How close is close

i am confused - you say call it a day and then another post is sent! :L-)

"if you are forced to pass close to every waypoint before it changes "

Nothing I have said stops you aiming close to every waypoint and being within the proximity zone for everyone.

There is no such thing as 'forcing' and for a number of reasons you can miss a waypoint by just 100 yards without having the alarm go - in such cases it would be better to accept the next waypoint than double back and then back again.

If you are miles out then this is your fault and has nothing to do with the GPS. In that case an alarm to offer you a choice is better than no alarm at all because if you are miles out it has nothing to do with anyone but you - it was not the fault of the GPS.

"If the GPS/plotter allows changes to the next waypoint when are at any distance from the present waypoint (albeit at 90deg to the track) then you are most definitely not on your predetermined track and you are going to have to look at your chart or plotter screen to check for hazards"

What Mike is missing is that you are out anyway - and not on your predertined track under all GPS options - he is somehow saying that by virtue of the fact that thge alarm at 90 degree will trigger this somehow leads to you being way out of your track and this is nonesense.

In either event - with Mike's no 90 degree preference or the standard 90 degree perference you are miles out - all that matters is what options and warnings you are offered when you are miles out. In both cases the charts/plotter has to be checked for hazards.

In practice I aim for each waypoint and normally have no problem in meeting each one but sometimes I have to miss it - maybe bat as little as 0.2Nm ( this miss is the same if you have the 90 degree warning or not and you are in off your route win both)- in those cases the 90 degree warning comes on and I can either head back in unplanned route to the waypoint OR accept the new waypoint also on an unplanned route - in either event I have to check the chart/plotter. At this point being offered the choice is good and no matter which one you take you have to check for hazards.

Using the method of Mike and you get 0.2 Nm out he has no alarm to state that you are now have a shorter route to the next waypoint and just has to double back and get to the waypoint through unplanned waters that have to be checked for hazards. The only difference is that he had no alarm and was not offered the choice!!!!!!


I am happy to leave it at that point - if Mike does not respond, I will not do so to him. If he chooses to go on I shall answer - I am totally frustrated by the simplicity of the logic here not being seen :-)

<hr width=100% size=1>Paul
 
You and I have both agreed that our respective logic is irrefutable. Actually I dont think either of us is wrong just not seeing the same situation from the same viewpoint so lets leave it at that

<hr width=100% size=1>
 
Sorry, no offence, I must have misunderstood what you meant

<hr width=100% size=1>
 
"You and I have both agreed that our respective logic is irrefutable. Actually I dont think either of us is wrong just not seeing the same situation from the same viewpoint so lets leave it at that "

I am sorry but I really think one of us is illogical. I wish it was just a differnt view point. However, I do agree to leave it at that though.

<hr width=100% size=1>Paul
 
OK, I'll put it another way, if you like. We both agree that we are both talking bollocks and leave it at that

<hr width=100% size=1>
 
Wow this started a debate, not sure I can follow all of it. But can say my Garmin 76 and 182 seem to follow Gludy's +/-90 degree rule. All works fine, I usually manage to pass within a few hundred feet of a waypoint, and it swtiches to next one smartly. When I have strayed off the straight and narrow, still works fine, a few miles. But the geometry means it doesn't switch quite so smartly.

Now the main reason I've posted, another GPS query. Used to have an old Magellan, and when achored it would indicate zero speed all the time. It's almost as if it had some code along the lines of,

If Speed < 0.5 then speed=0

But when I went to Garmin, I get readings of speed of 0.1 or 0.2 Kn as the boat swings about. That's fine, in theory, 'cause probably am doing 0.1 or 0.2 kn, and speed is not a vector quantity. The problem is, in my experience, that the anchor drag alarm seems to integrate the speed. So even if tied to firmly anchored buoy, the alarm goes off after a while.

This, as far as I can see, makes the anchor alarm useless. Anyone else found this, is there an answer? Increaing the speed averaging time doesn't seem to be it.

<hr width=100% size=1>Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'
 
Nearly every modern gps/plotter follows the 90 degree rule.

Beside the Raytheon system in the boat I have a hand held Garmin Map 76. As you say both have the 90 degree rule.

"The problem is, in my experience, that the anchor drag alarm seems to integrate the speed. So even if tied to firmly anchored buoy, the alarm goes off after a while."

The amchor drag alarm works by your position change - its your actual co-ordinates plus the allowance you set that determines if you have drag. It should be nothing to do with speed. So changing your speed parameters will have no effect.



<hr width=100% size=1>Paul
 
Doesn't seem to.

I can set the trip distance to zero, even when on buoy, and watch the trip distance increase, in feet to start with. If I set the anchor alarm to say, 250 feet, when the trip distance reaches 250 ft...Bleep....Bleep. If tied to a buoy with say 20ft of line, shouldn't stray by more than 40ft.

<hr width=100% size=1>Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'
 
That's what I thought. Both the 76 and 182 behave the same. Keep meaning to talk to Garmin about it.

<hr width=100% size=1>Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'
 
When at anchor the rising and falling tide alone will cause the boat to travel some fair distance around the anchor point. To base any alarm on a trip meter would seem stupid. To base it on tnhbe actual co-ordinates is the logical means of doing it.

I will check out my Garmin 76 at some point to see how that does it.

<hr width=100% size=1>Paul
 
In an ideal world

In an ideal world every electronic navigator would work the same way (provided that way was great) but they don't. Behind the electronic screen lies the very human hand of programmers, many programmers in fact. If you get lucky (Garmin actually being a good case in point the programming team set up protocols they tend to stick to but if you get unlucky you will find differences even between units in manufacturers' ranges.

In many cases Deleted User and powerskipper are right; you have to be within the set proximity to jump to the next leg. You can argue that this is a poor safety feature or in fact you can argue it is a very good one. If, for example, you were well off track and the unit simply switched to the next waypoint when your boat crossed an imaginary point the next track could be far more disastrous to the less well focused navigator than seeing the DTG count start to climb again and the bearing go haywire (which is a pretty fair indication that something is going wrong).

What happens next is also interesting. If you miss a waypoint and jump to the next one you also have to worry about the fact that you have shifted the intended track...well at least you certainly need to worry if there are any hazards nearby, or if the vis is poor and you expecting to see something downtrack and so on.

I wrote a (probably over-lengthy and over-complicated but phenominally interesting (to the author)) piece on this sometime back, looking at common electronic navigator errors where one of the highest was not understanding how the unit worked in relation to GoTo, follow track and route commands. Not quite sure when now but at around the same time we also looked at issues such as datum and even great circle versus rhumb line working (most navigators don't give you the choice but do you know how they navigate and does it matter?).

<hr width=100% size=1>
 
Re: In an ideal world

Kim
Whilst I can agree that there a variations in the manner in which untis are programmed, the issue here was initially because of confusion caused by the user not understanding the 90 degree rule. Once explained then what happned to the user who posted the question is fully explained.

"In many cases Deleted User and powerskipper are right; you have to be within the set proximity to jump to the next leg."

It was agreed on the thread that this happned to Mikes old autohelm unit. However, as you know nearly all, if not all modern units apply the 90 degree rule.

"If, for example, you were well off track and the unit simply switched to the next waypoint when your boat crossed an imaginary point the next track could be far more disastrous to the less well focused navigator than seeing the DTG count start to climb again and the bearing go haywire (which is a pretty fair indication that something is going wrong)."


I, and I think every person on the thread agrees with that one and noone at any time suggested that the 80 degree rule should switch you to the next leg without you choosing to accept that. This was never an issue.

Surely far better than seeing the DTG counter climb again is a warning that your intended waypoint is abeam and you have a choice - keep heading towards your intended waypoint OR accept the track to the next waypoint. In both cases you are off your intended route and need to check for hazards. Thsi option actually makes the situation much clearer to the user and avoids the frustrtation that Mike mentioned having with the old system. To the best of my knowlledge this is how all modern gps units work anyway. I think everyone has changed to having to confirm the next leg of the route before proceeding - an essential safety measure.

"What happens next is also interesting. If you miss a waypoint and jump to the next one you also have to worry about the fact that you have shifted the intended track...well at least you certainly need to worry if there are any hazards nearby, or if the vis is poor and you expecting to see something downtrack and so on."

But this was never accepted as safe by anyone on the thread - was pointed out as unsafe and is not how the 90 degree rule works.

"looking at common electronic navigator errors where one of the highest was not understanding how the unit worked in relation to GoTo, follow track and route commands"

This thread proves that. A question was asled about a unit that was in practice being triggered by the 90 degree rule and answers that were given about the proximity to the waypoint were wrong as this was not the trigger. Both the person who asked the question ands some who answered him were wrong - hence the long thread in what is a very simple point.

I know that Garmin, Raytheon, Simrad all have the 90 degree rule - my guess is that Furono has it as well. I will have this checked out. I think a good artcile is needed along the lines of ' you think you may know how to follow a eoute but do you'.

Cheers


<hr width=100% size=1>Paul
 
Re: In an ideal world

Having not been around for the last 24 hours I'm just working my way throught the replies - however whats this
"Both the person who asked the question ands some who answered him were wrong"
explaination please .......


<hr width=100% size=1>Adrian
<A target="_blank" HREF=http://www.arweb.co.uk/argallery/kelisha>More Pics of Kelisha</A> /forums/images/icons/smile.gif
 
Re: In an ideal world

Kim - I would like to ask a few questions directly to you.

Case 1 - Missing by 2 Miles
a) No 90 Degree rule - Given that when missing a waypoint proximity zone by say two miles for whatever reason with a system without the 90 degree rule - you would get no alarm and be offered no choice - your DTG would increase and so eventually you would have to head back un what are unplanned waters to the waypoint.

b) With 90 degree rule - now you get an alarm to say you are passing your waypoint and the indicators show it is 2 mile abeam. You are then offered a choice - go directly to the bext waypoint OR turn and head towards the original waypoint. In both cases, with both options you need to check for hazards as your XTE was 2 miles adrift.

Question - please explain what is unsafe about option (b).
Would you not agree that it may even be safer to get the warning and be given the options rather than no warning at all?

CASE 2- Missing by 100 metres (say because of cemmercial traffic)
a) No 90 Degree rule - First you get no alarm and so do not notice until your DTG starts increasing then you have to turn back to get within the waypoint zone and then back out again - this is regardless of traffic or tide conditions.

b) With 90 degree rule - First you get an alarm then you have the option to continue to the next waypoint or turn back. In front of you is your XTE and you can see that you only missed the waypoint zone by a whisper.

Please explain how option (b) can be less safe than option (a) without the alarm.

What really matters in using electronics is that the user understands the principles upon which the electronics work - there is no substitute for that. As it happens the 90 degree rule with the need to accept the options is clearly more logical and safer than not having it - however its use must be understood.

I look forward to your direct answers.


<hr width=100% size=1>Paul
 
Re: In an ideal world

"Both the person who asked the question ands some who answered him were wrong"
explaination please ......."

The person who asked the question made an assumption that the trigger to the next waypoint depended on distance. He asked what distance 6, 60 or 600 feet etc. This assumption was wrong because it did not depend on distance to the waypoint it was triggering on the point that the 90 degree line was passed.

He recieved answers about the trigger being the proximity alarm settings - this was also based on the same assumption and was incorrect.

Nothing personal is intended in stating maters of fact ... its just a matter of fact :-)


<hr width=100% size=1>Paul
 
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