NavPak Plotting Sheet Screen

The Plotting Sheet Screen is a Mercator plotting sheet for your celestial sights. The white cross shows the average of the intersections of the LOPs, and the position of the cross is shown in the upper left corner of the screen. In most cases, this will be your Fix, but you should not rely on a mathematical-only Fix, because they are susceptible to bad sights, so it is important to use the traditional graphic solution of a Plotting Sheet. This is a case where human judgment beats the computer. Touche' !

The traditional method of celestial navigation has wonderful, built-in error checking, trouble-shooting, and validation, and it all shows up on the plotting sheet. When you open the Plotting Sheet in NavPak Android/Kindle, it builds a Mercator grid (10 x 10 degrees) with your Assumed Position at the center and the sights plotted. The sights which are plotted are the ones which show up in the Sight Manager panel.

If there are 2 or more intersecting sights, then the white cross will be shown at the average of the intersections, and the Fix will be shown at the top of the screen. You will see the bad sights, because they will be far from the cluster, or the Assumed Position may be way off, or maybe all of your sights are nearly parallel, or your triangle or polygon (cocked hat) may be too big... All of these things can be seen with a glance at the plotting sheet, where various methods of math-only Fixes may or may not show these errors.

To fine tune your position, you can delete and add sights, and a new average is calculated each time you open the Plotting Sheet. After deleting any bad sights, the white cross should be close to the center of the cocked hat. If necessary, you can move the cross to your best estimated position using the Up, Down, Right, and Left keys on the Plotting Sheet. Tap [SAVE] to save the cross position as your new Assumed Position.

To be continued...

Terminology Refresher.

Cocked hat: If you plot 3 or 4 LOPs, you will probably end up with a triangle or a 4 sided polygon like a cocked hat. You may also end up with a triangle with a LOP through it. This is also a cocked hat. With more sights, the polygons have more sides, or you may have a few triangles on top of each other with a couple of LOPs going through the middle. All of these cocked hats, ranging from a triangle to a complicated polygon can be good fixes. After a little practice of shooting and plotting sights, you will recognize the good cocked hats from the bad ones.

One of the most significant traits of the cocked hat is the relative size. This tells you the overall accuracy of the fix and the consistency of the accuracy of the sights, at a glance. If you are on a stable ship, high above the water, and using a metal sextant, then your average cocked hat will be smaller than my average, because I'm splashing around in a small boat with a plastic sextant.