How to convert UK grid reference to latitude and longitude in R - r

Question

I have a vector of UK British National Grid references:
x <- c("SK393744", "SK442746", "SK376747", "SK108191", "SP169914", "SP206935", "SK173105", "SJ993230", "SK448299", "SK112396")
I need to convert this vector in WGS84 coordinates (latitude and longitude).
How can I do it using R?

Answer 1

Give these a go. If they work, I'll make a package with a few more of the other functions in that javascript library (which also has sister PHP & Java libraries, so it's fitting R shld have one).
# takes numeric east/north generated from the os.grid.parse() function
# i shld have made it take the vector the os.grid.parse() returns but
# we'll save that for a proper package version
os.grid.to.lat.lon <- function(E, N) {
a <- 6377563.396
b <- 6356256.909
F0 <- 0.9996012717
lat0 <- 49*pi/180
lon0 <- -2*pi/180
N0 <- -100000
E0 <- 400000
e2 <- 1 - (b^2)/(a^2)
n <- (a-b)/(a+b)
n2 <- n^2
n3 <- n^3
lat <- lat0
M <- 0
repeat {
lat <- (N-N0-M)/(a*F0) + lat
Ma <- (1 + n + (5/4)*n2 + (5/4)*n3) * (lat-lat0)
Mb <- (3*n + 3*n*n + (21/8)*n3) * sin(lat-lat0) * cos(lat+lat0)
Mc <- ((15/8)*n2 + (15/8)*n3) * sin(2*(lat-lat0)) * cos(2*(lat+lat0))
Md <- (35/24)*n3 * sin(3*(lat-lat0)) * cos(3*(lat+lat0))
M <- b * F0 * (Ma - Mb + Mc - Md)
if (N-N0-M < 0.00001) { break }
}
cosLat <- cos(lat)
sinLat <- sin(lat)
nu <- a*F0/sqrt(1-e2*sinLat*sinLat)
rho <- a*F0*(1-e2)/((1-e2*sinLat*sinLat)^1.5)
eta2 <- nu/rho-1
tanLat <- tan(lat)
tan2lat <- tanLat*tanLat
tan4lat <- tan2lat*tan2lat
tan6lat <- tan4lat*tan2lat
secLat <- 1/cosLat
nu3 <- nu*nu*nu
nu5 <- nu3*nu*nu
nu7 <- nu5*nu*nu
VII <- tanLat/(2*rho*nu)
VIII <- tanLat/(24*rho*nu3)*(5+3*tan2lat+eta2-9*tan2lat*eta2)
IX <- tanLat/(720*rho*nu5)*(61+90*tan2lat+45*tan4lat)
X <- secLat/nu
XI <- secLat/(6*nu3)*(nu/rho+2*tan2lat)
XII <- secLat/(120*nu5)*(5+28*tan2lat+24*tan4lat)
XIIA <- secLat/(5040*nu7)*(61+662*tan2lat+1320*tan4lat+720*tan6lat)
dE <- (E-E0)
dE2 <- dE*dE
dE3 <- dE2*dE
dE4 <- dE2*dE2
dE5 <- dE3*dE2
dE6 <- dE4*dE2
dE7 <- dE5*dE2
lon <- lon0 + X*dE - XI*dE3 + XII*dE5 - XIIA*dE7
lat <- lat - VII*dE2 + VIII*dE4 - IX*dE6
lat <- lat * 180/pi
lon <- lon * 180/pi
return(c(lat, lon))
}
# takes a string OS reference and returns an E/N vector
os.grid.parse <- function(grid.ref) {
grid.ref <- toupper(grid.ref)
# get numeric values of letter references, mapping A->0, B->1, C->2, etc:
l1 <- as.numeric(charToRaw(substr(grid.ref,1,1))) - 65
l2 <- as.numeric(charToRaw(substr(grid.ref,2,2))) - 65
# shuffle down letters after 'I' since 'I' is not used in grid:
if (l1 > 7) l1 <- l1 - 1
if (l2 > 7) l2 <- l2 - 1
# convert grid letters into 100km-square indexes from false origin - grid square SV
e <- ((l1-2) %% 5) * 5 + (l2 %% 5)
n <- (19 - floor(l1/5) *5 ) - floor(l2/5)
if (e<0 || e>6 || n<0 || n>12) { return(c(NA,NA)) }
# skip grid letters to get numeric part of ref, stripping any spaces:
ref.num <- gsub(" ", "", substr(grid.ref, 3, nchar(grid.ref)))
ref.mid <- floor(nchar(ref.num) / 2)
ref.len <- nchar(ref.num)
if (ref.len >= 10) { return(c(NA,NA)) }
e <- paste(e, substr(ref.num, 0, ref.mid), sep="", collapse="")
n <- paste(n, substr(ref.num, ref.mid+1, ref.len), sep="", collapse="")
nrep <- 5 - match(ref.len, c(0,2,4,6,8))
e <- as.numeric(paste(e, "5", rep("0", nrep), sep="", collapse=""))
n <- as.numeric(paste(n, "5", rep("0", nrep), sep="", collapse=""))
return(c(e,n))
}