Principles of Celestial Navigation

The Intercept Method » Assumed Position (AP) and Dead Reckoning Position (DR) » Setting Up the Plotting Sheet

Chief Sheedy: I’m now going to show you how to set up the Universal Plotting Sheet. The first thing I want to do is set up the latitude and longitude. The latitude is easy: in this case, it’s 32 degrees North. Label the corresponding latitude lines. The mid-longitude is 055 degrees West. Longitude’s a little bit tricky, but it’s easy once you get the concept. What I do is look at the compass rose. Since our mid-latitude is 32 I’ll mark out 32 on the compass rose, on both sides. I’ll set up with the mid-longitude, move the line over, and draw a line through those two lines, which goes through 32 on the compass rose. That’s our longitude. This is now 054 degrees West. I’ll do the same on the other side. I grab my dividers and draw the longitude line for 056 degrees West, and label.

Now on the longitude scale, the latitude scale is read by the latitude minutes that are given to you on the chart. Longitude is a little trickier, but it’s pretty easy. Now that you have the distance between 055 and 054 degrees West, bring it down to the mid-latitude scale. On the longitude scale, mark it. Parallel your longitude line up where it touches. This is your mid-latitude, 32 degrees. This is now 50 minutes longitude, 40 minutes, 30, 20, 10, and these are the seconds: 2, 4, 6, 8, 10.

In this case, our DR position is 31 degrees 25 minutes. I’m now going to plot that. I’m going to transfer this line over to the standard longitude line. The longitude was 54 degrees 12 minutes West. This is 12 minutes of longitude. 54 degrees, 12 minutes. This is our AP, Assumed Position.

Screen shot of Universal Plotting Sheet with Assumed Position marked. In this case, the Dead Reckoning coordinates are used as the Assumed Position.