#' Convert WGS84GEO coordinates to HK80GEO coordinates
#'
#' Convert WGS84GEO coordinates to HK80GEO coordinates
#'
#' This function uses the simplified relationship between WGS84GEO and HK80GEO.
#' See Page C4 on the explanatory note.
#'
#' @param latitude latitude in decimal degrees
#' @param longitude longitude in decimal degrees
#' @return \item{latitude }{latitude in decimal degrees}
#' \item{longtitude}{longitude in decimal degrees}
#' @note The coordinates should be within the range of Hong Kong. Providing
#' coordinates outside Hong Kong will lead to wrong results.
#' @author Jinlong Zhang
#' @seealso \code{\link{HK80GEO_TO_WGS84GEO}}
#' @references Survey & Mapping Office Lands Department, Hong Kong Government
#' (1995). Explanatory Notes on Geodetic Datums in Hong Kong, available at:
#' \url{http://www.geodetic.gov.hk/smo/gsi/data/pdf/explanatorynotes.pdf}
#' @keywords WGS84GEO HK80GEO
#' @examples
#'
#' options(digits = 15)
#' WGS84GEO_TO_HK80GEO(22.322172084, 114.141187917)
#' #### $latitude
#' #### [1] 22.3236998617778
#' ####
#' #### $longitude
#' #### [1] 114.138743472556
#'
#' #### Answer from the online conversion tool
#' #### http://www.geodetic.gov.hk/smo/tform/tform.aspx
#' #### 22.323701767, 114.138734989
#'
#'
#' @export WGS84GEO_TO_HK80GEO
WGS84GEO_TO_HK80GEO <-
function(latitude, longitude) {
#### The latitude and longitude should be both in decimal format.
lat <- latitude + 5.5 / 3600
long <- longitude - 8.8 / 3600
res <- data.frame(latitude = lat, longitude = long)
return(res)
}
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