How to calculate an Estimated Position

In this post I go through a step by step guide on how to calculate your vessels position by working up an EP on your chart.

A Completed Estimated Position

Based on a last known position, the EP is one of the only methods we can use to work out our current position when we haven’t got GPS or any visual references. In order to complete, we go back to the last position plotted on the chart and also the information provided by the log. You will need your chart, almanac and plotting instruments to work up an EP.

Step by Step Guide to working up an EP

1. Locate your last known position (LKP) and plot this on your chart

2. From your log, plot your Dead Reckoning Position (DR). This is done by converting your compass course steered back to degrees true, plotting this from your LKP and then striking off the distance traveled. This becomes your water track and is annotated with 1 arrow. Ensure you allow for the affect leeway has had on your vessel at this stage, before you plot your water track.

A Dead Reckoning position drawn on a chart

3. We now need to consider how the tide has affected your position. We do this by calculating the direction and rate of the tide, using either the Tidal Diamonds or Tidal Stream Atlas. Remembering to use the correct tidal hour and the standard port in which the diamonds or atlas refer too. The Computation of Rates table should also be referred to if the tidal range is outside that of the printed MHWN or MHWS.

4. Once we have our tidal direction and rate we can plot this from the end of the DR. Again plot the direction of the tide first, followed by its speed. This vector is marked with 3 arrows.

The completed EP

5. Your EP is now complete in its most simple form. The end of the tidal vector is marked with a triangle to denote an Estimated Position and the latitude and longitude can be read and recorded in the log book.

6. If required we are able to interrogate our plot further to work out both our speed and course over ground (COG & SOG). This is done by plotting our ground track, a line from our start position to the end of the tidal vector. If plotted, this line is annotated with 2 arrows to denote our Ground Track.

An EP with Ground Track plotted

7. Once plotted you can measure the direction of the Ground Track with a plotter to obtain your Course Over Ground (COG) and the length of this line with your dividers to obtain your Speed Over Ground (SOG).

Check out this tutorial video from Nomad Sailing to see a worked example on a chart.

Top tips to help you plot your EP’s

  • It is most common to work up an hour plot on the chart due to the frequency we usually fill in our log books and the tidal information being given in hours. It is however possible to plot an EP for any period of time, if this is the case ensure you divide both you speed and tidal rate by the time frame you are wishing to plot.
  • If completing a multiple hour plot, or have multiple courses (common when sailing upwind), ensure you plot all your DR’s first followed by all relevant tides from the end of this.
  • As the name suggests this plotting technique creates an estimate of your position. Whilst a perfectly reasonable method to use in the right circumstances we need to be aware of the increasing inaccuracies of these if we build multiple EP’s on top of each other. As such it best practice to verify your position by another means when possible.
  • We are also able to plot a Projected EP. This is useful for seeing where you will be in a given time frame and even more useful for working out if you are going to clear a hazard on our current course. This is constructed in the exact same way as above but ensure you reference the hour of tide ahead of you rather than the one that has already affected you. A projected EP is slightly less accurate than a normal one as we are predicting what we are going to do rather than obtaining what we have done from the ships log.

 

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