Grid overlay on world map?

[Elitechaos22]

Hi, not so much an issue, but i was wondering if there is a way to get a grid to overlay the entire world map, (not the height maps i download) so i can export multiple height maps for one area. I'm trying to get 20km x 20km maps so i can keep the detail but also trying to get the maps to line up properly.

Any help would be appreciated.

Thanks in advance.