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Old 14-11-2017, 12:16   #16
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Re: GPS drifting ? At anchor

Taking another look at your pic .... if the "+", is your anchor location, could you have possibly entered a direction/distance to you anchor, when you should've entered the direction/distance "from", your anchor?

If the "+", is your anchor location, then what would've happened if the wind had blown toward the land?
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Old 14-11-2017, 12:24   #17
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Re: GPS drifting ? At anchor

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Originally Posted by Dave_S View Post
So after a dodgy night's sleep I awoke to find I hadn't dragged but that the equipment was wrong.
How did you confirm this?

That looks exactly like what I'd expect to see with a dragging anchor.

Yes, you can get all kinds of GPS errors, small and large. None of them present themselves as a steady down-wind or down-current creep over time.

Are you sure your GPS position was 180m from where you'd started? That much error would be highly unlikely, even briefly.
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Old 14-11-2017, 12:25   #18
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Re: GPS drifting ? At anchor

what 3 sats give is a potentially poor(maybe very poor), location, and without a 4th sat, there is no check for the software to compare it to ... redundancy is actually very important.
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Old 14-11-2017, 13:44   #19
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Re: GPS drifting ? At anchor

Struck an interesting case when cruising in Fiji. A neighbouring yacht set the anchor alarm just before going to bed. Unfortunately in the meantime the yacht had dragged about 1nm along the beach unnoticed and passed through a reef where the anchor stuck. It was very dark at the time. In the morning they woke up and got quite a fright finding themselves an a little nook in the reef and no other yachts nearby. Very lucky .....
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Old 14-11-2017, 15:07   #20
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Re: GPS drifting ? At anchor

Cmon people, the real question here is "What sort of anchor were you using?"

That will soon flush out if you were dragging or not.

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Old 14-11-2017, 15:17   #21
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Re: GPS drifting ? At anchor

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Originally Posted by noelex 77 View Post
Yes, you were dragging.

This type of slow drag is quite common and it is not unusual for owners to unaware of what is happening. At least you suspected something was amiss, which is good to see .
Agreed. Time for a Rocna anchor :-) Best piece of kit on my boat and the anchor alarms never go off :-)
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Old 14-11-2017, 15:34   #22
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Re: GPS drifting ? At anchor

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Originally Posted by Alberg30Shill View Post
Agreed. Time for a Rocna anchor :-) Best piece of kit on my boat and the anchor alarms never go off :-)
I don't have a Rocna or an anchor alarm, and my anchor alarm never goes off either! ;-)
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Old 14-11-2017, 15:51   #23
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Re: GPS drifting ? At anchor

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I spent most of last night trying to work out if my anchor was dragging.

I was anchored in 2-4 meters of water on clean sand in strong current with 15 knots of breeze with 20m of chain. I set the anchor alarm location by scope and direction after I set the anchor rather than when i dropped it.

The alarm went off and the trace looked like tide change, I checked my visual markers and it looked Ok. I thought I had screwed up my anchor location so I reset it to the middle of the trace.

The alarm kept going off and the trace kept getting bigger and bigger, I don't like to blame the equipment when things don't match up but it said I had drifted over 180m and my visual markers didn't look like I had moved much but it was a moonless night. So after a dodgy night's sleep I awoke to find I hadn't dragged but that the equipment was wrong.

Does GPS do this sometimes or is the tablet I'm using crap at reading the satellites or is "anchor light" software crap?
Was your altitude all over the place also?
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Old 14-11-2017, 16:07   #24
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Re: GPS drifting ? At anchor

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Originally Posted by Dave_S View Post
Does GPS do this sometimes or is the tablet I'm using crap at reading the satellites or is "anchor light" software crap?
All of the above.
We used anchor lite as well, and would get false positives if we set the anchor alarm radius at less than ~30m greater than the scope we had out. The app will show you it's degree of uncertainty in position, which we often saw as greater than 20m - occasionally as much as 50m.

But your position plot looks a lot like you dragged. I'd suggest that your anchor tripped when the current changed direction (or wind overcame current) then grabbed again.

We only used the anchor alarm when I was unsure about the anchor holding - so mostly I viewed all the false positives as just a prod for me to go check for chafe and have a general look around.

Eventually we just dug the anchor in a bit harder, let out a bit more chain, and stopped worrying.

Mike
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Old 14-11-2017, 16:22   #25
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Exclamation Re: GPS drifting ? At anchor

Quote:
Originally Posted by Dave_S View Post
I spent most of last night trying to work out if my anchor was dragging.

I was anchored in 2-4 meters of water on clean sand in strong current with 15 knots of breeze with 20m of chain. I set the anchor alarm location by scope and direction after I set the anchor rather than when i dropped it.

The alarm went off and the trace looked like tide change, I checked my visual markers and it looked Ok. I thought I had screwed up my anchor location so I reset it to the middle of the trace.

The alarm kept going off and the trace kept getting bigger and bigger, I don't like to blame the equipment when things don't match up but it said I had drifted over 180m and my visual markers didn't look like I had moved much but it was a moonless night. So after a dodgy night's sleep I awoke to find I hadn't dragged but that the equipment was wrong.

Does GPS do this sometimes or is the tablet I'm using crap at reading the satellites or is "anchor light" software crap?
What was your rationale for not using the point where the anchor dropped as the anchor location?
It is so easy to hit the MOB button at drop point

When you used an anchor position for tracking that is in mid scope, you are tracking a position that is mobile as the tide and wind shift the anchor rode and the corresponding mid rode position.
With the anchor watch set on the actual anchor position on the seabed, there is a fixed point to compare as the boat swings around.
There will still be a variation in indicated position, but much less than in your case.

Are you aware that the inherent accuracy of the gps signal is 15 metres? Using differential gps the accuracy can be improved, but this is not the case on gps receivers at consumer prices.

The anchor alarm on my Garmin GPS has a resolution of 0.01 minutes which is plus/minus 18.5 metres!
I would not rely on any anchor alarm other than the number 1 eyeball using transits on shore objects.
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Old 14-11-2017, 16:31   #26
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Re: GPS drifting ? At anchor

Definitely a drag...
I don't see anything in your track plot that looks like GPS or software issues. w But that is definitely possible.
Erroneous GPS plots will show single-points that are bizarre. Your anchor alarm software might be able to filter out that one epoch in which you moved a few miles in one second and right back again. This would be good to know if it does.
Of course realize that your GPS might resolve positions 10-15 meters in any direction without you even moving, so you have to add that distance to the potential swing on the hook when using your alarm. Or don't add that potential swing if you want to be alerted to wind or tide changes where you want to make sure the anchor resets proper.
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Old 14-11-2017, 16:41   #27
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Re: GPS drifting ? At anchor

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Originally Posted by kish View Post
What was your rationale for not using the point where the anchor dropped as the anchor location?
It is so easy to hit the MOB button at drop point

I would not rely on any anchor alarm other than the number 1 eyeball using transits on shore objects.
Anchor lite lets you set it by saying the anchor is set xx meters at YYY degrees from the current position of the GPS. So you are setting the anchors actual position, not a random mid scope position. (Of course it takes a bit of experience to know exactly where the anchor is relative to the boat once you have finished digging the anchor in and set a snubber etc)

Personally I like dropping the anchor manually from the bow - lets me see the angle of the chain, count the scope out etc. Feel the chain while power setting the anchor. That's why I only set the anchor alarm later - I feel that an electronic anchor alarm is much lower priority than setting the anchor well. An after thought if you will.

No.1 eyeball likes to sleep. Electronics stay awake. Which is why I sometimes use an anchor alarm. (And tracking on the separate chart plotter - particularly if it'd cold on deck and I'm feeling lazy.

Mike
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Old 14-11-2017, 17:01   #28
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Re: GPS drifting ? At anchor

If the anchor position is indicated by the "+" sign, then you had actually set your anchor, almost on shore.

The fact that the "+"(if it's your anchor), remains on the convex of the track, whereas for certain it should have been on the concave of the track.

Being on the convex side indicates you are outside the circular tacking ... that can't be. You should have placed the anchor somewhere inside the circle.

So again .... might you have given the GPS the direction/distance "to the anchor", when you should've given the direction/distance "from the anchor" to your boat or visa versa?

Even for a dragging anchor, the track looks incorrect.
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Old 14-11-2017, 17:13   #29
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Re: GPS drifting ? At anchor

For all those pundits who advise not using an anchor alarm based on GPS, but rather use bearings to fixed objects: there are plenty of times that I have anchored where there are no suitable fixed objects, or when it is black as the inside of a cow. It is in those situations where I really appreciate an anchor alarm!

And FWIW, I particularly like the alarm built into our Vesper Vision AIS unit. Dunno just how they deal with GPS error, but we don't get alarms from anything other than actually getting outside the described circle. The software allows correction of the anchor's location after the fact, and records that location in a log for future use (don't know how permanent that record is). Part of the readout is distance to anchor, in feet. This is stable enough to show if you are slowly dragging while power setting the hook... another thing that can be done visually IF there are suitable range marks visible, but not so easily at night or in fog, etc. Good bit of kit...

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Old 14-11-2017, 17:15   #30
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Re: GPS drifting ? At anchor

It looks to me like you dragged throughout the tide change and ended up basically back where you started, but about 200 ft SSW of your starting point. Would explain your visual observation in the morning that you had "not dragged" and the track, which shows basically a rough but englongated circular track northward and then back southward.
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