将具有值作为因子的栅格图层转换为SpatialPixelsDataFrame

如何解决将具有值作为因子的栅格图层转换为SpatialPixelsDataFrame

我有一个以值作为因子的栅格图层和一个属性表。我想将其转换为SpatialPixelsDataFrame，但是当我尝试：

library(sp)
library(sf)
library(rgdal)
library(raster)
library(FedData)
library(rasterVis)

create_factor_rast <- function(base_rast,Spatial_polygon,fieldname) {
  nam <- unique(Spatial_polygon[[fieldname]])
  nam_df <- data.frame(ID = 1:length(nam),nam = nam)
  idfieldname = paste0(fieldname,"ID")
  Spatial_polygon[[idfieldname]] <- nam_df$ID[match(Spatial_polygon[[fieldname]],nam_df$nam)]
  new_raster  <- rasterize(x=Spatial_polygon,y=base_rast,field = idfieldname)
  new_raster  <- ratify(new_raster)
  # # Create levels
  rat <- levels(new_raster)[[1]]
  rat$names <- nam_df$nam
  rat$IDs <- nam_df$ID
  levels(new_raster) <- rat
  new_raster
}

spft <- "+proj=lcc +lat_1=40 +lat_2=43 +lat_0=39.83333333333334 +lon_0=-100 +x_0=500000.0000000002 +y_0=0 +datum=NAD83 +units=us-ft +no_defs"

SSURGO.dl <- FedData::get_ssurgo(template=c('NE043'),label = "Tau",raw.dir = "./SSURGO_r/RAW/SSURGO",extraction.dir = paste0("./SSURGO_R/EXTRACTIONS/","soils","/SSURGO"))

SSURGO.dl_shp<-SSURGO.dl$spatial

unlink('SSURGO_r',recursive=TRUE)

# extract the muaggatt table
muaggatt<-SSURGO.dl[["tabular"]][["muaggatt"]]

# get only the fields needed,must have mukey to do the join
muaggatt  <- muaggatt[c("mukey","hydgrpdcd","drclassdcd")]
SSURGO_merge <- merge(SSURGO.dl_shp,muaggatt,by.x = "MUKEY",by.y="mukey")

# Convert merged SSURGO polygon to Stateplane feet
SSURGO_merge    <- sf::st_transform(SSURGO_merge,crs(spft))
SSURGO_merge    <- as(SSURGO_merge,"Spatial")

grid_cell_size  <- 100   # ~30 meters


shp_extent      <- extent(SSURGO_merge)
r               <- raster(SSURGO_merge,resolution = grid_cell_size,crs=crs(SSURGO_merge))

r.drclassdcd  <- create_factor_rast(r,SSURGO_merge,'drclassdcd')

rasterVis::levelplot(r.drclassdcd)

r.drclassdcd_sp <- as(r.drclassdcd,"SpatialPixelsDataFrame")

最后一行给出以下错误：

h（simpleError（msg，call））中的错误： 选择函数'as.factor'的方法时评估参数'x'时出错：未定义的列已选择

是否有一种方法可以从栅格图层获取像素数据帧，并将因子转换回属性字段中的字符？