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Computing ferry angles to compensate for cross current and/or cross wind.


leong

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I hope you realize that you don’t need to concern yourself with ferry angles or ranges when you are using a GPS that is reliably working.

 

Of course, you should be familiar with ferry angles for when you won’t or can’t rely on a GPS.

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Then again, a GPS knows nothing about cross winds or cross currents and wouldn’t know a ferry angle from a ferry boat. So how does it help you keep your "course over ground" on the straight line from your position to the waypoint you desire to go to?

 

It cares nothing about its orientation; i.e., it even works if you point it up or point it down or face it backwards.

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I disagree. While I would never recommend relying on GPS  apps exclusively for all the obvious reasons, they can be more useful than you suggest. Let's assume you've been out to visit some islands out of Portsmouth and on your return a fog rolls in lowering visibility to 100 yds. With an app like Gaia you could go to your saved waypoints and select Jaffrey Point, at the entrance to Little Harbor. You then choose "guide me" and the app will show a straight line to the point and give a bearing, let's say 270 magnetic. If you have slack current and no wind, that bearing should bring you pretty close. You could check from time to time to see if your bearing has changed, or if it's calm enough you could secure your phone to the deck and try to stay on the line. 

After say 10 minutes of this you see that your bearing has changed to 275.This means you need a course over ground that is slightly more northerly because you have drifted to the south on the ebb. You make a correction in course to 280and paddle on. Eventually you find you can stay on a straight line course of 278 by holding your kayak compass on 285, having empirically established a ferry angle of 70

I can almost feel the breeze from the purists shaking their heads. I know. These things fail. They can't be relied on. You can't deal with a phone in conditions. But neither can you deal with a map and compass in conditions. I've never had to rely on this in a real situation but I have practiced it and am glad to have it as a tool when the fog rolls in.

Maybe we could do an island stabbing trip sometime using GPS only and see how we do. 

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Jim, I’m not sure you disagree and I don’t agree your way is very simple because it underutilizes the way a GPS works.

 

First some essentials of a navigation GPS (turn off compass if it has one):

The GPS knows the lat/long of where it is and it knows velocity it is moving (speed and direction).

 

Say you enter the location (lat/long or whatever map coordinates) of the place you want to paddle to; i.e., the way point. Assume, like in your example, it’s a foggy night, but also assume that the wind velocity, current velocity and paddling velocity are variable (Note velocity is speed and direction).

 

1.       You could estimate your paddling speed, the changing cross current speed and the changing cross wind resultant cross drift speed and do some complicated arithmetic to compute your ferry angle. Of course, it will change in accordance with the changing the wind velocity cross drift and current velocity.

2.       You can do as you said which will work fairly well even with variable wind and current velocity.

 

 

Now here’s the easy and most accurate way to paddle to your waypoint (don’t forget to turn off the GPS’s electronic compass): Just paddle so the arrow in the GPS compass page (see picture) is always pointing to the top of the display. After a while if the arrow is a little to the left of the top just paddle a little to the right to bring the arrow back to the top of the display. Similar for a right correction. Note some GPS models use a page with a straight line and a dot or something to keep on top of the straight line.

 

 

What could be easier and more accurate than this? In essence, the variable current velocity, your variable paddling velocity and the variable side drift due to variable wind velocity are all implicitly taken care of by the way a GPS works. The GPS doesn’t have to calculate any of the variables and use them to calculate a variable ferry angle.

 

 

I use this method whenever I paddle back from the Isles of Shoals to Little Harbor. It so good that on a windy day with a strong tidal current It would take me into the harbor’s inlet even just by following the GPS.

 

Of course, one should know how to use ranges, calculate ferry angles and use a compass.

 

 

compass page.jpg

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I think the main difference in how we're looking at this is that you're assuming a standalone GPS device and I'm assuming a phone app with additional navigational features. Couldn't hurt to have both. Incidentally, the app I referenced, Gaia, is having some trouble displaying NOAA charts as they are morphing into electronic only. It still works but without the previous chart detail.

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Most GPS “backcountry nav” phone apps including Gaia GPS do have some version of the feature Leon is talking about. It can take a bit of research to find the feature though. 

To access this feature on Gaia:

- tap your destination on the map. A details panel will appear at the bottom of the screen  

- tap the “…” button

- choose “Guide me” which displays a line on the map from where you are to the destination  

- put the map in compass mode by tapping the cross hair icon until the icon turns green and the map rotates dynamically with the phone orientation  

- maintain course such that the line always points straight up on the screen. (Not like shown below - I need to go more to the left!)
 

2D5A5039-BC0E-46F5-BD1E-BE140B936E2F.thumb.png.16578d394ad38c84c5bc46e3b0aa337d.png

Having said all that, I maintain that if you can see anything, using a visual range to maintain course via a ferry is always going to be just plain easier than trying to use a device that is not your eyes and brain. 

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2 hours ago, Joseph Berkovitz said:

Most GPS “backcountry nav” phone apps including Gaia GPS do have some version of the feature Leon is talking about. It can take a bit of research to find the feature though. 

To access this feature on Gaia:

- tap your destination on the map. A details panel will appear at the bottom of the screen  

- tap the “…” button

- choose “Guide me” which displays a line on the map from where you are to the destination  

- put the map in compass mode by tapping the cross hair icon until the icon turns green and the map rotates dynamically with the phone orientation  

- maintain course such that the line always points straight up on the screen. (Not like shown below - I need to go more to the left!)
 

2D5A5039-BC0E-46F5-BD1E-BE140B936E2F.thumb.png.16578d394ad38c84c5bc46e3b0aa337d.png

Having said all that, I maintain that if you can see anything, using a visual range to maintain course via a ferry is always going to be just plain easier than trying to use a device that is not your eyes and brain. 

You're right, Joseph - can't argue with that.

However, the point of my post was to make people aware of how a GPS really works. You don't need a phone or a phone app. Just (as in my picture) keep the arrow pointing to the top of the display and there is no need to think about any variables like ferry angle, wind velocity, current drift velocity or paddling velocity. The geometric physics of how a GPS works eliminates all of that. So, in the "goto" waypoint mode the GPS allows you to keep your velocity vector (course over ground velocity) directly on the straight line to the waypoint no matter any side movement caused by wind and/or current. The GPS doesn't know anything about the heading of your kayak, it just knows your velocity vector.

Nevertheless, I don’t know, but the Gaia GPS phone app may add some information useful for the paddler.

I hope everyone now realizes that the complications of ferry angles become irrelevant when you use a GPS.

However, I'm all for knowing how to use ranges and estimating ferry angles.

 

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I just found a similar thread I posted back on 8/7/13. Perhaps this text that I posted then will help clarify this new topic I started:

The GPS knows the kayak’s location and the waypoint’s location in some fixed earth coordinate system. It doesn’t know its own orientation (what direction it’s facing or even if it’s pointing backwards or straight down). It also doesn’t know the heading angle of the kayak. But it does know the Course-Over-Ground (COG) that the kayak is moving along as you paddle and drift with the wind and current. If the COG coincides with the beeline from your present location to the waypoint, the GPS’s arrow (at least it’s an arrow with my GPS) points to the top of the screen. If the kayak’s COG for this latest update is to the left of the present beeline, then the GPS’s arrow points to the right (indicating that you should change your heading to the right), and vice versa. Of course, your COG is the result of the vector sum of the kayak’s forward speed and the cross-velocity vector due to current, wind and wave action. So, the GPS doesn’t really have any idea of current velocity (I mean water speed and direction) or wind or anything else that’s pushing the kayak. All it really knows is the COG and the latest beeline to the waypoint. If you try to keep the arrow always pointing to the top of the screen (via heading changes as necessary) you will be following a straight line to the waypoint.”

I also said in response to someone’s question:

The arrow doesn’t point to the waypoint. It shows you whether your current COG is to the right or left of the COG to the waypoint. But when the arrow points to the top of the display your current COG corresponds to the COG to the waypoint, but it's not pointing to the waypoint. I’m mentioning the arrow because that’s what I use with my GPS. There are other screens like a line on a map that do the equivalent.

Also note that any GPS compass must be disabled.

 

 

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  • 1 month later...
3 hours ago, Joseph Berkovitz said:

If you have two or more people, each using Josko’s strategy, the results may be interesting…

Isn't that the common adventure model? ?

Edited by josko
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>>Most GPS “backcountry nav” phone apps including Gaia GPS do have some version of the feature Leon is talking about."

What I'm talking about is not a feature of a GPS no more than a steering wheel is a feature of an automobile. It's just how a GPS works. You don't have to add anything fancy to it unless it gives you something more useful.

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