光栅没有裁剪到shapefile

Question

我尝试合并shapefile中的某些状态，并生成raster我可以使用downstream。我已经合并了状态，但是当我创建一个空栅格以使用裁剪功能进行栅格化似乎失败了。我对GIS features中的R非常陌生，非常感谢您的帮助。 Shapefile is from http://www.arcgis.com/home/item.html?id=f7f805eb65eb4ab787a0a3e1116ca7e5

library(maptools)
library(shapefiles)
library(raster)
usa.states <- readOGR(dsn = "states_21basic/", layer = "states")
head(usa.states)
Co=usa.states[usa.states@data$STATE_NAME== "Colorado",]
Nm=usa.states[usa.states@data$STATE_NAME== "New Mexico",]
Az=usa.states[usa.states@data$STATE_NAME== "Arizona",]
Ut=usa.states[usa.states@data$STATE_NAME== "Utah",]
Corners= spRbind(spRbind(spRbind(Co,Ut),Nm),Az)
CRS="+proj=longlat +datum=WGS84"
Corners=spTransform(Corners, CRS(CRS))
> extent(Corners)
class       : Extent 
xmin        : -114.8218 
xmax        : -102.0372 
ymin        : 31.33563 
ymax        : 42.0023 
cor.ext=extent(Corners)
r<-raster(ncol=ncol(Corners), nrow=nrow(Corners), crs=CRS)
Corners.crop= crop(r,cor.ext, snap="out")

然后，当我接到'Corners.crop'的范围时，我会收到：

> extent(Corners.crop)
class       : Extent 
xmin        : -180 
xmax        : -36 
ymin        : 0 
ymax        : 45 

我对我失去的工作感到困惑。 我也希望有一个1Km的分辨率，如果更改空光栅上的分辨率或光栅化形状后更好一点。

Answer 1

library(rgdal)
library(raster)
library(rgeos)

usa.states <- readOGR("states.shp", layer = "states")

# Here we subset once
Corners <- usa.states[usa.states$STATE_NAME %in% c("Colorado", "New Mexico","Arizona","Utah"),]

# Dissolve polygons into one
Corners <- gUnaryUnion(Corners)

# Create a 20x20 raster using the extent of Corners
# The number of rows and columns can be change to increase/reduce the resolution
r <- raster(extent(Corners), ncol=20, nrow=20, crs=CRS(proj4string(Corners)))

# Rasterize
Corners.crop <- rasterize(Corners, r)