[英]Selection and extraction of points (shapefile) within a polygon in R
I have two shape-files;我有两个形状文件; one is a point file (some information for world) and the other is a 21-countries shape-file.
一个是点文件(世界的一些信息),另一个是 21 个国家的形状文件。
I need to extract the points which falls with in a country.我需要提取一个国家的分数。 I have to repeat this step in 21 times in QGIS or ArcGIS.
我必须在 QGIS 或 ArcGIS 中重复此步骤 21 次。
Is there any way to do this in R and if is in batch processing, that would be really great as I have to repeat this for other 4 data sets as well.在 R 中是否有任何方法可以做到这一点,如果是在批处理中,那将非常棒,因为我也必须对其他 4 个数据集重复此操作。 Thanks in advance
提前致谢
So as an example how to use over()
consider the following simulated data.因此,作为如何使用
over()
的示例,请考虑以下模拟数据。
library(raster)
library(sp)
#generate point distribution | with (x,y) coordinates
set.seed(42);pts <- SpatialPoints(data.frame(x = runif(30, 1, 5), y = runif(30, 1, 5)))
#create polygons
df<-data.frame(X = c(rep(1,5),rep(2,5),rep(3,5),rep(4,5),rep(5,5)),
Y = c(rep(seq(1,5,1),5)))
df$cell<-1:nrow(df) #polygon identifier (for later)
#make into spatial object (probably better way to do this)
coordinates(df) <- ~X+Y
rast <- raster(extent(df), ncol = 5, nrow = 5)
grid<-rasterize(df, rast, df$cell, FUN = max)
grid<-rasterToPolygons(grid) #create polygons
#plot to check
plot(grid); points(pts)
#and extract
pointsinpolygon = over(SpatialPolygons(grid@polygons), SpatialPoints(pts))
The last bit you can also do as最后一点你也可以这样做
df$pointsinpolygon <- over(SpatialPolygons(grid@polygons), SpatialPoints(pts))
to write the results directly to the dataframe.将结果直接写入 dataframe。
Here is an example polygons shapefile and how you can read it into R.这是一个示例多边形 shapefile 以及如何将其读入 R。
library(raster)
# example polygons filename
f <- system.file("external/lux.shp", package="raster")
pol <- shapefile(f)
I do not have a matching points file, so I generate some points for this example我没有匹配点文件,所以我为此示例生成了一些点
pts <- spsample(pol, 5, type="regular")
crs(pts) <- crs(pol)
To see what area the points fall in you can use over
or extract
.要查看点落在哪个区域,您可以使用
over
或extract
。 They both return the values in the order of the points, but extract also adds the (sequential) "point id" --- this is primarily for when you have overlapping polygons, such that a point might fall in two polygons.它们都按点的顺序返回值,但 extract 还添加了(顺序的)“点 id”——这主要用于当你有重叠的多边形时,这样一个点可能会落在两个多边形中。
over(pts, pol)
# ID_1 NAME_1 ID_2 NAME_2 AREA
#1 3 Luxembourg 9 Esch-sur-Alzette 251
#2 2 Grevenmacher 7 Remich 129
#3 3 Luxembourg 11 Mersch 233
#4 2 Grevenmacher 6 Echternach 188
#5 1 Diekirch 1 Clervaux 312
extract(pol, pts)
# point.ID poly.ID ID_1 NAME_1 ID_2 NAME_2 AREA
#1 1 10 3 Luxembourg 9 Esch-sur-Alzette 251
#2 2 7 2 Grevenmacher 7 Remich 129
#3 3 12 3 Luxembourg 11 Mersch 233
#4 4 6 2 Grevenmacher 6 Echternach 188
#5 5 1 1 Diekirch 1 Clervaux 312
If you just want the intersection, say of the points that fall in "Grenmacher", like you may want to do with your countries, you can如果您只想要交叉点,比如说落在“Grenmacher”中的点,就像您可能想要与您的国家一样,您可以
g <- pol[pol$NAME_1 == "Grevenmacher", ]
i <- intersect(pts, g)
i
#class : SpatialPointsDataFrame
#features : 2
#extent : 6.27111, 6.27111, 49.53585, 49.7887 (xmin, xmax, ymin, ymax)
#crs : +proj=longlat +datum=WGS84 +no_defs
#variables : 5
#names : ID_1, NAME_1, ID_2, NAME_2, AREA
#min values : 2, Grevenmacher, 6, Echternach, 129
#max values : 2, Grevenmacher, 7, Remich, 188
And perhaps write that back to disk like this也许像这样写回磁盘
shapefile(i, "test.shp")
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