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A globe

Explanation:

In order to find the shortest route between two places on Earth, it is useful to have a globe at hand. This is because the Earth is not flat, but rather a sphere. As a result, lines drawn on a flat map are not always as short or as long as they would be on a curved surface.

A great example is the shortest aerial route from Chicago to Seoul. On a flat map it appears that this would be over the middle of the Pacific Ocean, but the shortest distance between the two cities is actually a flight near the southern coast of Alaska. Why so far north? Because the Earth gets a lot slimmer towards the poles, the shortest route appears to curve on a flat map.