#' gravity
#'
#' Compute gravity at latitude phi
#'
#' @param phi latitude (in degrees)
#' @param elevation elevation (in m)
#'
#' @references
#' http://library.wmo.int/pmb_ged/wmo_8_en-2012.pdf
#'
#' @examples
#' ## gravity at equator
#' gravity(0)
#' ## gravity at pole
#' gravity(90)
#'
#' @keywords util
#' @export
gravity <- function(phi, elevation=0){
## gravity by latitude
gphi <- 9.80620*(1 - 0.0026442 * cos(2*phi/180*pi) - 0.0000058 * cos(2*phi/180*pi)**2)
## correct for elevation
gphih <- gphi - 0.000003086 * elevation ## assuming flat surface
## Helmert's formula
## g <- 9.780327 * (1 + 0.0053024 * sin(phi/180*pi)**2 + 0.0000058 * sin(phi/180*pi)**2)
return(gphih)
}
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