I dont do carthography very often but if im right i had the same issue on this map...
It´s like on any other map, go as close as possible to the unmaped part and try to bug yourself against the wall as often as necessary 
sucks to do that, but obviously no anet employee ever tried it otherways they wouldt have had made it that crappy
Toolbox offers an alternative minimap with a clear view of the ground you can put your feet on.
Sometimes there are very small and invisible spots you can go on to unravel map slivers.
It made things more practical for me when I did cartography titles. Tyria is the easier one though with some room for manoeuvre.
Hope it will help.
Toolbox propose une mini-carte alternative avec une vue dégagée du sol sur laquelle vous pouvez poser vos pieds.
Il existe parfois des endroits très petits et invisibles que vous pouvez résoudre pour éliminer les fragments de carte.
Cela rendait les choses plus pratiques pour moi lorsque je réalisais des titres de cartographie. La Tyrie est la plus facile avec une marge de manœuvre.
J'espère que ça va aider.
This is good advice. Texmod + Minimap of TB ++ You save time, especially for corner or other extreme little things .
If you use consos for more speed, move you on the boundary, facing you and not sideways, to slow your movements. Slow down your movements is better for discovering the map.
Whenever I do carto and those hard to reach mapping spots I aswell go really slow back and forth until I get it. You wont get it by just running casually on the edge of map. Sometimes I had to spend like 10min or 15min to hug those corners. There is that frost gate mission and there is a few spots there I spent like 2hours to try and map it and I still couldnt get it...
