Section 1: Terrestrial Position Fixing Flashcards
What are the types and phases of navigation?
Navigation begins from berth to berth.
Types of navigation are:
i. Coastal Navigation
ii. Ocean Navigation
Passage planning includes:
Voyage instructions:
- The ship, its equipment and available resources.
- Prevalent & expected weather.
- Safety / Security Hazards.
- Routeing, distances, ETA, fuel, stores, crew change etc.
- The primary task for every Watchkeeping Officer is to assist in ensuring the safety of life, property at sea and protection of the marine environment.
Pilotage or Harbour Phase:
This phase takes place from when the ship leaves the berth until it sails out of the port and disembarks the pilot. The same phase is repeated when a ship arrives at a port, embarks a pilot and then berths. The navigation in this phase is usually carried out by using a combination of visual means of position fixing, radar and GPS.
Coastal Navigation Phase:
Navigation within 50nm of a coastline or within 200m of water depth contour is known as coastal navigation. This phase is distinguished from other phases because of:
- An increase in traffic density.
- The availability of coastal features, including lighthouses and coastlines, for fixing position, together with instruments such as echo sounders, radar and compasses for visual bearings.
- An increase in the frequency of position fixing because of the close proximity to navigational hazards
- A possible increase in communication with other vessels and shore stations, such as port authorities and pilot stations.
Ocean Navigation Phase:
This phase begins when ships are generally away from the coastline and are in open seas. Therefore, the workload on OOW in open waters is greatly reduced because under-keel clearance is not an issue and traffic density reduces.
Usually, position fixing is only by GPS (or other equivalent satellite systems) or celestial observations.
If neither of these two methods is available, then the position is estimated by a method called dead reckoning.
The use of radar is restricted to the lookout for collision avoidance.
What are the two formats of navigational charts used by the professional mariner?
i. Electronic Charts
ii. Paper Charts
What are the different types of charts used by the professional mariner for safe navigation?
- Standard navigation charts (Mercator charts) - for Rhumb line sailing.
- Gnomonic charts - for Great Circle (GC) sailing.
- Routeing charts - for ocean sailing.
- Mariner’s routeing guide.
- Maritime security planning chart.
- Port approach guide.
How does the horizontal geodetic datum affect position fixing?
The Geodetic datum is a reference system for specifying positions on the earth’s surface.
There are hundreds of datums in use, each datum is associated with a particular reference spheroid that can be different in size, orientation and relative position from the others. Most datums are only valid locally which are used for higher accuracy to navigation within that particular area.
The World Geodetic System-1984 (WGS-84) datum is global in scope and positions are obtained by satellite navigation systems.
Most charts have not been configured to the WGS84 Datum. This means that positions obtained from satellite navigation receivers will not be directly compatible with the chart and must not be used without adjustment. Therefore, a correction needs to be applied for the WGS-84 GPS position to agree with the charts using other horizontal datums. Fortunately, most GPS receivers may be set to display positions in several other datums besides WGS-84 and perform the calculations automatically.
When known, the horizontal datum of the chart is usually named in the chart title. Since 1982 many hydrographic offices have been adding ‘Satellite-Derived Positions’ notes (usually situated close to the title) when charts have been revised. This note provides a latitude and longitude adjustment to be applied to positions obtained directly from satellite navigation systems (such as GPS) to make them compatible with the horizontal datum of the chart.
How does the tidal datum of the chart affect position fixing?
The tidal datum is a type of vertical datum and is a reference level from which all vertical measurements, such as depth of water, the elevation of a lighthouse, vertical clearance under a bridge etc., useful for navigation are measured.
Depths and drying heights are measured from the Chart Datum (CD), which is the lowest level up to which the water level is expected to fall. It is usually the Lowest Astronomical Tide (LAT), Mean Low Water Spring (MLWS) or Mean Sea Level (MSL) for non-tidal waters such as the Mediterranean Sea.
Elevations and vertical clearances are measured from the Highest Astronomical Tide (HAT) or Mean High Water Springs (MHWS).
These reference levels have been selected to provide the greatest margin of safety to prevent grounding and impact damage to bridges and other overhead obstructions.
If these vertical measurements are used for position fixing, appropriate corrections for the height of the tide at the time of observation need to be made. E.g. Sextant and Echo Sounder.
Which scale is to be used for measuring distances on a Mercator chart - latitude scale (vertical) or longitude scale (horizontal)? Explain the reason.
Only the vertical latitude scale on a Mercator chart can be used to measure distance.
This is because each minute of latitude equals to 1nm, while the distance between minutes of longitude decreases between the equator and the poles.
Describe the various position monitoring techniques using terrestrial observations. Mention the advantages and disadvantages of each technique.
- Visual fix - used when navigational marks or features can be visually observed.
The general limitation for a visual fix is that it cannot be used when visibility is severely restricted or away from the coast.
a) Passing buoys/buoyed channel
Advantages: Quick and continuous monitoring.
Disadvantages: Buoys can be out of position, signal not working.
b) Transit bearings
Advantages: Can obtain Line of Position (LOP or PL) quickly.
Disadvantages: Need to be prepared in advance as it lasts for a very short duration.
c) Leading lights
Advantages: Can verify/correct ship’s heading quickly.|
Disadvantages: Need to be prepared in advance as it lasts for a very short duration.
d) Visual bearings - simultaneous bearings/running fix
Advantages: Simultaneous bearing gives the vessel position instantly; the running fix needs only a single object to take a bearing from.
Disadvantages: For simultaneous bearings, need two observers, two azimuth rings and at least two objects to get an accurate fix. For a running fix, the Course Made Good (CMG) should be known.
e) Measuring angles - Horizontal Sextant Angle (HSA) or compass angle, Vertical Sextant Angle (VSA)
Advantages: Can be used even if compass error or sextant error is unknown.
Disadvantages: The ship has to be stationary during the observation. For HSA, at least three widely spaced objects are needed. For VSA, correction for the height of the eye and tide needs to be applied.
- Radar Fix
Advantages: Can be used mostly in any condition of visibility.
Disadvantages: Require use of appropriate range scale, tuning and target identification. Have blind zones. If gyro has an error, the error would be carried forward to the radar. Range and Bearing discrimination.
- Parallel Indexing
Advantages: Quick and continuous monitoring.
Disadvantages: Require use of appropriate range scale, tuning and target identification. Have blind zones. If constant alteration of course, difficult to monitor.
Can’t be used if the nearest land is more than 12nm.
- Echosounder
Advantages: Can be used even if the gyro is not working.
Disadvantages: Need height of tide and draft and squat correction as applicable. Very approximate method. Accuracy can be greatly improved if the gyro is working and a bearing can be taken.
List 6 common errors you need to be aware of when using terrestrial means of position fixing.
- Compass (Gyro and magnetic) errors affect visual bearings.
- Gyro error affects RADAR bearing as Gyro is an input for the Radar.
- Parallax error in reading visual bearings.
- Instrument errors on the azimuth ring, sextant (for HSA/VSA methods), parallel ruler etc.
- Errors of the echo-sounder.
- Human error in measurement and plotting.
The 2nd Mate has marked the Position Fixing Interval (PFI) or Position Fixing Frequency (PFF) as 30 minutes on a particular leg of the voyage. How will you determine if this duration is appropriate for that leg?
Position Fixing Interval (PFI) should be such that the vessel should not run into danger or be in a dangerous situation between any two fixes on that leg and the interval should allow for sufficient time to take action to bring the vessel back to safety.
The following factors need to be considered while determining PFI:
(i) Distance to the nearest danger.
(ii) Anticipated ship’s SOG on that leg.
(iii) The anticipated worst condition of wind and current.
As an example, in the above picture, it is clear that a 30-minute interval is not useful as it would be too late to take avoiding action. In this case, a 10-minute interval is better suited with more frequent position monitoring when closer to the shoal.
Though ECDIS makes life easier for the navigator, it is still a requirement that these GPS positions are verified (cross-checked) using independent methods. It is also worth noting that various industry guidelines emphasise the need to deploy traditional navigational skills in modern times, even with the changeover to ECDIS.
Frequency of Position Verification on ECDIS/Chart RADARs
Intertanko recommends the following position verification intervals:
(a) Open/Deep Sea:
While the vessel is at open sea, the accuracy of position verification is checked once every watch.
(b) Coastal Passage/Approaching, Anchoring and Berthing /Unberthing:
In these cases, the ship’s position on the ECDIS is compared with other means at least every one hour.
The guide also mentions that position verification methods include, but are not limited to, any or combination of the following methods:
- Visual observations.
- Radar observations.
- Parallel Indexing.
- Radar Overlay/ENC Underlay.
- Dilution of Precision (DOP) checking.
- Signal or Carrier to Noise Ratio (SNR or CNR).
Once the GPS position has been verified, it is also required that the verification is recorded for future reference. The methods available for plotting the verification on the ENCs vary depending on the options provided by each ECDIS maker. Commonly used options for recording the verification include ‘Entering Position’, ‘Event Mark’, ‘User Map Editor’, etc.
What is the general SOLAS requirement for updating Nautical charts and publications for the intended voyage? What are the specific requirements for electronic nautical publications?
In accordance with Regulation 27 of Chapter V of SOLAS, nautical charts and nautical publications, such as sailing directions, lists of lights, notices to mariners, tide tables and all other nautical publications necessary for the intended voyage, must be adequate and up to date.
Electronic versions of nautical publications, such as the above, can be carried as an alternative to hard copy nautical publications; however, such electronic nautical publications must be adequate for the intended voyage and up to date.
Note: Electronic versions of nautical publications should be:
(a) Officially issued by an administration, authorised hydrographic office, or other relevant approved organisation.
(b) Be accessible using a dedicated computer located on the bridge which is available at all times to the Officer of the Watch (OOW) and which is connected to the ship’s main and emergency power supplies.
(c) Be available on at least one other backup computer that can be made available to the OOW within 5 minutes.
In addition, all digital software and hardware used for accessing official electronic versions of nautical publications shall comply with the recommendations of MSC Guidelines for the onboard use and application of computers.
What are the uses of the ADMIRALTY Digital Catalogue (ADC)? Are corrections issued for chart catalogues?
All charts and publications for the upcoming voyage can be ordered with the help of the digital catalogue. It also gives contact details of the authorised distribution agents.
The ADMIRALTY Digital Catalogue (ADC) provides a comprehensive and up-to-date reference of ADMIRALTY Maritime Data Solutions, including Standard Nautical Charts and Nautical Publications, as well as AVCS, ARCS, ADMIRALTY ECDIS Services and T&P NMs. Users can search for these products by type, scale or a defined route, with weekly updates available online to keep the catalogue fully up-to-date.
Charts publications, e-NP, ENC, Raster charts
Your 2nd Mate informs you that everything is ready for the upcoming voyage. How will you ensure that the Admiralty paper charts and hard copies of the publications have been updated?
- If the vessel has subscribed to folio management services such as ChartCo, Digitrace etc., then select the voyage charts and publications and take out the list of permanent and T&P corrections. Randomly check the last few corrections to confirm if they have been correctly applied.
- If the vessel is not using a folio management service, check the chart and publication editions and list of permanent corrections from the latest cumulative list of Notice To Mariners. It should be noted that the cumulative list will only show the permanent corrections issued in NTMs prior to the publication of the cumulative list. All subsequent notices should be checked in the subsequent NTMs. The 2nd mate should have noted these in the paper chart maintenance record. The T&Ps can be checked from the Annual Notice to Mariners and subsequent weekly NTMs. Randomly check the last few corrections to confirm if they have been correctly applied.
What is meant by plain and parallel sailing?
Plane sailing (also, colloquially and historically, spelled plain sailing) is an approximate method of navigation over small ranges of latitude and longitude. The plain sailing method assumes that the earth is flat and does not take into account the curvature of the earth.
Hence, it can be used only for short distances (up to 600 NM).
Using this method, with the initial and final positions, we can calculate the course and distance.
Parallel sailing is a spherical sailing in which a course is either 090°T or 270°T(east or west direction) in a parallel of latitude.
What is meant by Difference in Meridional Parts (DMP)?
DMP between two latitudes is the length of a meridian between those latitudes on a Mercator chart expressed in units of longitude scale.
DMP between two latitudes may be obtained using the meridional part table as the difference or sum of the meridional parts of the two latitudes, similar to obtaining the d’lat.
What is Mercator sailing?
Mercator Sailing is another method of Rhumb Line sailing that takes into account the earth’s shape and is based on mathematical principles involved in the construction of a Mercator chart. It is used to find the course and distance between two positions that are in different latitudes from the large D. Lat. and distance. It is similar to plane sailing, except that plane sailing is used for small distances.
Also, in Mercator sailing, the D. Lat. is expressed in Longitude units, which is the difference of meridional parts (D.M.P.), whereas in-plane sailing, the D. Long. is expressed in latitude units or nautical miles.
A meridional part for any particular latitude is the length along a meridian on a Mercator chart, measured in units of the longitude scale, between the equator and the parallel of that particular latitude.
It can be extracted from nautical tables. The difference of meridional parts is the difference between the meridional parts for any two latitudes. The rules for finding D.M.P. are the same as for finding D.
Lat., i.e., same names, subtract; different names, add.
