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Old 03-02-2016, 10:04 PM   #41
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Twice before gps. Once in the Delaware Bay during a squall so bad we could not see the bow of the boat, Once in the Chesapeake taking the inside route north from Chrisfield and missed the turn into the bay itself. But the strangest was this past summer fishing the mouth of the Delaware Bay in an area called "The Rips" where the water shoals to 2-3' very quickly. That's when we found that gps is not exact. It was time to go after another "successful" day of catching Skates and Dogfish. The gps had us on the north side of the rips but we were actually 30' away on the south side. Luckily we were using the Parker with the outboard that day. Had to raise it almost out of the water to get by. If we were in the Mainship we would never have made it to the really shallow part.
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Old 03-02-2016, 10:14 PM   #42
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Greetings,
Mr. j. " That's when we found that gps is not exact." I think everyone needs to keep that in mind.
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Old 03-02-2016, 11:53 PM   #43
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GPS is sure a lot more exact now than when it was first made available to the public. No more induced random error for security purposes.

With a clear view of the sky and a decent receiver, I am continually amazed at how exact they really are.
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Old 03-03-2016, 01:55 AM   #44
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Believe GPS is more accurate than charts. For instance, electronic charts showed me on the shore of the upper Petaluma River/slough (west of Lakeville) while actually being in the middle of the channel. There was about a 50-foot difference.

Further up the river where charts were on the mark:

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Old 03-03-2016, 06:45 AM   #45
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As we were hand steering across the Gulf of Mexico a few weeks ago, one of my crew was on helm duty, and ended up 180 degrees of the course line. He steered that course for a while before he realized something was wrong and called for help as he was confused and feeling vertigo. I was asleep, so the third crew member who was close by got things turned around. This happened about 1:00 am.

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Old 03-03-2016, 06:57 AM   #46
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The GPS is accurate...it is the charts that aren't. Absolutely many small thoroghfares, creeks , etc aren't exactly "on"...but figure many are still on charts when they were measured by tape and transit...buy guys trudging through the swamp on foot.


The rips in Delaware Bay are a great example. I have fished them 100s of times and I would say every chart I have and some of the electronic charts have completely different representations of them. The only ones that are similar are the fishing charts that just mimic the NOAA ones.They are just large sand waves for the most part and change all the time...so I'm not sure what charts out there have really the best representation of them.


When visual clues are lost...the mind plays all sorts of tricks on you. Throw in vertigo and special orientation is under constant attack. Even with visual, optical illusions can kill you. A good example was in Biscayne Bay climbing out after a water approach at night...the nav lights of two airplanes diverging (one plane was taking off from Miami Int and another was on downwind) appeared to be nav lights of a single plane coming at us so close and fast, the other pilot cut power, banked and dove just a few hundred feet off the water. I saw the same thing which is unusual for optical illusions but possible. We just barely cleared the water and it took WAYYYYYY too long for both of us to get that visual out of our heads and figure out what it was. Our eyes saw the lights diverging but our minds painted in a small airplane between them because it was dark and we always expected the unexpected.


A great example of all of this is when you are sitting at a red light. You are looking down, out of the corner of your eye you see the car next to you creeping forward. You think you are rolling backward but in reality, you are still...it is just the other people moving for the green light. It is a paradigm shift that shows your senses can be easily overridden and like I posted before...only training and experience (or luck) will get you though the tough ones.
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Old 03-03-2016, 07:19 AM   #47
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GPS has become so reliable and accurate it's hard for me to imagine moving a boat without one. When folks see their chartplotter not showing their exact position in relation to their surroundings it's generally the charts within the plotter that are not exactly right. You have probably noticed this in some narrow channels and rivers where the plotter will show the boat on the shoreline when you are actually in the middle of the river or channel. The charts within the plotter are derived from govt data some of which have not been updated in many years. It amazes me that some of these older govt charts are as accurate as they are.

Spatial Disorientation is not getting lost but a complete loss of orientation. It can mean not knowing which way is up or down but can also mean a complete loss of your sense of direction. This happened to me many years ago when flying co-pilot for a regional airline. I was preparing to fly an approach to an airport in IFR conditions when unknown to me the Capt. changed the frequency of the nav aid (VOR) I was using without telling me. I was flying on instruments and when I looked back at the Nav aid (VOR) nothing made sense. He had changed the frequency from the VOR to the ILS, which is the nav aid used to guide the airplane to the runway in both azimuth and elevation. It was necessary to change the freq but I was still navigating using the VOR and not ready to switch over. We resolved the issue but today this would not happen. Coordination between crew members is much better established.
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Old 03-03-2016, 07:57 AM   #48
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While it is common to see errors on the chart plotter map where you're closer to land than you actually are, the navigation itself is very accurate. When I make a mistake, it's usually misinterpreting what I think I see. I do very well in the fog or at night as there is less to misinterpret and I rely more on the radar and plotter. A good example is returning at night to Ocean City, MD. Try to find the seabouy with it's blinking red light against a sea of lights from the amusement park behind it. It's very difficult until you almost on top of it. Fortunately the plotter and radar don't have that problem and it's not hard to spot it when your heading directly toward it. In unfamiliar areas, I now find myself taking reference points from the plotter / radar and matching them to what I see, as opposed to the other way around.

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Old 03-03-2016, 08:16 AM   #49
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Good point Ted...the more you use your tools the more they are an extension of your senses. I would guess those on the water all the time can relate.

I was taught that in navigation you are supposed to know where you are...navigation is really the art of confirming your "guess".

Whether a glance out the window or the glance at the RADAR gives you the info you need, the source isn't important. But you can see when it might be.
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Old 03-03-2016, 12:12 PM   #50
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[QUOTE=psneeld;420640]The GPS is accurate...it is the charts that aren't. Absolutely many small thoroghfares, creeks , etc aren't exactly "on"...but figure many are still on charts when they were measured by tape and transit...buy guys trudging through the swamp on foot.

Agreed, the charting up here is so poor that I often find myself "putting it in 4 wheel drive" as I run up the sides of glaciers and cutting overland through the mountains to get into some small bays.

If the visibility is good we just laugh about it, but when it's dark or foggy it's time to turn on the radar.
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Old 03-03-2016, 12:19 PM   #51
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Greetings,
So the GPS is right and the chart is wrong? Golly, what use is it then if the "blip" where you are, on the GPS, is on shore? So take away the incorrect chart and you're left with a "blip" overlaid onto a blank screen??????

Lucy...You got some 'splainin' to do.
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Old 03-03-2016, 12:35 PM   #52
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Quote:
Originally Posted by RT Firefly View Post
Greetings,
So the GPS is right and the chart is wrong? Golly, what use is it then if the "blip" where you are, on the GPS, is on shore? So take away the incorrect chart and you're left with a "blip" overlaid onto a blank screen??????

Lucy...You got some 'splainin' to do.
Your GPS still gives you all kinds of very accurate information that is useful. The only thing to remember is that the charts, and therefore the plotter on with your GPS information is overlaid, are not 100% accurate.

I am fortunate that most of the waters in which I travel are very well charted and there are few natural forces that are changing the landscape very quickly. Even so, I try to always remind myself that what I see on the chart plotter is someones best representation of what is in the real world, but it is NOT real. Still have to look out the window.
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Old 03-03-2016, 12:36 PM   #53
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Old 03-03-2016, 12:54 PM   #54
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Once you have a track set, the GPS repeats it perfectly. I have run in the dark to pick up my shrimp pots many times "on the track" not even able to see shore. I would swear the GPS is accurate to that track within a couple of feet. The charts on the other hand are often off by several hundred yards. I don't go poking around in the dark using the chart plotter as my guide. I kick on the radar and anchor very conservatively when sneaking into an anchorage I don't have a track into.
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Old 03-03-2016, 02:30 PM   #55
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As said above, once your GPS has laid down a track, it doesn't matter if some of it is on land or otherwise innacurate, it is very repeatable.

It is also easy to get used to the chart being off, by some distance, but mostly the same distance in the same direction. This is due to the datum being off, so a new track can be laid down, trusting the items on the chart to be correctly placed in relation to the inaccurate datum.

The most extreme example of this that I have experienced was while cruising in Turkey, on charts done by Beaufort about 200 years ago All the harbours were correctly drawn, but related to a datum that was off by as much as a nautical mile. This made things exciting for the navigator, below where he had no windows, while I, at the wheel, could see where I was and what else was around us. My view was a bit better for keeping us in the water, while taking a narrow passage out of harbour.
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