Historically, borders in inhospitable regions where the government cannot project power used to be vague.
Early Medieval European kingdoms usually had border forests, sometimes quite deep (50-100 km), where one end of the forest was definitely Bohemian and the other end definitely Saxon or Austrian, but the exact demarcation within it bothered no one.
There were only two kinds of people crossing them: invading armies and long-distance traders on very few walkable paths. The latter would be taxed prior to entering the city markets, not on the border. The former would be interested in sieging strongholds deeper in the enemy territory, not in taking a slice of wooded territory on the border.