rlang :: last_error（）in purrr :: map loop in r rlang :: last_error（）

Question

我正在使用一个函数来计算每个单元格的linestring长度，然后存储在列表中，将列表的每个元素转换为rasterlayer，然后将该列表转换为栅格堆栈，平均所有图层并获得单个光栅。

#function

#  build_length_raster <- function(one_df) {

  intersect_list <- by(
    one_df , 
    one_df$sub_id,
    function(subid_df) sf::st_intersection(grid2, subid_df) %>% 
      dplyr::mutate(length = as.numeric(sf::st_length(.))) %>%
      sf::st_drop_geometry()
  ) 
  
  list_length_grid <- purrr::map(intersect_list, function(x) 
    x %>% dplyr::left_join(x=grid2, by="cell", copy=T) %>% 
      dplyr::mutate(length=length) %>%
      dplyr::mutate_if(is.numeric,coalesce,0)
  )
   list_length_raster <- purrr::map(list_length_grid, function(x) 
    raster::rasterize(x, r, field="length", na.rm=F, background=0)
  )

  list_length_raster2 <- unlist(list_length_raster, recursive=F)
  
  raster_stack <- raster::stack(list_length_raster2)
  
  raster_mean <- raster::stackApply(
    raster_stack, 
    indices = rep(1,nlayers(raster_stack)), 
    fun = "mean", na.rm = TRUE)
#}

该函数提出了一个步骤，为了使ST_Intersection（）的结果网格具有与最初相同的单元格数，我使用Left_join（by =“ cell”列）。 na为0。当我从列表中运行一个数据帧的功能步骤时，它可以很好地工作，但是当我将其放入Map（）中以在列表中执行此操作时，我会收到此错误，这似乎是指dplyr函数：

final_list <- purrr::map(mylist, build_length_raster)

> rlang::last_error()
<error/rlang_error>
Join columns must be present in data.
x Problem with `cell`.
Backtrace:
  1. purrr::map(mylist, build_length_raster)
 15. dplyr:::left_join.data.frame(., x = grid, by = "cell", copy = T)
 16. dplyr:::join_mutate(...)
 17. dplyr:::join_cols(...)
 18. dplyr:::standardise_join_by(by, x_names = x_names, y_names = y_names)
 19. dplyr:::check_join_vars(by$y, y_names)
Run `rlang::last_trace()` to see the full context.

有没有办法解决此问题？

myData示例

library(tidyverse)
library(sf)
library(purrr)
library(raster)

#data example
id <- c("844", "844", "844", "844", "844","844", "844", "844", "844", "844",
        "844", "844", "845", "845", "845", "845", "845","845", "845", "845", 
        "845","845", "845", "845")
sub_id <- c("2017_844_1", "2017_844_1", "2017_844_1", "2017_844_1", "2017_844_2",
        "2017_844_2", "2017_844_2", "2017_844_2", "2017_844_3", "2017_844_3",
        "2017_844_3", "2017_844_3", "2017_845_1", "2017_845_1", "2017_845_1", 
        "2017_845_1", "2017_845_2","2017_845_2", "2017_845_2", "2017_845_2", 
        "2017_845_3","2017_845_3", "2017_845_3", "2017_845_3")
lat <- c(-30.6456, -29.5648, -27.6667, -31.5587, -30.6934, -29.3147, -23.0538, 
         -26.5877, -26.6923, -23.40865, -23.1143, -23.28331, -31.6456, -24.5648, 
         -27.6867, -31.4587, -30.6784, -28.3447, -23.0466, -27.5877, -26.8524, 
         -23.8855, -24.1143, -23.5874)
long <- c(-50.4879, -49.8715, -51.8716, -50.4456, -50.9842, -51.9787, -41.2343, 
          -40.2859, -40.19599, -41.64302, -41.58042, -41.55057, -50.4576, -48.8715, 
          -51.4566, -51.4456, -50.4477, -50.9937, -41.4789, -41.3859, -40.2536, 
          -41.6502, -40.5442, -41.4057)

df <- tibble(id = as.factor(id), sub_id  = as.factor(sub_id), lat, long)

#converting ​to sf
df.sf <- df %>% 
 ​sf::st_as_sf(coords = c("long", "lat"), crs = 4326)


#creating grid
xy <- sf::st_coordinates(df.sf)

grid = sf::st_make_grid(sf::st_bbox(df.sf),
                       ​cellsize = .1, square = FALSE) %>%
 ​sf::st_as_sf() 

#creating raster
r <- raster::raster(grid, res=0.1) 

#return grid because raster function changes number of cells 
grid2 <- rasterToPolygons(r, na.rm=F) %>%
  st_as_sf() %>% mutate(cell=1:ncell(r))

#creating linestring to each sub_id
df.line <- df.sf %>% 
  dplyr::group_by(sub_id, id) %>%
  dplyr::summarize() %>%
  sf::st_cast("LINESTRING") 

#creating ID list
mylist<- split(df.line, df.line$id)

#separating one dataframe of list to test function
one_df <- df.line[df.line$id=="844",]
one_df$id <- droplevels(one_df$id)
one_df$sub_id <- droplevels(one_df$sub_id)

Answer 1

由于Intersect_list在列表中有空的项目而引起特定错误，因此由于它们是空而无法加入的，因此无法加入列。如果您修改了映射功能以仅使用Intersect_list的非空项目，则不会收到该错误。

如您在注释中指出的那样，使用keep（Intersect_list，〜！is.null（。））删除空列表条目之前错误。

但是，我认为这不是解决此问题的最优雅方法。我可能会误解目标是什么，但是如果要从每个网格单元格中的总线路长度产生栅格，我认为不使用purrr可能会起作用的更简单的方法。

这与您的产品并不完全相同，但是我要保持更简单的RN来说明另一种方法。这是每个单元格中的长度的总和，例如start对象（类似于raster，但使用Tidyverse和sf）。

从您的对象开始

# Turn multiple lines into single MULTILINESTRING:
one_df %>% 
  st_union() -> 
  union_df

# Intersection of each grid cell with the MULTILINESTRING geometry:
grid %>% 
  st_intersection(union_df) -> 
  grid_lines 

# Get lengths:
grid_lines %>% 
  mutate(length = st_length(x)) %>% 
  st_drop_geometry() ->
  grid_lengths

# Join the calculated lengths back with the spatial grid, 
# most of which will have NA for length
grid %>%
  left_join(grid_lengths, by = "cell") ->
  grid_with_lengths

# Rasterize the length field of the grid
grid_with_lengths %>% 
  dplyr::select(length) %>% 
  stars::st_rasterize() ->
  length_stars
  
  length_stars %>% mapview::mapview()

我是 “>