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
What does LOP stand for?
Line of Position
What is the Three Point Fix?
Take three different bearings from the vessel to various different objects
How far apart should the objects you take bearings to be in Three Point Fix?
Between 30º and 150º
What is one thing you should avoid when taking bearings for Three Point Fix?
Buoys
What information do you need to work out Dead Reckoning?
Your starting position, the direction you are travelling and the distance you travelled
Why is accuracy for Dead Reckoning limited?
Doesn’t take into account tide
How do you work out Dead Reckoning?
Mark your starting position on a chart, draw a line in the direction you are heading using Portland Course Plotter and then use the ships log to work out distance travelled and use your dividers to mark out distance travelled and you have DR
Where do you find out tidal information?
Tide Tables and Tidal Atlasses
What information do you need to work out Estimated Position?
Dead Reckoning
Tidal information (Set and Drift)
How do you work out Course Over Ground?
Draw a line from the starting position to Estimated Position
True or False?
If the journey time from your starting positioning to DR is more than an hour you will need more than one tidal vectors
True - one tidal vector for each hour
How do you work out Speed Over Ground?
Divide Course Over Ground with how long it took to get there
What is Leeway?
The effect wind has on the vessel
When do we use Leeway?
Before we plot Dead Reckoning
When is Running Fix useful?
When travelling around a coast
When is Doubling the Angle on the Bow useful?
When theres no significant tide
How many arrows does a transferred position line get?
Two on each end.
How can you estimate leeway?
Compare the angle of the wake to your heading.
How many arrows does a tide line have?
3
If you are close-hauled or reaching in winds up to an F4, how much leeway should you use?
5 degrees
How many arrows does a water track have?
1
How do you do a Running Fix?
- Take a bearing on the charted mark.
- Draw line from charted mark out and note the time and log (AB). Put arrows on each end because it’s a position line.
- After an hour, take a bearing on the charted mark again and note the time and log (BC).
- Using anywhere on line AB, plot your course steered and length using your log readings.
- Add the effect of the tide to that line.
- From the end of the tide line, make a line from it to line BC that is parallel to line AB. Where it intersects is your position.
Mark it with a dot in a circle with the time.
How can you use a charted mark and a depth sounding to determine where you are?
Take the bearing of a charted mark.
Use the depth sounding to determine where you are on the map. Best if you can position yourself on a contour.
Be sure you include the height of tide in the total depth.
If you can take a bearing of a second charted mark, you have a good fix.
Name 8 ways to fix your position.
- Come along side a charted mark = Good
- Single point fix & depth = Average
- Two point fix & depth = Good
- Three point fix = BEST
- Transit and cross bearing = Good
- Radar range = Average
- Sector light and depth = Average
- Doubling the angle on the bow = Poor
How many arrows does a position line get?
One on each end
What is a Running Fix?
It’s a fix we determine by using one charted mark.
How do you calculate your Estimated Position?
- Covert magnetic course to true.
- If wind is on the port side, add leeway to course. If wind is on the starboard side, subtract leeway from course.
- Plot your course with one arrow for your water track. Use your log to see how far you’ve traveled within the past hour to know low long the line should be.
- Calculate the direction and speed of the tide. Add the line with 3 arrows to where your left off on #3.
- Mark the end of this line with a dot and triangle. This is your estimated position.
- If you join the start position to the EP, this is the course over the ground track and should be marked with 2 arrows. This is your COURSE OVER THE GROUND (COG).
- Measure the length of the COG. This is your SPEED OVER THE GROUND.
If you are close-hauled or reaching in winds more than an F4, how much leeway should you use?
10 degrees
How do you take a 3 point fix?
Take a bearing on 3 charted marks, ideally spread evenly.
TAKE THE BEARING OF THE MARK ABEAM LAST!
Where they cross is where you are.
Odds are that the lines won’t meet at a single point. They will instead create a “cocked hat” (triangle). You are somewhere in that cocked hat.
If the wind is on your starboard, should you add or subtract your leeway to determine your estimated position (EP)?
Subtract the leeway
What is a sector light fix?
When you use where a lighthouse light changes color and the depth sounding to determine where you are.
If the wind is on your port, should you add or subtract your leeway to determine your estimated position (EP)?
Add the leeway
What is an Estimated Position (EP)?
It’s the position you estimated you are after you’ve traveled from a known position.
What is Dead Reckoning (DR)?
The position obtained from the true course steered and the speed through the water (log). Does NOT include allowances for leeway or tide.
If the wind is on your port, should you add or subtract your leeway to determine your course to steer (CTS)?
Subtract the leeway from your CTS
How do you mark a fix on the chart?
A dot with a circle around it and the time the fix was taken
Mark the bearing lines with arrows on one end
Erase the bearing lines except for the parts right around the bearing.
How can you use a charted mark to determine your fix?
Come along side it and use the map to get the coordinates. Good way to get your fix.
What is Doubling the Angle on the Bow?
Take a bearing on a charted mark. Make note of your log reading.
When the charted mark bearing becomes 2x’s the original bearing, make note of your log reading.
Plotting these will show an isosceles triangle. The difference between the two log readings gives you the distance of two sides of the triangle.
USE WITH CAUTION because this does not include leeway or tidal data!
REALLY ONLY USED AT NIGHT WHEN THERE IS ONLY ONE CHARTED MARK TO USE
How many arrows does a ground track have?
2
If the wind is on your starboard, should you add or subtract your leeway to determine your course to steer (CTS)?
Add the leeway to your CTS
How can you use a transit to determine your position?
Sail down a transit bearing.
Look for another charted mark. When it is 90 degrees to your bearing, you mark it on the chart.
