geotag02.r

# function to get a parameter value from EXIF code,
# x = a vector of character strings representing the EXIF codes of a file
# parameter = character string representing the parameter
#             for which you want to extract the value
# the literal character string is returned for the requested parameter
get_EXIF_value <- function(x, parameter)
{
  line <- x[grepl(paste("EXIF_", parameter, "=", sep = ""), x)]
  if (!is.null(line) && length(line) > 0)
  {
    pos  <- gregexpr("=", line, fixed = T)[[1]]
    pos  <- pos[length(pos)]
    line <- substr(line, pos + 1, nchar(line))
    line <- gsub("[(]|[)]", "", unlist(strsplit(line, "([)]\\s+[(])|[)][()]")))
  }
  return(line)
}

# function to get the longitude and latitude from the
# character string obtained with the function above
# x = a character string with the coordinates
get_EXIF_coordinate <- function(x)
{
  lat    <- get_EXIF_value(x, "GPSLatitude")
  lat    <- as.numeric(lat)
  lat    <- lat[1] + lat[2]/60 + lat[3]/(60*60)
  latref <- get_EXIF_value(x, "GPSLatitudeRef")
  lat    <- ifelse(toupper(latref) == "S", -lat, lat)
  if (length(lat) == 0) lat <- NA
  lon    <- get_EXIF_value(x, "GPSLongitude")
  lon    <- as.numeric(lon)
  lon    <- lon[1] + lon[2]/60 + lon[3]/(60*60)
  lonref <- get_EXIF_value(x, "GPSLongitudeRef")
  lon    <- ifelse(toupper(lonref) == "W", -lon, lon)
  if (length(lon) == 0) lon <- NA
  return (c(lon, lat))
}

# for each file obtain the EXIF codes and extract the coordinates
GPStags <- apply(array(file_info$file.name), 1, function(x) {
  EXIF_code <- NULL
  try (EXIF_code <- attr(GDALinfo(x), "mdata"), silent = T)
  if (is.null(EXIF_code)) return(c(NA, NA)) else return(get_EXIF_coordinate(EXIF_code))
})