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Old 05-02-2017, 11:28   #1
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What is your preferred method for checking your nav gear for error?

Seems the most likely cause of nav gear reporting incorrect information will be user error, such as incorrect settings, followed by outdated chart data.

When driving, my phone and vehicle gps occasionally do not agree with reality or each other so significantly as to cause major problems with navigation. I don't want this on the water in poor visibility/conditions.

This brings me to developing a standard routine to quickly accurately "calibrate" my primary navigation tools so I can trust them.

Some ideas I have come up with are:
cross checking the chartplotter against radar would reveal quickly if there is a significant error in the settings.

depth should follow charted contours so following a series of depth readings against charted depth seems a good cross check also.

multiple gps against one another, ie all coordinate readouts should be very close from phone, handheld, laptop with puck and primary nav station

check actual depth at anchor against charted depth and sounding depth

Triangulate headland or navigation aid on actual chart, compare against nav station. (compare what you see with what your screen says)

With NMEA all in one MFD where radar, gps, depth, and chart are all wrapped into one display, I wonder how independent each part of the system is? Can I still run independent tests if everything is basically on one computer and running one type of software, possibly with centralized user settings causing an error to carry across all functions?

What method do you fine folks use when the rubber meets the road out there to check you nav gear? how often do you double check it?
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Old 05-02-2017, 13:01   #2
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Re: What is your preferred method for checking your nav gear for error?

First, I compare the COG from GPS with the heading from magnetic compass, taking variation and current into account.

Then I compare the fix with the position obtained from dead reckoning: if the fix was correct the last time and the course made good is correct, then the new fix must be correct, too.

Then, I compare the sounding with the chart, taking the tide into account if in shallow water. There is almost no silting (except in some rivers) in the area where I cruise.

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Old 05-02-2017, 13:23   #3
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Re: What is your preferred method for checking your nav gear for error?

By checking Nav gear, I assume GPS?
If so then almost all will self report a dilution of precision or other name as to stated accuracy, the GPS will be within this stated accuracy, any other inaccuracy other that will be from geo referencing the charts, or the accuracy of the charts themselves.
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Old 05-02-2017, 13:46   #4
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Re: What is your preferred method for checking your nav gear for error?

Really depends whether you are inshore or offshore

For double checking inshore or "near offshore" - i.e coastal, I find the best method is to use the Mark 1 eyeball and if that is not available due to darkness or fog etc, then radar.

Well offshore, it does't matter as much. A coupe of GPS units, monitor the magnetic compass, the direction of swell, wind and sun and that's about it.

But really as many possible inputs from different types of nav data and good (no, make that EXCELLENT) understanding of the navigational solutions and their individual weakness will help you stay in safe water.
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Old 05-02-2017, 13:51   #5
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Re: What is your preferred method for checking your nav gear for error?

Quote:
Originally Posted by nematon785 View Post
Seems the most likely cause of nav gear reporting incorrect information will be user error, such as incorrect settings, followed by outdated chart data.

Yep, that's it in nutshell!!!!!

........

With NMEA all in one MFD where radar, gps, depth, and chart are all wrapped into one display, I wonder how independent each part of the system is? Can I still run independent tests if everything is basically on one computer and running one type of software, possibly with centralized user settings causing an error to carry across all functions?

Yes you can - eyeball and brain

......how often do you double check it?

All the time
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Old 05-02-2017, 14:27   #6
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Re: What is your preferred method for checking your nav gear for error?

I think it depends a lot on whether you are in home turf or approaching a new area. Home turf you can swing up close to a buoy and verify it is where it should be on the chart plotter. That covers both GPS and chart accuracy. I also regularly compare depth sounder depth to charted depth.

If you are in new territory then it is harder to get to the high trust level for the chart plotter. Comparing a Radar overly is helpful provided the Radar is calibrated. That tells little about the quality of the charts in the area. Are the reefs where expected, are all the islands actually on the chart, is the GPS signal stable,....?
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Old 05-02-2017, 15:16   #7
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Re: What is your preferred method for checking your nav gear for error?

Always worth checking using eyes. Over 20 years ago (this is how I learnt the need to check) on a charter boat in Baltic Sea we were planning to head over to Gotland from Swedish coast, while we hadn't really checked compass there was a general concern that once we were out of sight of land that we would be relying on it as the boat did not have gps (it was that long ago). Anyway one of us decided it was an excuse to buy a new gadget, a handheld gps.........and a good thing we did as worked out the compass was way off, and while I can't recall how much I do remember that we did the calculation and had we just relied on the compass then we could have missed Gotland completely not going far south enough and ended up in Latvia/Lithuania.

So now I try to do a visual check when leave mooring on some distant landmark against the chart just to check the compass and where we think we are. It sounds overkill but I see it as a good habit to have checked while still have landmarks to use, before head too far offshore
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Old 05-02-2017, 15:24   #8
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Re: What is your preferred method for checking your nav gear for error?

FWIW, I don't think depth comparisons are very good checks on GPS accuracy. IF you compare measured vs expected depths and they agree, this does not mean that you are where you think you are... it only means that you are in an area with the same depth, and that can include a hell of a lot of places!

It does give some feedback if they do not agree, though, for that does mean that you ain't where you think you are!

But really, noting the GPS readout related to its accuracy is a pretty good check on t he instrument, as is noting the display of received satellites. Chart errors are far more threatening IMO, especially in areas without recent survey, and that includes a lot of the world!

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Old 05-02-2017, 15:30   #9
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Re: What is your preferred method for checking your nav gear for error?

Ha Jim, monitoring the depth once saved us from a big embarrassment when operator error screwed up the gps during a port entry... but its a pretty coarse check.

I check all instruments match and are correct when alongside. Then Monitor HDOP, test alarms, especially signal alarms and RAIM.

Underway I monitor DR position roughly. Add regular cross checks with radar ranges or PI's (if I have one) depth and another GPS. Plus of course visuals/and maybe hand bearing compass or transits, and an eye on HDOP/RAIM.

A good defence is a conservative passage plan and extra controls/checks at critical points.

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Old 06-02-2017, 08:25   #10
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Re: What is your preferred method for checking your nav gear for error?

I just head for open water because if something is going to go wrong, it will go wrong there... (To paraphrase Cap't Ron)
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Old 06-02-2017, 09:32   #11
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Re: What is your preferred method for checking your nav gear for error?

I check the sextant as per the manual, mirrors, alignment, etc.

I check the clock against the Internet or radio signal.

I check the depth against a line and a sinker.

I check the gps against the clock and against Google Maps and against a fixed object that is mapped on my maps.

I check the speedo against a cork and clock and the gps.

I check the compass against transits and sunsets and etc.

I must admit I have never found any errors in my instruments.

I did find some errors in my maps though.

Cheers,
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Old 06-02-2017, 10:22   #12
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Re: What is your preferred method for checking your nav gear for error?

The art of navigation is comparing the information from all available sources and confirming that the situation it describes is consistent. Where navigators get into trouble is steaming on when there is inconsistency.

You probably don't have to worry too much about your own boat in familiar waters, but it really pays to confirm consistency in new situations. The more sources you use the better, and relying on one source LIKE YOUR CHARTPLOTTER is a recipe for disaster.

Here are two well publicized examples of over reliance on a chartplotter, and I am sure there are others:

http://www.cpf.navy.mil/foia/reading...-grounding.pdf

Vestas Wind grounding report | Volvo Ocean Race

Personally, I have experierienced a BSB chart of Bermuda that was exactly 10 miles off in latitude.

I found a chartplotter 3 miles off, because it was set to use a Croatian datum. On another boat, the high end MFD had an accessible menu that allowed you to create your own datum by putting the cursor on the chart and telling the chartplotter that was where the boat actually was. That was not idiotproof, and the owner had managed to create a 25 mile error.
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Old 06-02-2017, 10:42   #13
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Re: What is your preferred method for checking your nav gear for error?

Quote:
Originally Posted by barnakiel View Post
I check the sextant as per the manual, mirrors, alignment, etc.

I check the clock against the Internet or radio signal.

I check the depth against a line and a sinker.

I check the gps against the clock and against Google Maps and against a fixed object that is mapped on my maps.

I check the speedo against a cork and clock and the gps.

I check the compass against transits and sunsets and etc.

I must admit I have never found any errors in my instruments.

I did find some errors in my maps though.

Cheers,
b.
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Old 06-02-2017, 15:04   #14
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Re: What is your preferred method for checking your nav gear for error?

Quote:
Originally Posted by Snowpetrel View Post
Ha Jim, monitoring the depth once saved us from a big embarrassment when operator error screwed up the gps during a port entry... but its a pretty coarse check.
Reminds me of the time a certain navigator miskeyed the co-ordinates of a waypoint which was supposed to be abeam Gubbins Reef (that was in the days of simple GPSs without chart displays). It was the sudden lack of depth that clued us in to the error
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Old 06-02-2017, 16:22   #15
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Re: What is your preferred method for checking your nav gear for error?

Anyone been to Coffs Harbour, in Australia?
Coming in from the south there, our chart plotter and Navionics both jumped, suddenly putting us a few nm north of where they had been/our actual location. Luckily there are good lead lights so we just used those. In happens on google maps while driving in your car there too..... When we departed, Chart plotter and Navionics were both fine and accurate.
Made me worried about sailing on dark nights in certain areas.
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