Calculating Compass Headings?

[JackD342]

No, nothing related to visibility, I meant that the magnetic declination value isn't a factor for swimming when fins underwater for the very short distances we cover under water. When you take a heading with a compass on the surface to your destination and then go down and follow the same course to the destination, magnetic declination isn't a factor. When you turn back to swim out on a reciprocal course, it is still not a factor.

If you are on a boat and using charts for navigation on longer distances, you will need to account for the declination for even small variance may cause great variance from the destination causing you to not find the exact point you were going to because of the magnetic declination.

Part of the thread concerns a wish to navigate from a true to scale map well marked map, rather than taking a heading. Or even creating such a map. At which point one needs to understand magnetic declination, and perhaps even grid north depending if starting with such a map.

 

[BlueTrin]

Part of the thread concerns a wish to navigate from a true to scale map well marked map, rather than taking a heading. Or even creating such a map. At which point one needs to understand magnetic declination, and perhaps even grid north depending if starting with such a map.

Correct I missed that, OP is strictly speaking about navigating without taking bearings at the surface and this is what we were talking about.

Now I understand BurhanMuntasser‘s post

 

[BoltSnap]

Part of the thread concerns a wish to navigate from a true to scale map well marked map, rather than taking a heading. Or even creating such a map. At which point one needs to understand magnetic declination, and perhaps even grid north depending if starting with such a map.

The map was created by compass navigation to begin with, all bearings are based on starting with a compass not a nautical chart.

 

[BlueTrin]

The map was created by compass navigation to begin with, all bearings are based on starting with a compass not a nautical chart.

Jack proposed this:

You can use Google Maps aerial view to get a more accurate image for the perimeter of the lake, and also correctly oriented. Then you can create your map to scale based on the various distance and headings from the billboard map, assuming they are accurate of course.

Also, assuming that Google is true north and your map photo is magnetic north, you can be more exact by adjusting for the local declination. Looks like magnetic north there is about -3° from magnetic north, but go ahead and double check that.

The discussion about correcting for declination comes from this post.

 

[JackD342]

The map was created by compass navigation to begin with, all bearings are based on starting with a compass not a nautical chart.

And the OP went on to say:

It would be nice to create a course without having to surface to grab heading from the surface buoy markers. I feel like this would be easy if I had a map that was to scale, but that is not the case.

 

[Altamira]

The map was created by compass navigation to begin with, all bearings are based on starting with a compass not a nautical chart.

This exactly. The issue of magnetic declination is a factor when planning a navigation course from one point to the next, or a series of points, plotted on a map with latitude and longitude lines. Without a map with accurate lat/long lines we are basically following a magnetic heading from point A to point B on whatever excuse of a map you are using. In a body of water with limited vis and no current about the best you will be able to do is try to keep a constant heading and have a method to determine how far you have finned from the start point. Add some current, then you will have to adjust your magnetic heading for the drift as well, just like a pilot has to adjust magnetic heading for wind drift.

 

[Ghost95]

Go to calculator.net and use the triangle calculator.

You'll have to remember that compass headings are different than degrees as far as the triangle calculations are concerned.

For example, the ski boat has a bearing of 10° from the dock and the bus of has a bearing of 33°from the dock. The angle that you need to input into the calculator will be the difference of those two or 23°. Then you put in your distances and hit calculate.

The program will build a triangle with angles and distances. You will have to apply those angles to the reciprocals of your given headings. If you want to apply to your given heading you have to use the outside angle who's is 360°- the inside angle you get from the calculator.

So after inputting what you have been given this is what I got.

Dock to bus is 105'@33°.
Bus to ski boat is 93'@344°.
Ski boat to dock is 180'@190°.

Just remember that the angles given by the calculator have to be applied to the reciprocals of your courses they are not the bearings between points.

I hope you can make sense of this. If you look at the triangle that the calculator builds and visualize it over your map I think you'll get it.

Someone may be able to explain this better for you though.

Good luck and safe navigating.

 

[Storker]

Two ways I can think of:
1. Use trigonometry and a calculator.
2. Redraw the map to scale using the info on it, and measure any angles/distances you want from your drawing. I'd recommend this mefhod!

I'd use trig.

It depends on how rusty your high school math is and if you're weird enough to find trig amusing, though...

 

[Ghost95]

If the map is to scale then measuring and using a compass should work.

If the map is not drawn to scale, but the range/bearing numbers are correct then you can get the numbers you're looking for by mathing it. You'll need to do it before the dive though.

Once you have the calculations you can draw your own map to scale with headings between all points. It's actually a fun exercise and will make you a better navigator.

 

[Dubious]

'd use trig.

It depends on how rusty your high school math is and if you're weird enough to find trig amusing, though...

I was never good at trig. I would have to brush up. I don't use too much math in my profession.

I use Google Earth to do this with great accuracy.

I will give this a try. I might even try from google maps first since I don't have easy access to google earth.