrix
D1 is then 1 minus
D1 <- as.dist(1-A)
The procedure for the choice of
alpha is repeated here with the new matrix
range.alpha <- seq(0,1,0.1) K <- 5 cr <- choicealpha(D0, D1, range.alpha, K, graph=FALSE) plot(cr)
The explained inertia calculated here with
D1 (red curve) is much smaller than the explained inertia calculated with
D0 (black curve). To overcome this problem, the normalized proportion of explained inertia (
Qnorm) is plotted.
cr$Qnorm # normalized proportion of explained inertia plot(cr, norm = TRUE)
D0 the curve starts from 100% and decreases as
alpha increases from 0 to 1. With
D1 the curve starts from 100% (on the right) and decreases as
alpha decreases from 1 to 0. This plot suggests to choose
The dendrogram obtained with the function
alpha=0.2 of is cut to get a new partition in 5 clusters.
tree <- hclustgeo(D0, D1, alpha =0.2) P5ter <- cutree(tree,5) sp::plot(map, border="grey", col=P5ter, main=" Partition P5ter obtained with alpha=0.2 and neighborhood dissimilarities") legend("topleft", legend=1:5, fill=1:5, col=P5ter)
P5ter is spatially more compact than
P5bis. This is not surprising since dissimilarities are build from the adjacency matrix which gives more importance local neighborhoods. However since the clustering process is based on soft contiguity constraints, municipalities that are not neighbors are allowed to be in the same
clusters. This is the case for instance for
cluster 4 where some municipalities are located in the north of the estuary whereas most are located in the southern area (corresponding to forest areas).
M. Chavent, V. Kuentz-Simonet, A. Labenne, J. Saracco. ClustGeo: an R package for hierarchical clustering with spatial constraints.Comput Stat (2018) 33: 1799-1822.
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