Running Fixes
One of the questions in the Yachtmaster Shorebased Chart Work exam that makes my students anxious is the 'Running Fix'. Actually, there is no reason to be anxious... the running fix is very similar to producing an Estimated Position (EP) - the most basic of our navigation procedures. However, instead of using boat speed/heading and tidal rate/set vectors to project forward a point, we do exactly the same to move forward a line... in this case a position line. Let's follow the process in pictures...
In the picture above, we can see that the navigator has taken too bearings from the Lighthouse on Colville point. Nothing else appears to be suitable to make a fix, so he decides to use the lines to produce a running fix.
We see, right, that the navigator now plots the heading and boat speed vector for 30 mins (or uses heading steered and log reading) from an arbitrary point on the first position line (anywhere on the line will work), and has then added 30 mins worth of tidal vector. In essence, they are creating an EP for an arbitrary point on the first line.
Our navigator now 'transfers'the first line to run parallel to the first line through the 'EP'. Transferred position lines are always denoted by a double arrowhead. Where the transferred PL at 1130 crosses the PL taken at 1130 is our fix.
Clearly we should be cautious with running fixes. We are using only one fix point so any errors with fix point identification, location or bearing measurement may be compounded ; we are also relying on an accurate log, compass and predicted tidal vector to produce our position. However, if we only have one fix point in sight it is better than nothing. The principle of the running fix also lies behind several astro-navigation techniques, so an understanding of how it works is always useful.