#' subset_county_polygons
#'
#' Returns county polygons from: Siczewicz, Peter. U.S. Historical Counties
#' (Generalized .001 deg) on the basis of year of interest
#' Dataset is subset on basis of area of interest
#'
#' @param areas chr, vector of codes indicating HUCs, counties, states, regions, aquifers, etc.
#' @param area.column chr, defines which column to use to specify area
#' @param year int, the year of interest to be mapped (defines historical basis for counties)
#'
#' @export
#'
#' @return hc.sub dataframe, a subset of county polygon data
#'
#' @examples
#' areas <- "Delaware" # 3 counties present day
#' area.column <- "STATE_TERR"
#' year <- 2010
#' hc.sub <- subset_county_polygons(year, area.column, areas)
#' hc.sub$NAME
#'
#' areas <- "Maine" # 16 counties present day
#' area.column <- "STATE_TERR"
#' year <- 2010
#' hc.sub <- subset_county_polygons(year, area.column, areas)
#' hc.sub$NAME
#' year <- 1850 # Maine had 13 counties in 1850; Delaware 3
#' hc.sub <- subset_county_polygons(year, area.column, areas)
#' hc.sub$NAME
subset_county_polygons <- function(year, area.column, areas){
test.date <- as.Date(paste0(year,"-12-31"))# year of interest converted to date
now.date <-format(Sys.Date(), "%Y-%m-%d")
# select area of interest
hc.sub <- histCounties[which(histCounties@data[,area.column] == areas),]
# substitute todays date for the last date of the histCounties dataset (2000-12-31)
hc.sub$END_DATE[hc.sub$END_DATE==max(hc.sub$END_DATE)] <- now.date
# assuming the counties defined in 2000-12-31 are the same now, use todays
# date to make sure the selection for contemporaneous counties works
hc.sub <- hc.sub[hc.sub$END_DATE >= test.date & hc.sub$START_DATE <= test.date,]# for year of interest, get the counties
return(hc.sub)
}# subset_county_polygons
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