Chartwork Flashcards
What is a Rhumb Line
A line that cuts all meridians at the same angle,
A straight line on a mercator projection
A curved line on a Gnomic projection
Whats is the stadard datum for GPS
WGS 84 (World Geodetic System)
Principles of mercator projection
Straight lines are rhumb lines, Great circles are curved
used for normal navigation
land features distorted as one moves north/South
Latitude scale used for distance, due to disortion, must use same latitude as the distance being measured
Parralels of longitude evenly spaced
Transvers mercator projection
Used for large scale (Zoomed in) charts, harbour plans
Particularly if have a North-South orientation
Same as mercator but rotated 90degs, so latitude is now evenly spaced, longitude is distorted
DIstance still measured on side as normal
Gnomic Chart
- Used for ocean navigation planning, polar charts
- Parallels of latitude are curved, parallels of longitude converge at poles
- Great circles are straight lines, rhumb lines are curves.
What is a great circle route, how do we use it
The shortest distance between 2 points on the surface of a sphere
* Plot departure and destination positions on the gnomonic chart; join two positions, since the great circle appears as a straight line on the gnomonic chart
* Choose the specific interval meridian along the track where the course will be changed. Then plot the positions of intersection of the track and the meridian chosen on the Mercator chart
* Join all the plotted positions on the Mercator chart by a series of rhumb lines. (These will make a curve)
What is a composite track
Combination of great circle and Rhumb line sailing
Efficient and safe
Follow the great circle, up to a certain latitude, beyond which it may be hazardous due to ice etc.
Travel due East/West at the safe latitude
Then finish the route on the other end of the great circle
How to correct charts and publications
- Look up current Cumulative List to check latest edition date against date of chart, or use ukho website
- Check for corrections that apply to the chart, listed in the Cumulative List.
- Check Weekly Notices to Mariners published since the Cumulative List for any more recent corrections and any new editions.
- Look at bottom left hand corner of the chart and see which corrections have or have not been applied to the chart.
What are depths measured to
Chart Datum, the approximate lowest Astronomical Tide
What are heights measured to
Underlined figures are drying heights above chart datum, all other heights are above MHWS. Except bridges
What are bridge heights measured to
Highest Astronomical tide, HAT
Explain a DR
From a Fix,
correct for Deviation and variation
measure the distance and bearing run,
Explain a EP
From a Fix
Adjust for Var and Dev,
Apply leeway if applicable
Mark distance and bearing run
get tide set and rate for time run
Apply tide with 3 arrows
Explain a CTS
From a Fix
Draw ground track through waypoint
get tide set and rate for time run
Apply tide with 3 arrows
From end of tide, mark 1 hour of distance on to ground track
Complete triangle to get CTS and SMG
Add or subtract leeway as required
Explain a running fix
Take a bearing of a known object at a known time
Run a straight course, monitor the log for distance
Take a 2nd bearing of the obect, note the log
Plot both bearings on the chart
From the 1st bearing, complete an EP
Parallel the 1st bearing to the End of the EP
Where the paralleled bearing crosses the 2nd, is the fix
What does FR mean for a light description
Fixed red, or FG Fixed green
List of types of Fix
3 point fix
Radar range and bearing
3 radar ranges
Bearing and depth contour
Transits
Running Fix
Dipping height
HSAs
VSAs
GPS
How to check the Echo sounders’ reading
Use a goddamn lead line
How to do use a horizon range (Dipping distance)
Measure height of eye
Get height of object from Chart (allow for tide difference from MHWS)
Use Norries to get distances for each height and add together
or (2.08√height of object) + (2.08√Height of eye)
Mark off bearing of object and distance on chart
How to HSA
Measure the 2 HSAs from 3 conspicuos objects
Draw the base line between them on the chart
Subtract the HSAs from 90ᵒ
Plot that angle from both ends of the base line,
if positive, The center of the circle is on the observers side of the base line, if negative, the far side.
Where the triangle meets is the centre of the position circle
Where both position circles meet is the position
How to VSA
Take a bearing of the object
Use sextant to measure the height of object to the waterline
Allow for dip (in Norries tables) if HoE is less than 12m
Correct for Index error in sextant if present
Account for tidal difference from MHWS
(if MHWS is 7m, current HOT is 5m. +2m to Height of object)
Distance Miles = (1.852 × height of object metres) / Sextant angle in minutes
Or use Norries tables
Mark distance and bearing on chart
What are the Fixed Sextant Errors
Graduation Error - play in the grooves
Collimation Error - telescope not parallel to plane of instrument
Centering Error - the centre of the pivot is not accurate
Optical/Prismatic errors (shade errors) - Shades not optically flat
Wurm and Rack errors
Graduation Error?
A graduation error occurs when the graduations (the Grooves) on the arc, micrometer and/or the screw thread are not cut accurately.
Collimation Error?
Collimation errors happen when the telescope is not parallel with the plane of the instrument.
Centering Error?
A centering error will occur when the pivot of the Index Bar is not at the center of the circle of which the arc forms a part.
Optical error?
Optical/Prismatic errors (shade errors)
An optical error is when the shades are not optically flat. Due to lack of parallelism of the two faces of an optical element, such as a mirror or a shade glass.
Wurm and Rack error?
particular micrometer (modern) sextants
Over time, the winding mechanism can get worn/tired
What are the Adjustable Sextant errors?
PSI
* Perpendicularly Error
* Side Error
* Index Error
Perpendicularity error?
A perpendicular error is when the Index Mirror is not perpendicular to the plane of the instrument.
To check, turn the sextant round, hold it horizontal and look into the index mirror.
If the true and reflected arcs do not line up, then the error exists. To adjust for this error, turn the screw at the top of the mirror.
Side Error
A side error is when the horizon mirror is not perpendicular with the plane of the instrument.
To check for this set the arc to zero, hold the sextant on the side and look through the telescope at the distant horizon.
If the true and reflected horizons do not line up, then the sextant error exists.
To correct this situation, slowly tighten or loosen the screw furthest from the frame at the back of the horizon mirror until the vertical line no longer appears to jump
Index Error
An index error is when the index and horizon mirrors are not parallel with each other when the arc is set to zero.
To check for this set the arc to zero, hold the sextant vertical and look at the distant horizon. If the true and reflected horizons do not line
up the index error exists.
To adjust for this error, you can either:
a) turn the screw at the top of the mirror and then re-check for side error an again for index error
or
b) turn the micrometer until the error is gone, and take the sextant reading and apply it to the Sextant Altitude. (If it off, it is on: if it is on, it is off.)
How to get you latitude with a sextant at sea
A meridian Passage - Mer Pass
At local Noon - When sun is due North or South
Use Almanac to find local noon
Watch until the sun reaches its heighest point to get sextant angle (ALT)
Use DR position and the Time to get Declination from Almanac
The 3 equations for calculation latitude with a sextant at noon
Always (90º - Alt)
Lat and Dec same Hemisphere, but Lat greater than Dec
Lat = Dec + (90-Alt)
Lat and Dec same hemisphere but Dec greater than Lat
Lat = Dec- (90-Alt)
Lat and Dec opposite Hemispheres
Lat = (90-Alt) - Dec
What 4 pieces of info do you need to get latitude with a sextant
At Mer Pass (Noon):
Use Sextant reading (ALT),
Declination from almanac
Time of sighting
Dead Reckoned Latitude
How to calc your Longditude with a sextant
Use almanac to find Merpass London Greenwich
Get time of Local Merpass (time it)
Difference in times is the difference in longditude
1 hour is 15º, 4 mins is 1º
Can use Arc to time table in Almanac for calculating
(tricky part is getting the exact time of local merpass)
