此示例应该是可重现的，第一部分（使用大多边形）有效，第二部分失败：
library(raster)
USA.altitude <- getData('alt', country='USA')
lower48 <- USA.alt[[1]]
Srs1 = Polygons(list(Polygon(cbind(c(-96,-95,-95,-96),c(40,40,41,40)))), "s1")
Srs2 = Polygons(list(Polygon(cbind(c(-97,-96,-96,-97),c(40,40,41,40)))), "s2")
spdf.large <- SpatialPolygonsDataFrame( SpatialPolygons(list(Srs1,Srs2)),
data.frame( z=1:2, row.names=c("s1","s2") ) )
plot(spdf.large)
usa.average.elev<-extract(lower48, spdf.large, fun=mean)
耶！一切似乎都好，是吗？现在用小多边形：
Srs3 = Polygons(list(Polygon(cbind(c(-95.01,-94.99,-94.99,-95.01),c(40,40,40.01,40)))), "s3")
Srs4 = Polygons(list(Polygon(cbind(c(-94.01,-93.99,-93.99,-94.01),c(40,40,40.01,40)))), "s4")
spdf.small <- SpatialPolygonsDataFrame( SpatialPolygons(list(Srs3,Srs4)),
data.frame( z=1:2, row.names=c("s3","s4") ) )
plot(spdf.small)
usa.average.elev<-extract(lower48, spdf.small, fun=mean)
因错误而失败：
pmax错误（上限（10 / dim（r）[1：2]），c（1,1））：未找到对象'r'