Of course you can. You can proceed with your idea even if the other guy is still making his map.
Example: when I started on my Nordic Countries map, there was an older project called Scandinavia, which was at the time in the recycling bin. When Nordic Countries was in graphics, the Scandinavia map was continued, and for a time we both worked on our maps at the same time. Well, I kept working on my map and it's now finished a long time ago. Scandinavia is at the bin again, I think... (and it's not even because I finished first, he could still have finished his map too, but I guess rl got in the way.)
The thing is, you can't copyright an idea. Even if some other guy has posted his map idea first, he can't claim any right to that idea, and anyone is free to make a map of the same idea.
Go ahead and publish your idea.