Single and Three Point Fixes
This is particularly useful at the beginning and end of a passage i.e. passing a buoy at a river entrance. The requirements are:
- You can see, positively identify and pass very close to the object
- The position of the object is shown on the chart
This is the most common method of fixing your position. Using three objects in turn measure the bearing and then plot on the chart to see the intersection. You can also use transits and contour lines. Make sure you:
- Use landmarks that are easily identified
- Take the bearings as accurately as possible and quickly (as you will be moving)
- Choose objects that are as spaced apart as possible and are not to far away
5. The Course over the Ground is from point A to your Estimated Position (remember: 2 arrows = 2 feet on the ground!)
1. Take a bearing and draw it on the chart, together with the time and log reading;
2. You know you are somewhere on this line;
2. Sail for a set length of time (i.e. 15, 30 or 60 minutes) such that you can have a changed bearing of i.e. 30°;
3. Take a second bearing, and plot it on the chart. Note the time and log reading;
4. Choose a convenient point on the first position line and plot your Course steered Through the Water and the distance travelled;
5. At the end of this plot on your Tide (like when you do your Estimated Position - see above);
6. Plot a line parallel to the first position line that intersects your plot in step 5;
7. Where the second bearing (in step 4) and the transferred line meet is your position
Using a rising and dipping table
In the back of the Reeds Almanac, and also in the RYA Training Almanac you can find a rising and dipping table.
If the lighthouse you are looking at is far enough away that you can get it to just "dip" below the horizon, an you know the height of your eye above sea level you can use table and calculate the distance off the light house you are (or if you don't have the table use distance off = 2.08 × (√ (height of eye h metres + √ height of light H metres )
This range together with the bearing of the light house taken with a hand bearing compass can be used to calculate your position.
Using a sextant
A sextant is not just used for calculating the position of a star or the moon in astro-navigation.
With a sextant you can calculate the angle to the top of a chartered object - and from this the distance away from it you are.
- use the sextant and measure the "vertical sextant angle"
- correct for index error
- read off the chart the height of the chartered object (i.e. a lighthouse), remembering it is above MHWSThe distance between you and the lighthouse = 1.856 x (height of chartered object taking into account tide / the angle in minutes)
i.e. if the measured angle is 1° 19', index error is +6': angle = 73'; the height to MHWS is 3m; the height of the lighthouse is 80m then 1.854 x (80-3 / 73) = 1.96 nm
This distance together with the compass bearing can be used to plot your position.