PLOTTING POSITION
Now we are ready to proceed to the plotting sheet. On my boat, I use a navigation log book with squared pages, and make life easy for myself by making my plotting sheets on the book, this way everything is nicely documented and can be checked by the organisers of the races in which I participate.
My approximation for a plotting sheet is that every square is five minutes of latitude, so 12 squares up or down equal 1° of latitude. Between latitudes 0º–20°, the longitudes are on the same scale. Between latitudes 20º–35º I use one square for six minutes, which means 10 squares for one degree of longitude. For latitudes higher than 35° I use the scale of 7.5 minutes per square, which means eight squares per one degree of longitude. With my accuracy of sextant use, I have found this sufficient, though perhaps not precise enough if you want more accuracy.
SETTING UP THE PLOTTING SHEET
You first plot the morning sight. When you entered the Sight Reduction Tables, you chose an estimated latitude of whole degrees, the one closest to your DR position. Now you mark that latitude at mid height of your plotting sheet. On the vertical scale mark 10 minutes of latitude per two squares, both north and south. Then mark the closest whole degree of your DR longitude in the middle of the horizontal scale and, depending on your latitude, mark either 10, 12 or 15 minutes per two squares of the sheet to the left and right (west and east).
ESTIMATED POSITION
Then you start by marking down your Estimated Position (EP) on the sheet. As you entered the Sight Reduction Tables using an whole degree figure for latitude, that is your estimated latitude. Your estimated longitude is the interpolated value of longitude, which you used in order to get a whole number of degrees when you calculated the LHA. After getting the EP1 (for the forenoon sight) on the plotting sheet, you now take the chart protractor and draw a line across the EP1 at the angle of the azimuth z/n with an arrowhead at the end towards the sun and the line continuing also across the EP1 away from the sun. Mark the number of minutes of the Z/n next to the arrowhead, so it will be easy to check everything afterwards.
Next you will mark the length of the Intercept on the line of azimuth, which is the difference of H/o and H/c measured in nautical miles. If H/o is greater than H/c, the Intercept will be drawn towards the sun (where the arrow is pointing at), if H/o is less than the H/c, the Intercept will be away from the sun. When you make the subtraction in your calculations, write either towards or away right away next to the answer, so you remember to mark it correctly.
NOON POSITION
As we are finding our position at local noon time, the next step is to take the movement and heading of the vessel between the time of the sight and the local noon into consideration. You draw a vector (arrow) from the end of the Intercept in the direction which equals the true heading of the vessel and the length equals the distance travelled between the forenoon sight and local noon.
Then remains only the marking of the forenoon position line, which is a line perpendicular to the azimuth and going through the end of the DR vector you just marked from the end of the Intercept.
AFTERNOON SIGHT
The plotting process for the afternoon sight is the same, except that the DR vector is drawn at an opposite (+/- 180°) of the heading of the ship to bring back the afternoon position line to our noon position. If you take the forenoon sight 2.5 to 1.5 hrs before the noon sight, and the afternoon sight a similar time after the noon, the lines will cross each other in a nice, almost 90° angle, giving you a fairly exact position.