Nautical mile is nmi or NM since nm is nanometer, which is rather small.
Just implement things in the straightforward way. You need that implementation when you're zoomed in anyways. For when you're zoomed out far enough, you might (or might not) need to merge lots of low-res versions of textures and use those since you probably can't have 100,000 texture binds per frame. You can just pass in a uniform to say where each tile maps to in the low-res texture mapping, then adjust your uv coords in the shader. You really won't know at what point you need to abandon the normal mipmap magic for a custom solution until you actually have it up and running though.
If you have no moving parts and don't allow rotating the view, you could also just render different zoom levels to texture and basically blit a single texture to screen. This works in quite a few game situations where it wouldn't work on Google Earth or such.
But just do the obvious solution like any other tile-based map until it stops working. You might be amazed just how much punishment that method can take.