I must admit to being slow to recognise the significance of this map, I saw it a few days ago from a comment by Tonzi in the Cipher Explanations page and didn't realise what it was.If you haven't finished the book yet you may not want to read on.
Then last night @MyTheseus on Twitter pointed out a new blog post on WhoIsStraka.WordPress.com which had the same conclusion, although arrived at via a different thought process.
Firstly the map is quite important as it explains the relationship of the coordinate system to the codes used, they are linear as you can see and these aren't locations from the book, these are all coordinates that can be found when LONOE is in the wheels window.
When you look at the map you can see that Maraú in Brazil (blue pin) is close to this line, on the postcard dated the 20th of April (insert on page 201) Eric writes that he is near this place.

So the conclusion is that the missing letters L O N O E are actually a pointer to the location of FXC should Straka have ever picked up the message hidden in the footnote cipher.
Tonzi very kindly created this as a Google map for us all to use and see this linear relationship.

