Basically, they're lines that you can draw on the ARPA screen that will stay fixed on the ground as you move along. In other words, they will move past you, but will stay the same relative to the ground.

 

Why are these useful? Well I'm glad you asked. If you have a GPS or some other position fixing device input to your ARPA, you can use Nav Lines to actually draw your course line, channel edges, boundary areas, and more on the ARPA screen.

 

For example, if you are on a liner vessel and come into a certain port two or three times a week, you could use Nav Lines to draw out your track line throughout the port and save these lines either to a hard drive or floppy disk. Now, every time you come through that port, you have your track lines all laid out for you on the ARPA screen.

 

Nav Lines were essentially the predecessors to Radar Charts and ECDIS, so, despite the fact that there are better systems out there today to keep you on track, these things were awfully nifty for a long time.

 

 These are actually really easy on most ARPAs. All you have do do is usually click on the "Nav Lines" button, make sure that you are in the Nav and not the Index mode, then use the cursor to scroll across the screen and drop points at the Lats and Longs that you choose. Basically that's it, though on some models the process might be a little more complicated.

 Some models even allow you to construct jointed nav lines, so that if you want to draw a square boundary area you only have to click 5 times to draw the 4 sides rather having to draw each side as an individual line. Basically, this allows you to click once to drop the first point, click again to drop the second, then click again to drop a third point and the second and third will automatically be connected by a line.