Created
December 19, 2017 21:07
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Map a lat/lon point to a county or other geography
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#' Get county/state name from lat/long pair. | |
#' | |
#' Maps a lat/long pair to a geography. | |
#' | |
#' @importFrom maps map | |
#' @importFrom maptools map2SpatialPolygons | |
#' @importFrom sp over CRS SpatialPoints | |
#' @export | |
#' | |
#' @param lat list of numbers or number of the latitude in degrees. | |
#' @param lon list of numbers or number of the longitude in degrees. | |
#' @param projargs a CRS string. Default is "+proj=longlat +datum=WGS84". | |
#' @param shape either "county" or "state". | |
#' @return a shape name. | |
latlon2shape <- function(lat, lon, | |
projargs="+proj=longlat +datum=WGS84 +ellps=WGS84 +no_defs", | |
shape=c("county", "state")) { | |
shape = match.arg(shape) | |
# Prepare SpatialPolygons object with one SpatialPolygon | |
# per county | |
shapes <- map(shape, fill=TRUE, col="transparent", plot=FALSE) | |
IDs <- sapply(strsplit(shapes$names, ":"), function(x) x[1]) | |
shapes_sp <- map2SpatialPolygons(shapes, IDs=IDs, | |
proj4string=CRS(projargs)) | |
# Convert pointsDF to a SpatialPoints object | |
df_points = data.frame(x=lon, y=lat) | |
pointsSP <- SpatialPoints(df_points, | |
proj4string=CRS(projargs)) | |
# Use 'over' to get _indices_ of the Polygons object containing each point | |
indices <- over(pointsSP, shapes_sp) | |
# Return the county names of the Polygons object containing each point | |
shapeNames <- sapply(shapes_sp@polygons, function(x) x@ID) | |
res = shapeNames[indices] | |
res = toupper(strsplit(res, ",")[[1]][c(2,1)]) | |
res = list('county' = res[1], 'state' = res[2]) | |
} |
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