Watchkeeping, Charts

Q. What is the difference between Geographical and Nautical Mile?
Geographic Mile is equal to 1855.4 metres & is the length of 1’ of the equatorial arc. Nautical Mile in latitude is the length of arc of the meridian there, which subtends an angle of 1’ at own center of curvature. Its value varies with latitude & the mean value at 45° is 1852.3 metres.

Q. What is a great circle?
Great circle is a circle drawn on the surface of a sphere, the plane of which passes through its center. If a sphere is cut along a great circle sphere will be divided in two equal halves. Distance measured along great circle is the shortest distance between any two points on it.

Q. What is geographic latitude?
Geographic latitude is angle made between a line drawn from a point on surface of earth to the center of curvature of that part of meridian & plane of equator is called geodetic or geographic latitude of that point.

Q. What is a rhumb line?

Rhumb line is a line which when drawn on a chart makes equal angle to all meridians on the way.

  • On Mercator chart it appears as straight line.
  • On a Gnomonic chart, all ‘rhumb-line courses’ except north-south and east-west courses converge towards pole.

Q. What is the significance of the ‘date of new edition’ on a BA chart?
The date of new edition is marked below the south border of the chart, to the right of ‘Date of publication’.  When the chart has major changes such as inclusion of a new traffic separation scheme or is revised & modernized in style, an up-to-date edition is published. Thus, a chart is known by its new edition date as to whether it is valid or not. Entries regarding the small corrections subsequent to the date of new edition may appear at left hand corner, outside the south border of the chart.  If a chart is printed say two years after the date of new edition then record of small corrections for that chart for a period between date of new edition & date of printing is found on the chart at the usual place, in printed form.

Q. What is source data in reference to a chart?
‘Dates of Surveys’ & ‘Source Data’ are found within the Source Data diagram, which is placed in some convenient part of the chart, not to obscure essential navigational information. Different areas covered by the chart may have been surveyed; by different surveying authorities at different times; using different methods / techniques; on different scales. These details are indicated on the chart in the source data diagram. The user may be guided by above information in respect of the degree of reliability to place on the chart while navigating on it.

Q. How is reliability connected to it?
The user is aware as to which part of the chart area was surveyed more recently as compared to the other areas. Most of the earlier surveys were mainly exploratory, trying to discover new lands. Later surveys paid more attention to depths, used improved instruments & techniques.
Prior to 1864 AD British survey ships were sailing ships. There after the steam replaced sails. Prior to 1935 lead & lines were the only means of obtaining soundings. Later the echo sounders were introduced. Earlier survey-ships assumed the maximum draft of 15 m & did not foresee the deep drafted ships drawing up to 30 meters.  

Q. What is datum, as referred to a BA chart?
Uniform datums w.r.t. which the height & depths are indicated on the chart may be found under the title of chart. On BA charts the depths indicated are usually below one of the lowest of tide levels & is normally Lowest Astronomical Tide (LAT). In non tidal waters such as Baltic, chart datum is usually Mean Sea Level. Heights indicated are normally above Mean High Water Springs (MHWS). Because of above datums the measured depths or heights at any given time should be normally, more than charted depths & heights respectively.

Q. How do you decide which side to pass a buoy?
Conventionally, the ‘general direction of buoyage’ is along the course taken by a mariner when approaching harbour, river, estuary or waterway from seaward. Wherever possible, buoyage authorities have taken the principle of following clockwise direction around the continent. While proceeding along the general direction of buoyage in the channel, the starboard hand buoys have to be passed on starboard side & vice versa. On the reverse passage i.e. while going in a direction opposite to the general direction of buoyage the starboard hand buoy have to be passed on port side.

Thus, it is very important to know the general direction of buoyage in an area, especially where the buoys mark a short stretch of deep water or shoal or where there are two channels emerging from same point. The Buoyage Authorities determine the general direction of buoyage. The same can be found in Admiralty Sailing Directions or the relevant Admiralty chart, where the conventional direction of buoyage may be indicated by magenta arrow symbol.

Q. Suppose a chart is published in 1958. Its date of new edition is Jan 1985. Date of printing is Jan 1994. The chart is purchased from the depot in Jan 1996. What entries are likely to be found on charts to indicate that all corrections are in order?
While receiving a new chart, it must be ensured that the new edition date on chart is the current new edition date as per the latest cumulative notices to mariner or the information available. The entry regarding small corrections on the chart would be as follows:
LH Bottom corner: Numbers of weekly notices from Jan 1985 to Jan 1994 will be in original (electronically) printed form. The record of relevant corrections with year, are found on the chart in (electronically) printed form. Additionally, hand written entries for the corrections, between Jan 1994 and (say) 1236 of 1995 (May 1995) are also found in this part of the chart.

At thumb label on the reverse of the chart within the seal of the Chart Depot: An entry, stating that the chart has been corrected up to say BA notice 35 of 1996, may be seen indicting that while the chart was at depot, the corrections up to notice number 35 were done. Thus, between May, 95 and Jan 96 there was no correction affecting the chart.

Q. What do you do to a chart enroute, if it is a voyage chart and is cancelled but not replaced?
An entry stating that the chart is cancelled must be written on the back of chart. Thus, insert remark ‘Cancelled vide say WNM 321 of 1996’, ‘New ed. pub. 15.04.96’ (in large letters), a positive way of identifying such chart must be there but should not be destroyed or removed away till a replacement is found. After a replacement chart is found the navigation officer should write replaced with the date on the back of the chart. Relevant entries must be made in correction log. In the mean time the following may be done:

  1. Keep a track of all the recent notices; navigational warnings; changes in lights or buoys; chart inserts, etc.
  2. The cancelled chart must be used as reference chart only without putting a total reliance on it.
  3. This being an exceptional situation a high resolution copy of the chart may be requested of the current edition to find out if any major changes are made to the chart.

Q. How are the temporary and preliminary corrections made?
The ‘in force’ T & P notices affecting a chart are given in ‘Annual notices to mariner’. The chart is corrected for the same in pencil. In addition to above the weekly notices to mariner of the current year is also checked for new notices (since the first notice). The entry regarding same is made in pencil in the space next to the entries for small corrections, e.g. 1996 726(T) or 431(P).

Q. Give examples of temporary and preliminary corrections.
Temporary Correction: Special marks are placed to indicate firing practice area.
Preliminary corrections: A wharf is planned to construct between given coordinates. Thus, a wharf may be constructed as planned or with some changes.

Q. How will you correct a chart, not corrected for about two years?
A WNM has two indices, viz.

  1. Charts affected & notices affecting them. &
  2. Index of notices with page on which the same is detailed.

As every notice indicates the number of previous correction, it’s advisable to first search the latest WNM on board to see if any chart is affected in that week, instead of searching the WNMs in serial order.   Cumulative notices to mariners, which is published quarterly, lists all the charts with notices affecting the chart for a period of one and a half years. The numbers of notices can be picked up from there. A link of corrections must form from the last small correction entered on the chart and the latest correction affecting the chart. Once all the correction numbers are listed in a rough paper the actual corrections must be done from old to new.

Q. What, if cumulative notices to mariners is not there?
The index of charts affected must be seen in the latest WNM. If not found the WNM before the latest is seen. This process is continued till the notice affecting the chart is found. From this notice the last correction is found. This is repeated till the last correction entered on the chart is found. Once the list is prepared in a rough paper the corrections are done from old to new.
Thus, in proceeding as above only 5 or 6 out of over 100 WNMs had to be opened. Moreover it’s confirmed that no intermediate correction has been omitted. Entry on chart or correction log must always be made only after corrections have been carried out.  

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