Originally Posted by noelex 77
Most terrestrial maps show areas you can explore, but marine charts are a little different, their job is primarily to show the no go areas.
Multiple map sources are therefore very useful. Even if they are less than 100% reliable like the consumer input layer on electonic maps, (which on occasion I have found very helpful), or a smudge on google
earth. Taking the most pessimistic of the sources is safer than hoping the "official" chart has all the answers.
The bottom line is that false positives on marine maps are not much of a concern, it is the false negatives that will cause you grief.
I hope I'm not the only one thinking this is a characteristically wise observation from N77.
I didn't fully realise this myself until a minor challenge in the mid 1980s.
On checking the actual charts on a boat I'd recently been flown in to join as navigator (because their Satnav had failed!), I found we didn't have the chart for Suva harbour, out next port of call. (It was listed in the Rolodex as being on board)
We were about to retrieve our docklines in Nuku'alofa, and if we delayed more than an hour or two, we would lose the narrow timeslot during which the sun would be from the right angle to be able to con safely through the nearby reefs
, whose nav aids were in some disarray. So it would cost us an extra day.
I normally carried large sheets
of tracing paper for such contingencies but on this occasion I think I had a limited baggage allowance, so I seem to recall
plundering the galley
for baking paper which I joined with masking tape.
I took off in the dinghy
and found a friendly sailor on a nearby boat, who was happy for me to copy his chart. When I saw just how much information it contained, I quickly realised I wasn't going to have time to transcribe sufficient detail for a daytime entry to be viable, so I had a brainwave, and the realisation mentioned above lodged in my consciousness.
First, though, I concentrated on carefully capturing the locations and characteristics of the various lights. This took a while. However, because they can be unambiguously identified at night, and only three are needed at any one time for an unambiguous fix, this was the key to the success of my idea of a night entry using limited information. (Even if, as usual in this part of the world, some of the lights were not lit on any particular night)
-- I've since found that German merchant naval captains used similar reasoning to draft
usable charts from the "List of lights", or just from the Pilot, when they found themselves having to divert to neutral or friendly ports
- eg South America
- when various world wars broke out --
I then marked in the details pertaining to the channel (including approaches) and the quarantine anchorage, and plotted any particularly tall landmarks or peaks which would stand out against the sky. These tasks took hardly any time at all.
Armed with the above realisation, all I had to do then was to draw a bold line around everywhere the water
started to get too thin for our draft
, areas containing moorings etc, and put a similarly bold circle around any isolated NO GO areas, like rocks, coral
heads, piles, and such. I didn't need to know why
not to go there.
So the 'chart' showed nothing of the shoreline, the shore- and dock- side facilities, or any extraneous info of any sort; most would be of zero interest when entering at night even if time had permitted.
We arrived in the vicinity in the early afternoon, so we hove to off the beautiful island to the south of Suva whose name I forget, and swam and snorkelled until dusk. The entry was straightforward and uneventful - the rudimentary, but purpose-built chart made it a doddle. It also didn't hurt that the entire crew kept their eyes peeled for things not to hit (which of course they should always do when entering crowded waters at night)
I suppose the short version of this story would be "IF you know where you are, and where you must NOT be, that's all you strictly need to know"