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I am trying to add district labels for a country, which I am plotting using levelplot (to use worldclim precipitation data) and adding boundaries using spplot.

I am having trouble adding text labels of the districts onto the map. I have the following code:

tan <- getData("GADM", country="Tanzania", level=1)
alt_world <-getData('worldclim', var='alt', res=10)

tanalt.sub <- crop(alt_world, extent(tan))
tan.sub <- mask(tanalt.sub, tan)

p1 <- levelplot(tan.sub, col.regions=colorRampPalette(brewer.pal(9, 'Blues')),margin=FALSE,main ='Tanzania rainfall (mm/year)')
p2 <- spplot(tan)

p <- p1+p2

I have tried the following to add labels:

labels <- layer(sp.text(coordinates(tan), txt = tan$NAME_1, pos = 1))
p2 <- p2 + labels

But get an error regarding 'inherited function for coordinates'.

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Managed to solve it using ggplot2, however still looking at how to use labels with levelplot.

tan.adm.spdf <- getData("GADM", country="Tanzania", level=1)
tan.adm.df <- fortify(tan.adm.spdf, region="NAME_1")

tan.adm.centroids.df <- data.frame(long = coordinates(tan.adm.spdf)[, 1],lat = coordinates(tan.adm.spdf)[, 2]) 
tan.adm.centroids.df[, 'NAME_1'] <- tan.adm.spdf@data[, 'NAME_1']

alt_world <-getData('worldclim', var='alt', res=2.5)
tanalt.sub <- crop(alt_world, extent(tan.adm.spdf))
tan.sub <- mask(tanalt.sub, tan)

test_spdf <- as(tan.sub, "SpatialPixelsDataFrame")
test_df <- as.data.frame(test_spdf)
colnames(test_df) <- c("value", "x", "y")

p <- ggplot() + geom_tile(data=test_df, aes(x=x, y=y, fill=value), alpha=0.8) + geom_polygon(data=tan.adm.spdf, aes(x=long, y=lat, group=group),fill=NA, color="black", size=0.90) + geom_text_repel(data = tan.adm.centroids.df, aes(label = NAME_1, x = long, y = lat, group = NAME_1), size = 3)

Edit, found a solution:

tan.adm.labels <- SpatialPoints(tan.adm.centroids.df[,c("long", "lat")])
p1 <- levelplot(tan.sub, col.regions=colorRampPalette(brewer.pal(9, 'Blues')),margin=FALSE,main ='Tanzania rainfall (mm/year)', alpha=0.5) + layer(sp.points(points, pch=21,col="black", fill='red',cex=0.95, alpha=0.7)) + spplot(tan.adm.spdf, alpha=0.7, col='#808080') + layer(sp.text(coordinates(tan.adm.labels), txt = tan.adm.centroids.df$NAME_1), scale=0.5)

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