I have a large data set where each station has the same latitude and longitude. In the data set some rows are missing the lat and lon and instead say 'unknown'. I need to fill in the unknowns with the lat long from other stations where that data is not missing.

In this example I would want row 5 to have 3 and 8 inserted for lat and lon:

```
> station <- c("a","b","c","c","c")
> lat <- c("1","2","3","3","unknown")
> lon <- c("6","7","8","8","unknown")
> data.frame(station,lat,lon)
  station     lat     lon
1       a       1       6
2       b       2       7
3       c       3       8
4       c       3       8
5       c unknown unknown

```

There are a million rows in my data set, if it takes a couple minutes to complete that is fine since this only runs one time before the analysis starts. I'd prefer to not install another package unless really necessary.