我正在为美国东北部绘制地图。地图背景必须是海拔图或年平均温度图。我有两个来自Worldclim.org的栅格，这些栅格为我提供了这些变量，但我需要将它们裁剪到我感兴趣的状态。
我该怎么做？ far：
#load libraries
library (sp)
library (rgdal)
library (raster)
library (maps)
library (mapproj)


#load data
state<- data (stateMapEnv)
elevation<-raster("alt.bil")
meantemp<-raster ("bio_1.asc")

#build the raw map
nestates<- c("maine", "vermont", "massachusetts", "new hampshire" ,"connecticut",
  "rhode island","new york","pennsylvania", "new jersey",
  "maryland", "delaware", "virginia", "west virginia")

map(database="state", regions = nestates, interior=T,  lwd=2)
map.axes()

#add site localities
sites<-read.csv("sites.csv", header=T)
lat<-sites$Latitude
lon<-sites$Longitude

map(database="state", regions = nestates, interior=T, lwd=2)
points (x=lon, y=lat, pch=17, cex=1.5, col="black")
map.axes()
library(maps)                                                                  #Add axes to  main map
map.scale(x=-73,y=38, relwidth=0.15, metric=T,  ratio=F)

#create an inset map

 # Next, we create a new graphics space in the lower-right hand corner.  The numbers are proportional distances within the graphics window (xmin,xmax,ymin,ymax) on a scale of 0 to 1.
  # "plt" is the key parameter to adjust
    par(plt = c(0.1, 0.53, 0.57, 0.90), new = TRUE)

  # I think this is the key command from http://www.stat.auckland.ac.nz/~paul/RGraphics/examples-map.R
    plot.window(xlim=c(-127, -66),ylim=c(23,53))

  # fill the box with white
    polygon(c(0,360,360,0),c(0,0,90,90),col="white")

  # draw the map
    map(database="state", interior=T, add=TRUE, fill=FALSE)
    map(database="state", regions=nestates, interior=TRUE, add=TRUE, fill=TRUE, col="grey")

海拔和平均温度对象是需要裁剪到嵌套对象的区域范围的对象。

评论

您是否有可能通过从具有相同范围和分辨率的随机数据创建栅格来使此结果可被其他人重现？
为什么不跳过裁切片并直接遮罩？

#1 楼

我将使用maps软件包删除并找到一个状态shapefile。然后使用rgdal将其加载到R中，然后执行一些多边形叠加工作。 

library(raster)
# use state bounds from gadm website:
# us = shapefile("USA_adm1.shp")
us <- getData("GADM", country="USA", level=1)
# extract states (need to uppercase everything)
nestates <- c("Maine", "Vermont", "Massachusetts", "New Hampshire" ,"Connecticut",
         "Rhode Island","New York","Pennsylvania", "New Jersey",
         "Maryland", "Delaware", "Virginia", "West Virginia")

ne = us[match(toupper(nestates),toupper(us$NAME_1)),]


# create a random raster over the space:        
r = raster(xmn=-85,xmx=-65,ymn=36,ymx=48,nrow=100,ncol=100)
r[]=runif(100*100)

# plot it with the boundaries we want to clip against:
plot(r)
plot(ne,add=TRUE)

# now use the mask function
rr <- mask(r, ne)

# plot, and overlay:
plot(rr);plot(ne,add=TRUE)


怎么样？ gadm shapefile非常详细，您可能想要找到一个更通用的文件。

#2 楼

这是使用来自extract()软件包的raster的方法。我使用WorldClim网站上的海拔高度和平均温度数据对其进行了测试（我将此示例限制为海拔高度，温度工作类似），并且可以在此处找到包含州边界的美国适当的shapefile。只需下载.zip数据并将其解压缩到您的工作目录。
您需要加载rgdal和raster库才能继续。
library(rgdal)
library(raster)

现在让我们使用readOGR()导入美国shapefile。设置shapefile的CRS之后，我创建一个包含所需状态的子集。请注意大写字母和小写字母的使用！ >作为最后一步，您需要确定高程栅格中位于给定状态多边形边界内的那些像素。为此使用“掩码”函数。
state <- readOGR(dsn = path.data, layer = "usa_state_shapefile")
projection(state) <- CRS("+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs")

# Subset US shapefile by desired states
nestates <- c("Maine", "Vermont", "Massachusetts", "New Hampshire" ,"Connecticut",
             "Rhode Island","New York","Pennsylvania", "New Jersey",
             "Maryland", "Delaware", "Virginia", "West Virginia")

state.sub <- state[as.character(state@data$STATE_NAME) %in% nestates, ]

结果非常简单：
elevation <- raster("/path/to/data/alt.bil")

# Crop elevation data by extent of state subset
elevation.sub <- crop(elevation, extent(state.sub))