Posted: Dec 05, 2011 - 06:41 PM
We use magnetic in normal areas, true when going up over the polar regions,(as the isoganals are converging down into the center of the earth) & sometimes "grid nav" depending on what escape chart you have.
In my bush pilot days we used to use astro compasses in the high arctic, taking star & sun shots in order to get a "true heading", that would not be doable on a Cat. You can also obtain pretty accurate true headings by converting time to longitude, knowing that the relationship is 15.04 X the sine of the latitude. That sounds rather complex to figure out a heading, right? You can dumb it down by multiplying your hour angle, (converted to "zulu" or GMT by 15, that number is your true heading, point right at the sun, & set it on the compass. Of course it sucks if it's heavily overcast, or the sun has set, or you don't have time to point right at the sun, aka in a race.
You can also point mickeys little hand at the sun, 1/2 way between his hand & the number 12 is true south.
Nearly all GPS data in North America conforms to World Geodetic System 84, (WGS84) & most aviation & marine maps are Lambert conformal conic projections, with two standard parallels, but you don't need to know any of this.
Do you see where I'm going with this?
For the entire USA, including Alaska, ( the isoganols are far enough apart not to matter) the entire argument isn't worth a mouse fart in a hurricane.
Use the KISS principle... Keep It Simple Stupid!
If you are poor use a compass, preferably liquid filled so as to provide some damping. If you have more cash get a low end GPS. If you have a little more funds get a GPS with the electronic compass, you don't have to be moving to get a bearing.
The important thing is planning BEFORE you sheet in. I've tried a Garmin 76CX, you just don't have time to look at that little screen & make adjustments as it changes. It is very easy to get suckered into flying/sailing heads down, constantly watching the compass. That will get you killed, literally, quite quickly in my business. Mark your course out on your marine map, & pull the headings by a parallel rule, or just eyeball it with a pencil & the compass rose on the chart, you will be within a degree or two. That is the magnetic track to make good. Line the boat up on that track, & keep it there, sailing heads up by sighting on a distant object, or even a distant cloud, realizing you will have to adjust your heading to account for tides & leeway, to make good your intended track.
It doesn't matter in your part of the world if you use true or magnetic, as long as you are consistent. If you pull the courses off the chart in true, set your GPS to true, or convert. I think it is easier to do it all magnetic, as all charts have a compass rose on them that is oriented to magnetic, & it saves remembering "east is least, West is best, & doing mental gymnastics to convert on the fly.
As stated by others, I wouldn't waste time making it more complex than need be. In the days before GPS, across the featureless barrens,(much like being out of site of land) we flew for hours on simple "dead reckoning", euphemistically know as the "bout there" method. You already know the heading from point A to point B is xxx. Set out on that course with a quick reference to your compass, then stay heads up, & maintain the course by keeping the sun, landmark, anchored boat, distant cloud, at the same relative bearing. You look down to adjust something, look back & notice the relative bearing has changed. Turn the boat til your crew yells, "bout there, that looks right" & drive on. You will maintain a consistent course by looking outside, not staring at a compass. Even after several hours, if you have maintained a consistent heading you will only be "out" by the tide drift or leeway.
Edited by Edchris177 on Dec 05, 2011 - 06:47 PM.
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