Today, I read this overview of the grammar of American Sign Language. One of the most interesting aspects, to me, was the section about referring to subjects that you can’t point to, because they aren’t currently around. According to this website…

If the person is not there, if you have identified him by spelling his name or some other method of identification, (like a “name sign”), then you can “index” him to a point in space. Once you have set up a referent, you can refer back to that same point each time you want to talk about that person.

So, when communicating routes, travelers could build imaginary maps in the air thought a series of hand gestures. Then, the person receiving the route information would project that imaginary map over the sky to determine where they should go.

For example, you could gesture to one spot in the air and sigh “HERE WE.” Then you point at a specific star in the sky above you and gesture to another spot in your imaginary air map, saying “HERE THAT STAR.” Then you follow up with “FROM MARK ON THAT STAR THIRD PLANET, NEXT STAR HERE.” And during that last “here” you could gesture to a blank spot in the heavens where, if you were standing on the right spot, next to a mark on the third planet of the second star of your map, the next star in the route would be. And then you add that star to your imaginary air map and continue…