Positon fixing Flashcards
What is a ships log
any device used for measuring speed through the water and distance travelled
What is a logbook
A document documenting the passage of a vessel
what information would you find in a log
Time, course, the distance the ship has travelled, depth of water, wind speed and direction, barometer and remarks
When on a transit, what 2 options does the skipper have for obtaining a position fix
taking a bearing off the side of the vessel and plotting where that lines up with the transit, or taking the depth, accounting for the height of the tide, and then plotting where the transit crosses a contour.
when taking a 3 point fix how far apart should the objects used be
at least 90 degrees
which bearings should be taken first when making a 3 point fix
the ones off the bow or stern as they will change the least
when noting a fix in the logbook what else should be added
time, boat speed, distance travelled and the course being steered
when a 3 point bearing produces a triangle what is this called
a cocked hat
when working with a cocked hat, what should you take as the actual position fix from a 3 bearing fix
the centre, unless any dangers are nearby, in which case you should take the part of the triangle closest to the danger as this the worst-case scenario
what are the 4 steps to creating a 3 bearing fix
First choose 3 things you can see both outsides and on the chart. these should have a spread of at least 90 degrees. Take bearings from each of them and note them down. Then convert the magnetic bearings to true bearings and plot them on the chart. Lastly, take note of the produced fix and error size and add it to the logbook.
When using a hand compass to measure the bearing of an object which way is the bearing pointing
towards the boat
when using a GPS to get a bearing, which way is the bearing facing
away from the boat
How can you use the compass rose and GPS to obtain a fix?
by setting the centre of a compass rose as a waypoint in the GPS, you can then get range and bearing to that waypoint, allowing you to draw a line from the centre of the compass rose out the range distance and obtain a fix. note that you have to flip the bearing 180 from a gps
what is dead reckoning
The position estimated by taking course steered and the log, not taking account of the tide
what does dead reckoning not take into account
tidal currents or leeway
how do you convert a dead reckoning to an estimated position
you add the tidal speed and set for that period, giving you an estimated position
how can you use an estimated position to get speed over ground
take the distance of the ground track, and divide it by the number of hours the estimated position covers
how many arrows mark the true course when marking the estimated position
1
how many arrows mark the ground track when marking the estimated position
2
how many arrows mark the tidal vector when marking the estimated position
3
when does a circle of error apply to an estimated position
when in fog or no other way to work out the accuracy of the estimated position
when creating a circle of error, what size should it be
10 per cent of the total journey up to that point
when first installing a depth gauge, how should it be checked
by comparing it to a reading from a leadline
what are good ways to help check the estimated position
depth contours, bearing fixes and gps
if sailing for 2 hours how do you account for the tide when calculating the estimated position
draw the two tidal vectors tip to tip and use that as one large tidal vector
if sailing for less than an hour how do you take account of the tide when working out the estimated position
divide the tidal set by the fraction of an hour sailed
what is a projected estimated position?
a projected estimated position is an estimated position drawn up using anticipated speed and course steered, to work out the course over ground and final position
