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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 04-03-2008
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Sometimes at night, even with great charts and lots of eyes, you can't really tell where you are. There's just too much background noise (cars, streetlights, water towers), the buoys get lost in the clutter, and the GPS is busy giving numbers that aren't on the charts.

I came up the Race (east end of LI, NY) that way once. For some reason the night was just too damned dark to see anything, the shore clutter made the marks useless, and since we were racing we weren't about to slow down and take our time plotting GPS numbers. I wasn't totally rash--once I realized the depthsounder wasn't being obstinate. It was just saying "_ _ _" feet of water meaning "more than the 199 I can count to" and that put us firmly in the deep water channel, which was where we needed to be.

But without that extra tool, the charts would have been useless, without a lot of time and effort to plot GPS numbers every couple of minutes on the charts. (Pre-computer nav that trip.)
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  #22  
Old 04-03-2008
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sailaway21 is just really nice sailaway21 is just really nice sailaway21 is just really nice sailaway21 is just really nice
I've no quibble with radar. If you're using a GPS/chartplotter to enter a strange harbor though, by night or by day, you're deluding yourself that you're navigating. billyruffin's last post is a nice summation of how piloting should be done when entering port or working in close quarters. Those chartplotters, especially those where the charts are over a year old (90%?), are going to start getting more and more people killed. Visual is real. Radar is real. Chartplotters are a cartoon that may or may not be real.
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  #23  
Old 04-03-2008
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The chart plotters are just another tool in the arsenal of the navigator. To not use one would be as silly as relying completely on one. It sure is nice when the radar paint and the chart overlay match up. Warm and fuzzy!
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Old 04-03-2008
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I've been fortunate on numerous night landfalls, particularily pre-GPS days, to have mostly entered jettys on the Gulf Coast at night. Generally once you pick up the sea bouy, the range lights are pretty clear (fog is not much of an issue down here) and easy to follow into the jettys.
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Old 04-07-2008
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thanks for this thread guys, I'm currently planning a trip with unxperienced crew from Mallorca to Ibiza and return in May (including at least one nicht hop) . Your experiences emphasized my opinion not to arrive at night (almost) no matter what, and if I absolutely have to do it then how to do it.

Cheers from Germany!
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  #26  
Old 04-12-2008
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Going into a new port at night, you may be safe relying on the GPS for navigation, but it is a good idea to back it up with visual markers or, at a minimum, radar.

Last year I was coming up to the Marieta Islands near Puerto Vallarta sometime before daylight. Several miles out, I noticed that the radar showed the island over a mile in the wrong place. I thought the radar might be broken.



I slowed down to arrive after dawn. (You can see the jags in the boat track at the bottom of the screen where we were floating around.) It turned out that the GPS data (Navtech) and the admiralty charts show the islands more than a mile off their true position. I am glad I noticed this sooner rather than later, because the shore is rather rocky.

Marietas Islands, west of Puerto Vallarta

The Rains' Boating Guide to Mexico shows the islands in the correct place.
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  #27  
Old 04-13-2008
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Bob, could that have been chart datum error? GPS and Admiralty charts using different chart datums? Or GPS and radar not being set to use the same display, i.e. DD.MM.mm versus DD.MM.SS perhaps?
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Old 04-13-2008
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Actually GPS has shown a number of smaller islands to have been historically mis-plotted. Most of these are now fairly well known and charts have been updated.

I'd point out via Bob's excellent example that, when it comes to cartography, GPS, and visual/radar navigation the visual/radar is to be assumed the most trustworthy. The value of the coastal chart is not in it's precise correlation of terrestial masses with an accurate latitude and longitude but in the accurate depiction of their relative locations. In simpler terms, once landfall is made latitude and longitude cease to be of great navigational significance. The island is exactly where you see it as being, either visually or on radar, regardless of whatever the discrepancies may be in assigned latitude and longitude. Perhaps the best correlation may be made with the fathometer, or depth sounder, versus the charted depth. Which are you to believe? (After all, cartographers and surveyors have off days also!)

Ignoring the effects of radar super refraction, which manifest themselves only at very long ranges, usually within a temperature inversion, the radar is to be trusted above the GPS. I'd have been inclined to think, were I in a similar circumstance, that the GPS was "broken" not the radar.

If I'm reading your display correctly, and there is some doubt in my mind as to that, the error is on the order of minutes not seconds, ie.. miles off. Are there more islands off screen that I'm not seeing and what is the nature of the topography of the islands. I'm having trouble resolving the two images I'm seeing as being the same-but then, I guess that's the whole point, eh? (g)
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  #29  
Old 04-13-2008
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If the destination is close enough to get there in daylight hours, I leave early enough to get there well before dark.

If not, I leave late enough in the evening to sail comfortably through the night and get there at daybreak.

If the voyage is longer than 24 hours and my ETA looks like being in darkness hours, I slow the boat down to stage the arrival at daybreak anyway.

If all else fails, I stand off until daybreak but I don't go into a strange anchorage in the dark no matter how well lit or charted. When I'm close to things that can sink my boat, eyeball navigation has no substitute

Chart plotters may be pretty good these days but they're still plotting on images of charts that are often nowhere near as accurate as we would like and unless you have the latest editions, they may contain inaccurate info.

JMHO

Andre
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Old 04-13-2008
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Quote:
Originally Posted by hellosailor View Post
Bob, could that have been chart datum error? GPS and Admiralty charts using different chart datums? Or GPS and radar not being set to use the same display, i.e. DD.MM.mm versus DD.MM.SS perhaps?
The datum (WGS84 vs. NAD27, for example) usually only makes a couple hundred feet difference, and the electronic chart "informs" the chart plotter of its datum. The radar and GPS were set correctly, as they aligned properly before and after visiting the Mariatas with no change in settings. Also, the land was where the GPS claimed it was before and after visiting the Marietas.

If you compare the charts (electronic and otherwise) with Rains's Mexico Guide, you can see that there is a difference in the lat/lon between the two.
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