Basically you set up lines around your ship that float along with the ship and provide a visual indication of whether or not you are on course or if you have come to a waypoint. These differ from Nav Lines because they are fixed relative to the ship, wheras Nav Lines are fixed relative to the ground.

 

So, if you were attempting to stay within a channel just offshore, parallel indexing would come in awful handy. What you would do would be to draw an index line parallel to your ship's course and tangent (just touching) to a visible point of land.

 

For example, if you were coming into Los Angeles from the north and wanted to stay within the channel, you could set up a parallel index line a certain number of miles from your ship that just touches a point on shore. If you were drifting out of the channel, this line would either come away from the land or would cross over and touch more land. Simply by looking at the radar screen, you could know whether you were left or right of your track line.

 

If however, you didn't want to run down the center of the channel, but simply wanted to know when you were leaving the channel, you could set up 2 index lines, as far apart as the channel is wide.  Set the first one to represent one side of the channel, set the other to represent the other side of the channel. When you draw them, fix them so that some point of land or a rock or some fixed object is located in the center when you are in the center of the channel. As you move along, wherever that object is relative to the lines you drew is where you are relative to the edges of the channel.

 

If you're chugging along and know that you're coming up on a turn or a danger area, you can also set up an index line to let you know when to change course. Unlike the previous Options, the lines you set up for this wont be parallel to your course, that would defeat the point. Instead, these will probably be perpendicular to your course, or will be aligned with the course that you will be coming to next. If you set these lines ahead of your ship along the course line, when they hit the object you previously chose, you will know that it is time to turn.

 

If, for example, you're within a channel that doglegs up ahead and you want to know when to start putting some rudder on, you could set up an index line that will cross a fixed object at the time you need to turn. It's easiest to set this up on a chart and then transfer it over to the ARPA unit.

-First, find your wheelover point or waypoint

-Next, from that point, find some notable land feature somewhere ahead of your track line. (It can be behind as well, just remember this when you are maneuvering.)

-Find the distance and bearing to that object from the wheelover point

-On the ARPA, draw in a line at the distance you just found, but perpendicular to the bearing (add 90 to the bearing)

When this line crosses the object, you are at your wheelover point and should begin to change course