Quick and Easy
Here is a quick and easy explanation on how to find your estimated position or your “EP” based on just your course heading and one bearing. This method is used when you are making way on a steady course and you use an un-moving landmark to sight a bearing angle from your ship’s compass (or handheld). Do not use a buoy, ship or other floating object!
Practice Question
This is typical of what you may find on your coastal navigation portion of your exam. Or if you are taking the ASA105 course. Thank you Tom Tursi at the Maryland School of Sailing for this practice question.
The sample images below, are based on the video tutorial at the end of this blog. You will see we are using the Chart 1210Tr to plot this question’s answer. We are provided the Deviation table for this ship, and we have already calculated that the Magnetic Variation for the year 2002 in this area of the world is 16°W.
Q24 – At 0900 on September 26, 2002 you depart the “1”Fl G 4 sec BELL buoy located east of Noman’s Land Island, and set a course of 080° psc at 6.4 knots. At 1000 you take a visual bearing of 319° psc on the Spire at Chilmark on Martha’s Vineyard. What is your 1000 estimated Latitude and Longitude position?
1 - Plot Your Dead Reckoning Positions
Remember we plot all lines on the chart in TRUE! So you need to use your TVMDC conversion table to find the True angles to plot. When labeling the plotted lines, you can use either True values or psc values, you just must stay consistent when writing the exam (and better also in real life practice). I like to use psc values on the chart as this is what I reference in real life when navigating from the the cockpit and referencing the results from my chart on my nav table.
In the sample question, we figured out previously that the Variation is 16°W and the provided Deviation table tells us that at our ship’s heading of 080°psc our Deviation is 4°W. We will use the TVMDC table to convert our ship’s course to TRUE which ends up being 060°T.
We are leaving at 0900 and taking a bearing an hour later at 1000, but we will also plot the 0930 DR (half circle over a dot).
2 - Sight and Plot Your Bearing
At 1000, we take our bearing on the landmark. Again, we choose a non-movable object. In this question, we use a spire. Using the ship’s compass, we sight the spire over our port side at 319° or 299°T.
Plot this line of position (LOP) from the spire across the ship’s course line on the chart.
3 - Never Where You Think You Should Be
Well, we have fallen quite short of where we thought we would end up at 1000! It could be because there is a current, or it could be the tide. Or, maybe, you did not maintain a constant speed of 6.4 knots for the entire hour.
In any case, to find our EP (estimated position), we draw a line from our plotted 1000 DR position to our spire bearing LOP (line of position) making sure to form a right angle or 90° with this LOP.
4 - Square And A Dot
Where those two lines meet is your estimated position (EP). Mark it with a square and a dot, then find the latitude and longitude of this dot!
This is the 2nd least accurate of position fixes after dead reckoning positions. For a hierarchy of accurate position fixes, sign-up to the blog and this series, and you’ll find out!
Want the cheat-sheet of this method? Sign up below the video and I will send it to you!
In the meantime, follow along with the video demonstration of the entire process using the test chart 1210Tr.
How far off course is your boat if you were trying to maintain a 060°T track? What is causing the boat to go off course? How do you compensate for this drift and “slowing down”? What Course should you have steered to maintain your track at 060°T?
For the answers to these questions, sign up for my blog and subscribe to my youtube channel! I will be working out these answers in the coming coastal navigation tutorials and practice questions!