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Old 12-02-2013, 23:11   #61
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Re: Coordinate Math Question

Then I don't see what the grid has to do with the Tyee Shoal Light, if it can't be used to layout a desired point, whether that point is arrived at in cubits, yards or 64ths of an inch. Following this logic, if I had asked how to start a gasoline engine, you would start by explaining the chemical makeup of diesel. Or, more likely, invite me to read a chemical treatise on the development of diesel.

I suppose that (one of) the difference(s) between us is that I would have no problems laying out rafters in fathoms.
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Old 12-02-2013, 23:49   #62
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Re: Coordinate Math Question

May I suggest you get a good book like "Navigation for the rest of us". Study it and see if you don't come up with an answer.
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Old 12-02-2013, 23:58   #63
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Re: Coordinate Math Question

I'll look into that. On that same subject, the book I believe everyone is referring to as "Bowditch" is now a free PDF from the Coast Guard. (The U.S. Coast Guard.)

So far, it hasn't done anything except ensure that it will be two weeks before my wife cooks for me again.

Your post comes as a refreshing breeze. I was expecting someone to write about the economic and political effects of the telegraph at any moment.
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Old 13-02-2013, 01:32   #64
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Re: Coordinate Math Question

Quote:
Originally Posted by Jammer Six View Post
Alright, let me re-phrase.

If I want to do latitude and longitude as separate operations, how do I set my dividers for 200 yards of latitude after laying out my grid to read in degrees-minutes-seconds?
Not sure this hasn't been answered simply yet
200 yards is 600 feet.
There are 6080 feet to a nautical mile, so 600/6080 = 0.09868etc.
Lets call that 0.1
Therefore 200 yards is very very close to 0.1 nm.

For all intents and purposes here, 1 nm = 1 minute of latitude. So 200 yards is very very close to 0.1 minute of latitude.

Therefore set your dividers off the latitude scale to be 0.1 minute apart.

Use the same method for any other number of yards to convert to latitude.

For longitude there is an extra step involved in that you have decrease the value of the distance by the cosine of the actual latitude in question.

For instance, if you are talking about 200 yards at the equator, multiply the distance by the cosine of 0 degrees. Now of course that is a special case as the cosine of 0 degrees is 1. Therefore 200 yards at the equator = .1 minute of longitude.

Now consider 200 yards at latitude 45. To convert to degrees of longitude, multiply the distance by the cosine of 45 degrees. IIRC the cosine of 45 degrees is 0.707, thus 200 yards now becomes 0.07 minutes of longitude.

Of course it remains as .1 minute of latitude.

Hope this helps .
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Old 13-02-2013, 01:33   #65
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Re: Coordinate Math Question

Jammer, on a chart the lat and long scales are not in degrees/minutes/seconds, but in degrees/minutes/decimalminutes. When you use lat or long in a mathematical operation you normally convert the minutes to decimal degrees by dividing by 60 and adding that fractional part to the whole degrees.

The idea with the gif file I posted is you save it to your computer and print it out.

Pick any point on the sheet for a reference, and set your lat and long from that. If you like, your Tyee Light can be at the dead center. It doesn't matter. You label the lat and long however you like. Convert your yards to nautical miles. This way you can use your latitude scale for distance. 6080 feet in a nautical mile, for our purpose.

Bowditch is the Navigator's Bible. It is dry reading, but it is complete and authoritative. I urge you to plod through it and work all the example problems, then when you get to the back cover, repeat.
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Old 13-02-2013, 03:30   #66
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Re: Coordinate Math Question

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Originally Posted by hellosailor View Post
Length of 1 degree of Longitude = cosine (latitude) * length of degree (miles) at equator
Ah, there we go, ta V much. not linear.

back to post 6..

not:

New Long= Old long + (sin(course)*dist*90/oldlat)

but:

New Long= Old long + (sin(course)*dist/cos(lat)

Which seems to tally up pretty well with openCPN over short distances anyway. (lat) should be halfway between old lat and new lat.

Seems to be a tiny bit out, maybe some spherical stuff coming into play or rounding up in Opencpn ?
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Old 13-02-2013, 05:41   #67
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Re: Coordinate Math Question

Here is my plot sheet generator program.

http://www.thevoicestoldme.com/Nauti...r_32_Setup.exe

It is a self extracting self installing exe file. Should run on anything from W98 to W7 64 bit. The screen does not display at full resolution, but the printed output will be very nice because functions are sent directly to a virtual printer object, and resolution is whatever your printer resolution is. Does not work for all printers, but most.
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Old 13-02-2013, 09:24   #68
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Re: Coordinate Math Question

Quote:
Originally Posted by Jammer Six View Post
Then I don't see what the grid has to do with the Tyee Shoal Light, if it can't be used to layout a desired point, whether that point is arrived at in cubits, yards or 64ths of an inch. Following this logic, if I had asked how to start a gasoline engine, you would start by explaining the chemical makeup of diesel. Or, more likely, invite me to read a chemical treatise on the development of diesel.

I suppose that (one of) the difference(s) between us is that I would have no problems laying out rafters in fathoms.
I would FIRST tell you to not put gasoline into a diesel engine ....
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Old 13-02-2013, 10:02   #69
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Re: Coordinate Math Question

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Originally Posted by HappySeagull View Post
I would FIRST tell you to not put gasoline into a diesel engine ....

i already done it. on my golf sometime in 1980's. can not remember.
but it was the other way round. diesel into petrol engine

it stalled about 100 yards down the road
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Old 13-02-2013, 12:04   #70
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Re: Coordinate Math Question

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Originally Posted by Jammer Six View Post
Alright, let me re-phrase.

If I want to do latitude and longitude as separate operations, how do I set my dividers for 200 yards of latitude after laying out my grid to read in degrees-minutes-seconds?
Use the easy measures it offers you. degrees,minutes,seconds,or maybe "cables"( a tenth of a minute)
oh look. even tailors can go to sea. Not the "cables" in the mile scale above.
Click image for larger version

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or this

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Take your dividers and compare these scales to the latitude marked on the sides of the chart.
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this is because it's a MERCATOR chart.

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note the central latitude. You will need to know this when you advance to geometry on a MERCATOR chart. proportion cos lat=long

do not be confused by this: it is for PLOTTING increments of longitude.

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horse to water, can't make it drink.

wherever you plot to, in feet, you will only be there for a split second, if at all. Current and wind will move you off it. By the time you know you are on it, you won't be. So, unless the scale of your chart is very very large scale, you are as well to plot "wide" or in the middle of dangers with seconds or tenths of cables (a circle 60 feet dia) as plot inside a few feet which are pretty useless on most charts, except as soundings...as to that...well again it's a matter of chart scale.
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Old 13-02-2013, 16:47   #71
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Re: Coordinate Math Question

Okay, Growly, I think I'm beginning to see. The rectangles created by the circle are the correct ratio of height to width for a given latitude.

The reason I think my charts are in degrees, minutes and seconds is because there are sixty subdivisions between the minutes, not ten or one hundred. I'll post a picture of it as soon as I go downstairs.

I appreciate the circle you posted, but the self extracting .exe won't work for me, as I run Macs. I think I'm going to look into a pad of the circles at Captain's Nautical Supply. Glancing through Bowditch, if I'm really going to work the examples, I'll probably need it anyway.

Reading the definitions in Bowditch was like reading the first few chapters of the bible, about who begot who. (Wait, I don't care. They're probably all dead, now, and reading it is making me older.) I think I'll save the definitions for when I need one, and then I'll go back and look just the definition I need up, and parse it word by word.

Conachair, Wotname, thank you. I see how the formulas will work, they're actually similar to jack-rafters.
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Old 13-02-2013, 17:57   #72
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Re: Coordinate Math Question

Lol! Well, I think I did warn you that Bowditch can be dry reading! Take it in small bites. Remember, once you know where something is in there, you can always refer back to it at a later time. Memorizing everything is only slightly better than simply being familiar enough with the book to be able to flip right through to the correct page or chapter. It is a good textbook but it is an even better reference book.

Okay on your charts... if there are 60 divisions between minutes, yeah, that is seconds. Decimal minutes has been the standard for a long time now. But if there is any question, by all means, scan an edge and post it. Right or Left edge... the Latitude scale. The reason for marking charts in decimal minutes of Latitude is it is more convenient to work with decimal miles than with 1/60 miles, and the Latitude scale is where you usually pick off distance measurements with your dividers.

And yeah, that's exactly right, about the ratio of Lat to Long spacing. That's why the 30 degree angle is used to set the Longitude spacing correctly for 30 degrees of latitude. Or 40 or 57 or 13 or whatever latitude.

Sure, a pad of commercially printed plot sheets will do you fine. You can use them for practicing radar plotting problems, too. Time/Speed/Distance problems, lots of different stuff where a graphical vector solution is desired.

Now you are starting to learn some of the cool stuff about navigating. Feels good, huh? You will be shopping for a sextant before you know it!
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