R/sp_transform.R

In MohoWu/ricardor: Collection of plotting functions used in Ricardo projects

#' Convenience function to transform a spatial object's projection system to
#' WGS84 latitude and longitude.
#'
#' `sp_transform` is a simple wrapper for [sp::spTransform()] which has
#' been written so spatial objects can be transformed quickly without the need
#' to remember the WGS84 proj4 string. `sp_transform` will force projections
#' when the spatial object contains no projection information.
#'
#' `sp_transform` will also work for raster layers.
#'
#' @param sp Spatial object which is to be transformed.
#'
#' @param to A proj4 string for the projection-transformation. Default is the
#' WGS84 string.
#'
#' @param warn Should the functions raise a warning when the projection is forced?
#'
#' @author Stuart K. Grange
#'
#' @examples
#' \dontrun{
#'
#' # Load a shape file of canal locks for the UK
#' sp_locks <- sp_read("uk-canals", "locks")
#'
#' # Print projection string
#' sp_projection(sp_locks)
#'
#' # Convert the shape file's projection (UK's Ordnance Survey National Grid)
#' to WGS84 latitude and longitude
#' sp_locks <- sp_transform(sp_locks)
#'
#' # Print projection string
#' sp_projection(sp_locks)
#'
#' }
#'
#' @import sp
#'
#' @export
sp_transform <- function(sp, to = NA, warn = TRUE) {

  # Default
  if (is.na(to)) to <- projection_wgs84()

  if (!grepl("raster", class(sp), ignore.case = TRUE)) {

    # If no projection
    if (is.na(sp_projection(sp))) {

      # Warn
      if (warn) {

        warning("Spatial object had no projection. The projection has been forced.",
                call. = FALSE)

      }

      # Now force
      sp::proj4string(sp) <- to

    } else {

      # Otherwise convert projection system
      sp <- sp::spTransform(sp, CRS(to))

    }

  } else {

    raster::crs(sp) <- to

  }

  return(sp)

}