Author Topic: General Questions  (Read 45556 times)

Randi

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Re: General Questions
« Reply #135 on: August 15, 2015, 03:14:34 pm »
That sounds good to me.

HatterJack

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Re: General Questions
« Reply #136 on: August 15, 2015, 04:33:17 pm »
I think listing as an observation is appropriate, and your google fu is quite strong. You were looking for Summerline, when the actual term for what they did is determination of longitude by Sumner Line (so named for Captain Thomas H. Sumner, who devised the method when his ship and crew were in grave danger during heavy weather and rough seas, which prevented making observations in a regular and timely manner), which involves using observations of altitude of known celestial bodies to triangulate position when likelihood of error by dead reckoning is likely to be in error, but direct observation is impossible. It is surprisingly precise when you've got a capable navigator, but can be disastrous if calculated incorrectly.

Randi

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Re: General Questions
« Reply #137 on: August 15, 2015, 06:03:34 pm »
 8)
copied to the OWpedia ;)

Hanibal94

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Re: General Questions
« Reply #138 on: September 16, 2015, 08:59:33 am »
What should a man do when the location says "Based on D.R. and bearings"?
http://oldweather.s3.amazonaws.com/ow3/final/USS%20Vicksburg/vol011of023_jpg_clean/vol011of023_123_0.jpg

I know bearings usually means Observed - but combined with DR? This is very odd.

Maikel

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Re: General Questions
« Reply #139 on: September 16, 2015, 09:18:23 am »
Since at least part of it is DR, I'd go for DR.

HatterJack

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Re: General Questions
« Reply #140 on: September 16, 2015, 10:53:53 am »
Generally I've only ever seen a combination of DR + bearings done when there's literally nothing available to observe, but the skipper (even when it's the same person navigating, which is pretty common in modern civilian shipping) demands that an observation be made (e.g. post sunset, in a storm, during a new moon in the open ocean).

It's similar to Sumner Line observations, but isn't as reliable. In theory it can be more accurate than the Sumner Line, but relies entirely on instrumentation being precise (fun fact, it usually is anything but). It's possible, given the way that latitude and longitude are calculated, to use nothing more than the ship's compass and knowledge of how fast you're moving, and in which direction, to get a reasonable approximation for where you're at in the world, provided you have a DR or direct observation as a reference point. The *big* problem with this is that if you're not moving at the speed you think you are, or if your compass isn't corrected (or at least the calculations for your compass bearing isn't corrected for any deviations) you can be off by hundreds, if not thousands of miles (depending on how fast your ship can travel, and how long you've gone between observations.

Still, even with the inherent potential for winding up *way* off course, a good navigator working with reliably accurate instrumentation, can calculate out a reasonably accurate position, generally within a few miles or so of the actual location. This sounds pretty bad, when you look at something like a sat-nav being accurate within 30-40 meters (or better), but when you're talking about human beings, calculating all of this out by hand (since calculators didn't exist yet), being within a dozen or so miles of the actual position, in a target area of thousands of square miles, that's pretty good. At the very least, the position they calculate for would be visible from the deck.

Randi

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Re: General Questions
« Reply #141 on: September 16, 2015, 01:20:30 pm »
What should a man do when the location says "Based on D.R. and bearings"?
http://oldweather.s3.amazonaws.com/ow3/final/USS%20Vicksburg/vol011of023_jpg_clean/vol011of023_123_0.jpg

I know bearings usually means Observed - but combined with DR? This is very odd.

Strange indeed, but probably safest to go with DR.

Hanibal94

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Re: General Questions
« Reply #142 on: September 16, 2015, 02:22:50 pm »
Strange indeed, but probably safest to go with DR.

A mod has spoken, and I shall do it that way.
I fixed the few pages where I had entered it as Observed - only had to fix three locations, because I only do 5 dates per day on this vessel.