如何使用 sf 读取 .csv 多边形数据？

问题描述

我正在尝试读取一个 .csv 文件，其中每一行都包含有关多边形的信息。多边形几何信息存储在 shape 列中，例如 'longitude latitude altitude accuracy; ...'。

这是一个带有单个正方形多边形的简化示例：

structure(list(field = "parcel_a",shape = "5 50 0 0.5; 20 50 0 0.5; 5 40 0 0.5; 20 40 0 0.5"),class = "data.frame",row.names = c(NA,-1L))

如何在 R 中使用 sf 读取这种数据格式？我也很想知道这种格式来自哪里。

解决方法

这里有一些东西可以帮助您入门。

# Load packages
library('sf')
library('stringr')

# longitude latitude altitude accuracy;
v <- structure(list(field = "parcel_a",shape = "5 50 0 0.5; 20 50 0 0.5; 5 40 0 0.5; 20 40 0 0.5"),class = "data.frame",row.names = c(NA,-1L))

# Function to take the string,split it up,and turn it into a polygon
mkshp <- function(x) {
    
    x <- str_trim(x)
    
    y <- unlist(strsplit(x,split=' '))
    
    # Reshape it to a matrix
    m <- matrix(as.numeric(y),ncol=4,byrow=TRUE)
    
    # Just want the first two columns - long,lat
    m <- m[,1:2]
    
    
    # Need to reorder the matrix because the coords are not in order
    # This bit will need to be rewritten to accomodate your real data
    m <- m[c(1,2,4,3),]
    
    
    # Need to close it by adding the first row to the end
    m <- rbind(m,m[1,])
    
    # Create a polygon object
    st_polygon(list(m))
    
}

# shps will be a list of polygons (one for each row in the data frame)
shps <- lapply(strsplit(v$shape,split=';'),mkshp)

# Set these polygons as the geometry column in our data frame
st_geometry(v) <- st_as_sfc(shps)

# Set a Coordinate Reference System if you wnat
# st_crs(v) <- ...

plot(v,axes=T)

r r sf