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Old 12-01-2009, 11:04   #1
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Why Waypoints?

Ok, this may be dumb, but here goes:
Why do we need waypoints when going from here to there? I notice J. Cornell's world cruising book is full of waypoints for various ocean passages.
Surely you will never steer to a point in the ocean unless it is your intended port of call.
If you're going from here to there don't you just steer the best course possible, given sea, hazard and wind conditions. Take a reading on your GPS once a day and plot the Long. and Lat. on your paper.
I ask these questions knowing very little about GPS navigation except that it tells you where you are. What else do I need?

Pete.
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Old 12-01-2009, 11:20   #2
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No question about sailing is dumb.

In fact it shows incredible intellect that you've chosen the right sport to ask about:-)

Re the question. Having a minimum of 2 waypoints allows you to plan your route.

When underway steering a fixed heading over a typical Cornell route is rarely even possible but knowing the course from A to B by a great circle (the shortest) makes good sense. Your 'shortest mile' or rhumb line course would then be a series of smaller steps, each with a different compass heading

Even on other one off / relatively short (ie non Cornell) routes, steering on a single fixed heading might not be the best thing to do. As you've already recognised, as you'll be influenced by other external things.

If there were no tides or currents between the two closer waypoints and you were under power - a fixed course might get you close. But in the real world it rarely does.

Finally it is good sense to log your position from GPS say each hour rather than once a day. No need to plot it on a chart - just a note book / log works fine. This is just in case the electronic bits let you down - and if so you are at least reasonably confident on where you are and can move to plotting on charts using DR from that point onward.

Enjoy
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Old 12-01-2009, 11:50   #3
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A Mercator chart is not the actual shape of the earth. I'm sure you know that. Over great distances a great circle drawn on a Mercator chart is actually a straight line as drawn on a sphere. Look at a small scale chart which shows an entire ocean and run a line from one side of the ocean to the other. Now take a globe and run a string from the same place to the other same place. You will see quickly that the string and the straight line do not cover the same part of the Earth. You will see that the string in the northern hemisphere passes closer to the north pole. This is why a great circle drawn on a Mercator chart always bends towards the closest pole.

The other reason we have waypoints is sometimes the straightest route is not always the best route. Sometimes there may be land masses in the way. Sometimes the safest or fastest or most comfortable way of crossing the water is not always the shortest distance...frequently in fact.
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Old 12-01-2009, 12:01   #4
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Thanks boys.

I am still puzzled about this idea of steering to a waypoint.
If I were under engine power all the time it would make sense. But with sailing, the damn wind blows from where you want to go and it is a never-ending compromise of headings one steers when trying to make the objective.
Where do waypoints come in?

Open mind, but vacant.

Pete.
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Old 12-01-2009, 12:09   #5
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Your heading is the direction your bow is pointing. Your course over ground is the actual path you are taking across the earth. Your course to steer to make your destination or waypoint, is rarely the same as your heading. Your course made good is looking back at the path you took over the earth to get to where you are now from a place where you were earlier.

Your waypoint is your goal in a sense. Its what you steer the boat to get to.

Sometimes with a sailboat you have to make a series of tacks in order to get to your goal...or your waypoint. Sometimes with a sailboat you can steer the boat directly towards your goal.

If you are serious about getting yourself to a place you cannot even see, you pick a course to get you to your next turn (a waypoint) or your destination (also a waypoint in a sense).

Does that answer it?
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Old 12-01-2009, 12:21   #6
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how does a gps work out its route between 2 waypoints, is it great circle or a fixed heading, i've only ever used a gps with distances short enough that it wouldn't matter.
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Old 12-01-2009, 12:34   #7
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VMG

Another reason to use waypoints; it allows for the use of VMG (velocity made good) which will give you an indication of which tack is more favoured.

Jack
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Old 12-01-2009, 12:58   #8
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Ok, Jack

The VMG deal makes sense. On a trip from Victoria to Honolulu there would be times when the destination (my one and only waypoint entered, e.g.) would be faster reached by one tack or another, according to the VMG result.
Do most/all GPS units give you VMG?

I last sailed 30 years ago, from Vancouver to Brisbane and used the damn sextant. It worked with a noonsite, when possible. I'll be glad to be rid of it.

Many other things have changed as well and all for the better and easier.

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Old 12-01-2009, 13:00   #9
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Quote:
Originally Posted by rigormortis View Post
Ok, Jack

Do most/all GPS units give you VMG?


Thanks,
Pete
Yes, most marine GPS units provide VMG; among a whole host of other bits of info.

Jack
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Old 12-01-2009, 13:46   #10
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I now understand your screen name To get from where you are to where you want to go, you need to know two things: where you are and the quickest possible route to follow to where you want to go. Neglecting the effects of weather and geography, a great circle route would be shortest. It is impractical to represent a great circle route as a continuous curve (implying constant, miniscule course changes). Rather, plot it as a series of waypoints. Next step, prior to your voyage, adjust those waypoints to take advantage of favorable current or weather patterns, thus modifying the "shortest" route into the "quickest" route. Once underway, you have the choice of either adjusting your course made good to hit the waypoint, or adjust the waypoint to better suit the weather. At the most basic level, all the waypoint gives you is a better estimate your most efficient course to make good. How efficient depends on how good you are at factoring in weather/current to set your waypoints. Waypoints out of books can only factor in geography and prevailing winds/currents. It is up to you to modify the route based on actual conditions.

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Old 13-01-2009, 00:34   #11
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Go use a chartplotter for a day or 2 and you will never use a: "GPS once a day and plot the Long. and Lat. on your paper."

We just came over a very dangerous bar into a tidal estuary - Southport Seaway for the Aussies
There are leading markers on the rocks and buildings and Nicolle was going to use them and I was using my plotter.

We didn't even SEE the physical markers!

But on the plotter we surfed a coupla waves right into the Seaway. Hairy stuff! Great adrenalin rush! But in safety as the computer had it down to the merest inches.

To do that you need waypoints. A waypoint is just a term for Latitude and Longitude. How you use them are up to you. Some waypoints we enter with a little graphic of a skull and cross bones. they're the bits to stay away from

Remember you don't have to follow a path along or to waypoints.

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Old 13-01-2009, 00:43   #12
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Old 13-01-2009, 08:14   #13
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Quote:
Originally Posted by martinworswick View Post
how does a gps work out its route between 2 waypoints, is it great circle or a fixed heading, i've only ever used a gps with distances short enough that it wouldn't matter.
Honest, you'll have to read the manual to know for sure.
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Old 13-01-2009, 08:26   #14
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Quote:
Originally Posted by rigormortis View Post
Ok, this may be dumb, but here goes:
Why do we need waypoints when going from here to there? I notice J. Cornell's world cruising book is full of waypoints for various ocean passages.
Surely you will never steer to a point in the ocean unless it is your intended port of call.
If you're going from here to there don't you just steer the best course possible, given sea, hazard and wind conditions. Take a reading on your GPS once a day and plot the Long. and Lat. on your paper.
I ask these questions knowing very little about GPS navigation except that it tells you where you are. What else do I need?

Pete.
The real world is a little more complex than what's seen on a simple chart or world map (look at pilot charts, for example, for a comparison). Cornell's routes take into account things like ocean currents (for example, heading south down the US East Coast, why jump into the north-setting Gulf Stream, even if it's the shortest distance?) and major weather features (why not grab a lift from the winds around the Bermuda high?).

Pulling one fix a day (whether GPS or celestial navigation) would mean you might not find out you're stemming a foul current (possibly an eddy from a major current or just something set up by the wind) for some time - possibly time wasted going nowhere in a hurry. Checking your position and SOG (speed over ground) hourly would help with this issue.

However, you're right about asking "do I really have to sail to that specific spot on the chart". Of course that's not necessary. But the basic rhumb lines presented are more than just arbitrary lines drawn on a big map. Think of them as guidelines (literally and figuratively).

HTH
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Old 13-01-2009, 10:31   #15
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Quote:
Originally Posted by martinworswick View Post
how does a gps work out its route between 2 waypoints, is it great circle or a fixed heading, i've only ever used a gps with distances short enough that it wouldn't matter.
I think most can do either, depending on the set-up; check your user's manual. If that doesn't help, then punch in two waypoints; use the exact same latitude, but put WP2 100 degrees West of WP1, then calculate the course. If it's 270 then you have a rhumb line; if it's 3-hundred and something, then you've got a Great Circle departure. Don't use zero for Lat.
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