在R中裁剪栅格

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。我有来自Worldclim.org的两个栅格，这些栅格为我提供了这些变量，但是我需要将它们裁剪到我感兴趣的州的范围。有关如何执行此操作的任何建议。这是我到目前为止所拥有的：

#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")

高程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。任何输入都会有所帮助

我会放弃使用该maps包，并找到一个状态shapefile。然后使用将其加载到R中rgdal，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，您可能想找到一个更通用的文件。

这是extract()从raster包中使用的一种方法。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即可。

您需要加载rgdal和存储raster库才能继续。

library(rgdal)
library(raster)

现在使用导入美国shapefile readOGR()。设置shapefile的CRS之后，我创建一个包含所需状态的子集。注意大写和小写字母的使用！

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, ]

接下来，使用导入栅格数据，raster()并使用先前生成的状态子集的范围对其进行裁剪。

elevation <- raster("/path/to/data/alt.bil")

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

最后，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“遮罩”功能。

elevation.sub <- mask(elevation.sub, state.sub)

这是一个非常简单的结果图：

plot(elevation.sub)
plot(state.sub, add = TRUE)

干杯，
弗洛里安