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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » INLAND AND COASTAL NAV 1-16, 1-17

   
Author Topic: INLAND AND COASTAL NAV 1-16, 1-17
RROBA


 - posted January 04, 2006 06:18 PM      Profile for RROBA           Edit/Delete Post 
I have just started the course and having fun. I got a 4.1nM for 1-16A. Did I do something wrong?
Also, when directions are given such as in 1-17 "south of buoy", is it safe to assume that all such directions are true unless otherwise indicated? Thanks

From: NJ
David Burch


 - posted January 05, 2006 12:10 AM      Profile for David Burch           Edit/Delete Post 
Glad to hear you are enjoying the course.

The answer in the book is 4.01, so 4.1 is off by 0.9/4.01= 22%. That is really larger than it should be if the answer in the book is correct. We have several ways to check that.

First, we can do a search on 1-16 to see if anyone has questioned it before, and also check the Tech support section to see if any typos have been reported. (One of the topics you get when you do this search has a related discussion about a similar question).

and we can also compute the precise answer as a better check, although these are supposed to be done by plotting not computing. (There is a link to the free computers we offer in that same link as above to Resources\library\freeware and shareware\NIMA calculators, with a note on this ap)

next we can do some other reasoning... which could in fact be more easily done by just measuring from the chart, but i do not have the chart at the moment.

The chart we are working on has a scale of 1:80,000. That means that one inch on the chart is 80,000 inches on the water.

so, 80,000 inches/12 = 6666.7 ft and 6666.7/6000 = (about) 1.1 nmi. so one inch on the chart is 1.1 nmi, and 0.9/1.1 = 0.8 inches. so this seems to say that a 0.9 nmi error is a plotting error of 0.8 inches, which is a lot. so this is too big an error. had this come out to be 0.1 inches, then we are probably still too large for some cases, but well within reason if we have to draw really long lines.

we point this reasoning out as something one might use away from the classroom in evaluating work done underway.

the problem at hand now must lie in the plotting of the positions or the reading of the final result from the lat scale. Please double check and post more if this does not work out.

somewhere also, we have a short article online about accuracy, or how close should the answers be. When i find this i will post a link here.

==================

Yes, you are right. if we read just "south" in almost any application, we can fairly assume true. if this should not be the case, then whoever wrote what we are reading, was not careful enough. Put another way, it is very important to always specific the units, especially if we said 180, instead of "south." a key task in navigation is developing and using unambiguous communications.

in our case, if this is not done already, we should expand our glossary to include this explanation. thanks for the reminder.

From: Starpath, Seattle, WA


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