Trust Your GPS?

I have just recently had several disconcerting experiences with my chart plotter. Both times were on narrow upper river channels, where my plotter allegedly showed the channel. I have gone up rivers beyond where the chart indicated as navigable, but this was different. I would be going up the Chehalis, North River, the Wishkah, and watching the chart. My boat icon would be right on the edge of the river per the plotter, but I was actually in the middle of the river. Many times I would take the outside of a bend in the river, that appearing to be the deepest channel, and it would be unnerving to look down and see that the boat icon was 100 feet on shore per the chart plotter. I even turned it off/on to see if it would re-calibrate or something. Nope, it showed me on shore.

I learned to ignore the plotter in these situations, but it did make me wonder about narrow channels elsewhere. I have been amazed at the accuracy when on salt, accurately indicating the finger piers at Westport, so maybe it's just when 5 miles up a twisty river.

The other spooky part was returning down the river. Most times I had slowly gone up following the deepest channel, regardless of what the plotter said. Now when descending I could get on plane at 15 knots and follow my plotter track back down. As long as I stayed on track, I'd be returning downstream in safe water where I had already traveled. But the track that I had made on the plotter, and now needed to follow back down, went up onto shore in several places. Another odd feeling.

Turns out that it is what's outside the window that is reality. Seems the woman in the car forgot that.

At points where there were submerged snags or I got into water less than 4 feet deep I marked with way points and man overboard indications. Then I knew to slow down at those locations on my return. I always found that returning on my track is very accurate, so I don't think that the error is GPS to GPS, but rather chart to GPS in some situations.

Mark
 
Marco Flamingo":25r434q7 said:
the track that I had made on the plotter, and now needed to follow back down, went up onto shore in several places.
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The track drawn by the plotter is made by joining a continual series of dots pinpointing a specific location. The spacing of these dots can be adjusted in your plotter setup menu. Most boaters using more open water like lakes set the spacing farther apart to conserve the memory as you will be limited to X # of pinpoints. Or you can erase tracks that are not needed. If you are navigating rivers with hairpin turns then I would set the point spacing as close as you can. Especially if you need to return on your track after dark. With the spacing set too far apart it will show you crossing land when rounding points because it is joining 2 dots with a straight line.

Agree - nothing beats keeping a sharp look out. Conning and common sense were used long before the gps was invented.

Regards, Rob
 
It is rare that it is the GPS that is inaccurate, but rather the charting. All the chart makers get their info from government surveys... and some waters haven't been surveyed in decades.

Our home is on an island, on a canal. To get to our place from the ICW, you go up one canal and down another. Every GPS device we have used in over a decade shows that we are on land as we get close to the end of the first canal... it is the charting.

Of course, you should be looking outside the boat when navigating. And just as important: use what you have. If you are in VFR conditions (good visibility), you can compare what you see to the chart. If you are in fog, night, smoke, or other low vis conditions, you use the chartplotter AND the depth finder AND radar AND a compass. In those conditions, you should be going slow, so you also use your eyes and ears outside the boat. You can hear other boats, feel wake, hear the surf breaking, hear the bells, chimes, or horns on buoys. With all that going for you, there is less reliance on any one device.

There is a generation that hasn't seen paper charts (maps), and never had to get around without the benefit of GPS (or LORAN C before that). It really is impressively accurate IF the charting is current/accurate.

If you have spent any time out on the water, you have seen people running too fast for the conditions. A GPS may show you the way (within the bounds of the chart accuracy), but it doesn't show you what else is in the water.
 
I have an old Raymarine C80 that for some reason began showing the land with square lines. Although the coordinates are still accurate it shows my slip as being a mile inland. I can still anchor within a few feet of my dive sites but the map is driving me crazy.
 
Frist I always turn on my chart plotter before I leave the dock/ramp, to be sure it is working properly. Second, have some back up GPS (in my case I have 3 plotters, but only use 2--one for AIS, the second for navigation.

As Jim notes, the problem is not the GPS--it is very accurate 99% of the time. (The exception is that GPS can be spoofed, and in close proximity of both military bases and US Naval ships, I have on occasion lost GPS--I have not had that experience with GLONASS (Russian GPS).
 
Jim nailed it. I trust my GPS to be exactly as accurate as it is telling me that it is being. My lat/long are my lat/long within the bounds of the current accuracy. the basemaps, software, and other gizmos that analyze these location data and convert that into some kind of information or instruction is quite suspect though.

I recall an ongoing discussion I had with Garmin regarding the location of an airstrip on a remote island in Northern Alaska. We had looked for it where it was supposed to be, and it wasn't there, we started scouting nearby islands, and we found it. I walked the runway, as well as the perimeter of the island it was on and sent that information to Garmin, noting that they had the airstrip improperly located. The locals knew about this issue very well.

Garmin did not believe me at all, and insisted that their data QA-QC was so thorough, that I must have missed the real airstrip, and that what I thought was an airstrip (the thing I landed on, walked, and then traced the island, was not an airstrip).

Not to rag on Garmin (I like them, have several handheld GPS units, and a plotter on my boat). Just point out that there is great certainty behind the bad info in those basemaps!
 
A GPS is really nice to look at, BUT

I am from the "old school" I rely on paper charts,
a compass, look at my surroundings and
most of all I pay attention.

The price of good seamanship is constant vigilance!
 
There is an entire other source of information which is being disregarded in this discussion. That is aerial maps/satellite maps. I often print these out ahead of time, and put waypoints on them--especially in areas where the charts are poor, or old. Also there are lots of changes along any coastline or waterway

Be sure that you have the right DATUM selected on your chart plotter.

When we were near the headwaters of the St. Johns River, we found that the charts were worthless--both paper and chart plotter. Enter the I pad and Google Earth. We quickly found the way up the tributary. As we went on up, we identified where we were by the various bends, power lines, the rare structure--all of which were quite accurate. There are a number of Topo map, satellite images which are available on line, and in printed format.

I am also "old school" and do much of my planning on the computer or charts. However, the GPS plotters, I pad are far better. Also I can think of many situations where there was poor visibility, due to darkness, fog etc. This is especially difficult when approaching a coastline with no visibility. I sailed to Catalina Isand many times at night, long before any electronic aids were available. I compensated for the small current, and would often arrive early in the AM--to an area, with no lights. One night I thought I knew where I was, and turned right. After going 4 miles (about an hour against the windI) I realized I was at the West End of the Island. Turning around, I found my cove (which I had been to hundreds of times, only a few hundred yards to the East (left) of where I first made land fall. The night the current was just enough different to make the difference.

Often maps are way out of date--and cannot be relied on for accuracy---especially outside of the US. When I have found major discrepancies, I have reported them, with documentation, to NOAA. Yes, many of the charts on the plotters are off--no cartography company is without fault. Over 80 % of maps have my house address in the wrong location--including our Country GIS system.
 
I spend a lot of time boating in places where the charts say "use local knowledge" and where narrow channels through large expanses of water, big Gulf of Alaska Swells interact with moving bars, and big tide changes and currents developing most of the time. It's a somewhat predictable chaos. The charts don't even try to show you the bottom.

I have been very impressed with Lowrance's insight genesis toolkit and have been converting my sonar data (I log sonar sidescan sonar data as I travel the area) into my own personal basemaps. This feature works very very well, especially in areas where maps are poor, or conditions change frequently. The maps are incredibly detailed and very very adaptable, and these have become a wonderful tool in navigating these areas.

I think there are other options now for these features to be gained in non-proprietery software, such that one could log sidescan sonar data on any number of manufacturer's units and use the data to make one basemap. I haven't tried it but it looks cool:

http://www.navionics.com/en/sonarchart
 
So as I read this ,and see that someone is having a problem navigating with the gps on a local river, My first two thoughts are? how deep in the canyon the river is in, what if any view do you have to the south( where most of the satellites are? and when was the river charted last? Rivers move. Your chart and gps are probably both operating fine in the situation you have put them in. remember that when in doubt look out ( the window ) because that is where reality is. Not on your plotter.
 
Aurelia":urmawjdu said:
thataway":urmawjdu said:
Over 80 % of maps have my house address in the wrong location--including our Country GIS system.
Click to expand...

What is an example of the 20% that get in right? Are you in a city?

Greg
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Greg, the Garmin Street map is wrong only by address, same with Google maps. I believe it is Tom Tom which does get it right. Trulia has both the photo and map correct. Garmin, and several of the other GPS maps have the address wrong. When the boat is at the dock, or on the trailer, its position is correct on the Garmin Chart plotter.
 
I have a RayMarine C120 and I also have a small Garmin as a backup. In new areas, where close and detailed navigation is required, if they don't agree, I get real finicky about watching depths and clearances.

There are some places that, as mentioned, the plotter data is off. (Don't try coming into John Wayne Marina by IFR without LOOKING at the entry. It could be a very hard landing.

Harvey
SleepyC :moon

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