What is your preferred method for checking your nav gear for error?

[SV Windrush II]

I have Navionics and Plan2Nav on my tablet, Raymarine Navionics plotter and radar at the helm, and a Furuno GPS at my Nav station.
When approaching land I cross check all these and compare what I see visibly. If something looks strange I go to my paper charts and triangulate my position if possible. So far sailing into all the Micronesian atolls and reef passes at least one of my instruments was dead on.
The only one that was off at times was my Navionics charts, which was not accurate from Marshalls on to the Philippines.

What I found was that it is not the GPS that is off, it is the electronic charts themselves, as I compared Plan2Nav on the same gadget and was dead on verses Navionics.

Also when I can get internet signal I overlay the charts with google earth which I found was always accurate

[StuM]

Originally Posted by zeehag

my gps placed me in a parking lot 500 ft away from my actual location, and my spot placed me on other side of a channel from my stationary location. yup all are off about 500 ft off in different directions. i love it.
guess i wont be finding amelia earhart......
i love watching folks entering the lagoon at barra de navidad for first time, using gps for the tricky and narrow channel--they invariably turn sharp left instead of going from the point at the marina to the little beach visible from that point.
excellent entertainment, as all become mud wrestlers.

Do the GPS co-ordinates of your GPS and Spot vary by that amount? (I doubt it)

Much more likely that they use two different chart sources and the charts are off. Where do you end up if you plot your GPS/Spot position on a paper chart?

[Cadence]

Originally Posted by StuM

Do the GPS co-ordinates of your GPS and Spot vary by that amount? (I doubt it)

Much more likely that they use two different chart sources and the charts are off. Where do you end up if you plot your GPS/Spot position on a paper chart?

Some of the charts are probably circa Captain Cook paper or plotter.

[Paul L]

Originally Posted by Hydra

Sometimes, the GPS is on but wrong. Last summer, I entered a river mouth, passing close to the west of a lighthouse and the GPS showed the correct track. The next day, I came out, passing again to the west of the lighthouse. But this time, the plot was to the east, by a few tens of meters.

Alain

10s of meters is not really that bad. I've had issues with GPS when correction data was applied incorrectly. See this thread http://www.cruisersforum.com/forums/...-174482-4.html

The poster was referring to Mexico, where the charts are often off in the non-commercial ports.

[SV Windrush II]

Also the accuracy of the GPS is dependent on the latest Antenna frequency to communicate with the new satellites.
Make sure you are using a newer antenna. For example, I am using a Raystar 130 which according to Raymarine, guarantees its accuracy to within 5 meters, which I found to be true.

[Pelagic]

Originally Posted by Stumble

Practically speaking, the GPS is correct, everything else is wrong. If you want to confirm the GPS is correct get out another GPS.

LOL Stumble, perhaps semantics, but GPS tells you nothing of practical value, except on your return visit

#1Your eyes and other senses do.
#2 Chart renditions of surveyors who visited 200 years ago do
#3 Pilot references that report observations from the many mariners who visited before do

Granted GPS, if working, will give you a very accurate geo/spatial position, but it is only useful it terms of practicality, in that it offers accurate 'return to' values, once proven by traditional methods of observation.

For the newbie, claiming GPS infallibility in terms of safe navigation, is a dangerous thing

[SV Windrush II]

It is interesting that there is still a fear that a GPS would quit or not be accurate as compared to paper charts.

I had a full set of paper charts with me covering California, Hawaii, Marshall's and the Philippines as I crossed the Pacific. I think I pulled them out a few times to plan my trip and then rolled them back up again.

I cant even imagine approaching a narrow pass over a reef and pulling out a paper chart to try to run a triangular position to figure out where I am in the process, plus the fact that even the best paper charts are not detailed enough to give you accurate points to go off from to run your position on most Pacific Atolls.
I know all you old timers have done it but its still a major risk.
Reading some of the old books before GPS, most of the sailors just would not try to navigate in areas where the conditions are rough or at night when they could not see. What I have experienced is, the weather or daylight is usually not in the right conditions to have very good visibility at the time I need it.

Many times I have entered a channel or anchorage on just on instruments at night and several times in a gale that I would never try trusting paper charts and observing physical navigation aids which by the way do not even exist for the most part after Hawaii.

Plus the fact that what I have proven from the times I have used Paper charts, especially in the western pacific, they are really far off compared to my GPS plotter.

Bottom line is, I would rather trust my GPS plotter any day over a paper chart position. and if my GPS quits?, well... that's why I have 3 plotters, a tablet, and a laptop aboard, and also back up antennas So far I only had 1 corroded connection that failed and fixed it within minutes.

If the world comes to an end and they shoot down all the satellites, well maybe my position would not matter at that point hahaha

[StuM]

Originally Posted by Pelagic

LOL Stumble, perhaps semantics, but GPS tells you nothing of practical value, except on your return visit

UNless you use a Google Earth overlay

[StuM]

Originally Posted by SV Windrush II

It is interesting that there is still a fear that a GPS would quit or not be accurate as compared to paper charts.

I had a full set of paper charts with me covering California, Hawaii, Marshall's and the Philippines as I crossed the Pacific. I think I pulled them out a few times to plan my trip and then rolled them back up again.

I cant even imagine approaching a narrow pass over a reef and pulling out a paper chart to try to run a triangular position to figure out where I am in the process, plus the fact that even the best paper charts are not detailed enough to give you accurate points to go off from to run your position on most Pacific Atolls.
I know all you old timers have done it but its still a major risk.
Reading some of the old books before GPS, most of the sailors just would not try to navigate in areas where the conditions are rough or at night when they could not see. What I have experienced is, the weather or daylight is usually not in the right conditions to have very good visibility at the time I need it.

Many times I have entered a channel or anchorage on just on instruments at night and several times in a gale that I would never try trusting paper charts and observing physical navigation aids which by the way do not even exist for the most part after Hawaii.

Plus the fact that what I have proven from the times I have used Paper charts, especially in the western pacific, they are really far off compared to my GPS plotter.

Bottom line is, I would rather trust my GPS plotter any day over a paper chart position. and if my GPS quits?, well... that's why I have 3 plotters, a tablet, and a laptop aboard, and also back up antennas So far I only had 1 corroded connection that failed and fixed it within minutes.

If the world comes to an end and they shoot down all the satellites, well maybe my position would not matter at that point hahaha

You've obviously never sailed in an area where the GPS/plotter charts are badly offset. I don't trust my GPS plotter to take me through a narrow passage unless I have previously verified the offset by a physical sighting of a reference point.

Just a couple of anecdotes from a couple of early trips around the coast here:
1. Eyeballing my way into Cloudy Bay, I was about 20m from the outer reef marker when the plotter said it was still 1/2 mil north east of me. Knowing that offset allowed me to locate and follow the subsequent channel markers, even though my plotter track showed me a long way from the channel.

2. Going into the very narrow Sewa Bay, the chartplotter told me that we were sailing a couple of hundred meters to the East, straight over the headland.

[SV Windrush II]

Originally Posted by StuM

You've obviously never sailed in an area where the GPS/plotter charts are badly offset. I don't trust my GPS plotter to take me through a narrow passage unless I have previously verified the offset by a physical sighting of a reference point.

Just a couple of anecdotes from a couple of early trips around the coast here:
1. Eyeballing my way into Cloudy Bay, I was about 20m from the outer reef marker when the plotter said it was still 1/2 mil north east of me. Knowing that offset allowed me to locate and follow the subsequent channel markers, even though my plotter track showed me a long way from the channel.

2. Going into the very narrow Sewa Bay, the chartplotter told me that we were sailing a couple of hundred meters to the East, straight over the headland.

Yes I have had that problem with Navionics, but it is not a GPS position problem, it is the plotter map showing the GPS position in the wrong spot.

That is why I use at least 2 different sources for my plotters.

When I was approaching Lamotrek Atoll, my Navionics showed me on the reef when I was in fact 1 mile away still. I switched to my Plan2Nav software which is C-map based and gave me a accurate location on the plotter. if I can use the google overlay then it will show how far off the plotter is also.

So to not get us confused, GPS positioning i find to be very accurate, plotting software that places your position on a digital map can be and often times is incorrect. is this a better explanation?

[StuM]

Originally Posted by SV Windrush II

So to not get us confused, GPS positioning i find to be very accurate, plotting software that places your position on a digital map can be and often times is incorrect. is this a better explanation?

Yep, that's the crux of it.

[Paul L]

Originally Posted by SV Windrush II

Yes I have had that problem with Navionics, but it is not a GPS position problem, it is the plotter map showing the GPS position in the wrong spot.

That is why I use at least 2 different sources for my plotters.

When I was approaching Lamotrek Atoll, my Navionics showed me on the reef when I was in fact 1 mile away still. I switched to my Plan2Nav software which is C-map based and gave me a accurate location on the plotter. if I can use the google overlay then it will show how far off the plotter is also.

So to not get us confused, GPS positioning i find to be very accurate, plotting software that places your position on a digital map can be and often times is incorrect. is this a better explanation?

And just to delve down a little deeper on the not to be confused. It is generally not the GPS or the plotting software which causes the error. It is the digital chart not accurately created/adjusted to the GPS datum by the chart creator.

[conachair]

Originally Posted by Paul L

And just to delve down a little deeper on the not to be confused. It is generally not the GPS or the plotting software which causes the error. It is the digital chart not accurately created/adjusted to the GPS datum by the chart creator.

Aren't the inaccuracies more likely to be from the original paper chart data on which digital charts are based on?

And again, this is where Google earth images as kap files can be very useful.

[SV Windrush II]

Originally Posted by conachair

Aren't the inaccuracies more likely to be from the original paper chart data on which digital charts are based on?

And again, this is where Google earth images as kap files can be very useful.

Not sure if digital charts are based on paper anymore. NOAA does not even print paper charts any longer. furthermore my Plan2Nav plotter software on my Apple tablet is consistently more accurate than my updated paper charts. At least in Micronesia and the Philippines that is.

Yes the google earth overlay is a great feature

[conachair]

Originally Posted by SV Windrush II

Not sure if digital charts are based on paper anymore. NOAA does not even print paper charts any longer. furthermore my Plan2Nav plotter software on my Apple tablet is consistently more accurate than my updated paper charts. At least in Micronesia and the Philippines that is.

Yes the google earth overlay is a great feature

.... "original paper chart data "

If the last survey was 50 years ago then that's what you're stuck with.

Something sounds odd if the paper charts are out compared to cmap with the latest NTM's applied...

*Note that C-MAP frequently updates its electronic charts as Hydrographic Offices and other sources provide additions and/or modifications