He must have pieced it together from clues scattered through the whole history of the Grail quest. A map with no names.
7 seconds sound clip from the Indiana Jones and the Last Crusade movie soundboard.
You can hear this line at 00:41:57 in the Blu-ray version of the movie.
Quote context
[...]
- It's the name of a city. Alexandretta?
- Your father did know. Look. He made a map.
- Now, he knew there was a city with an oasis due east, here.
- He knew everything except where to begin, the name of the city.
[...]