Can you navigate without using GNSS?

[MADRIGAL]

Yes still like to sit down and DR on paper charts, I use the GPS to see how far out my calculations are.

Or, how far out the GNSS was. On a cruise last week-end, a look back at the track on the Garmin showed that we had left certain buoys to port, when the mark 1 eyeball said we had left then to starboard. It was not a problem in the good visibility we had; we were in chart and compass mode, ticking off buoys on the passage plan as they appeared where expected. It was a good reminder not to rely on GNSS for absolute accuracy in fog, but to leave a margin for error in dangerous waters regardless of the navigation technology in use.

 

[Buck Turgidson]

Or, how far out the GNSS was. On a cruise last week-end, a look back at the track on the Garmin showed that we had left certain buoys to port, when the mark 1 eyeball said we had left then to starboard. It was not a problem in the good visibility we had; we were in chart and compass mode, ticking off buoys on the passage plan as they appeared where expected. It was a good reminder not to rely on GNSS for absolute accuracy in fog, but to leave a margin for error in dangerous waters regardless of the navigation technology in use.

But how can you be sure that the GPS was in error and not your chart?

 

[Bilgediver]

I can not afford GNSS so I am stuck with my old slightly less accurate GPS . It gets me there when it finds a signal otherwise it is the Mk 1 eyeball and pencil and chart.

 

[Buck Turgidson]

I can not afford GNSS so I am stuck with my old slightly less accurate GPS . It gets me there when it finds a signal otherwise it is the Mk 1 eyeball and pencil and chart.

GPS is GNSS.

 

[Fantasie 19]

GPS is GNSS.

GPS is a GNSS.. apparently....

I had no idea by the way before I had a google on "GNSS"

 

[MADRIGAL]

But how can you be sure that the GPS was in error and not your chart?

Good point; the charts loaded into the plotter (based on the same surveys as the paper chart) are likely not as accurate as the GPS fix, which could therefore show us on the other side of the buoy. In this case, however, we saw the buoy and know which side we
passed it on. GPS accuracy is generally excellent, but not absolute, and still requires a safety margin.

 

[capnsensible]

GPS is a GNSS.. apparently....

I had no idea by the way before I had a google on "GNSS"

Could always look at post #6.....

 

[Fantasie 19]

Could always look at post #6.....

I was replying to #64 .. but you have a point...

 

[Buck Turgidson]

Good point; the charts loaded into the plotter (based on the same surveys as the paper chart) are likely not as accurate as the GPS fix, which could therefore show us on the other side of the buoy. In this case, however, we saw the buoy and know which side we
passed it on. GPS accuracy is generally excellent, but not absolute, and still requires a safety margin.

Your GPS position is referenced to the WGS84 datum. Your visual position was referenced to the Buoy. Who knows where the buoy on your chart is referenced to!
In this case as the buoy is the reference for safe pilotage it definitely has priority, especially if someone moved it last week to take into account movement of the channel.

 

[capnsensible]

Your GPS position is referenced to the WGS84 datum. Your visual position was referenced to the Buoy. Who knows where the buoy on your chart is referenced to!
In this case as the buoy is the reference for safe pilotage it definitely has priority, especially if someone moved it last week to take into account movement of the channel.

I was taught to never use floating navigation marks for fixing coz they drift around a bit on the tide and to keep a sharp eye on the echo sounder in pilotage waters.
I think I've just drifted the thread I started, fnarr.
I've found that a lot of gnss doodads I've used have the facility to select a number of datums, but I've rarely found a chart of any nationality that doesn't use wgs84 these days.

 

[Chiara’s slave]

Your GPS position is referenced to the WGS84 datum. Your visual position was referenced to the Buoy. Who knows where the buoy on your chart is referenced to!
In this case as the buoy is the reference for safe pilotage it definitely has priority, especially if someone moved it last week to take into account movement of the channel.

Never mind that, if theres any depth of water, the buoys move, 4 times a day, and a surprising distance too. Just yesterday, Cowes week had to warn competitors that the start line ODM was about 50m on the course side of the line, owing to the tide. Kind of them, when you race in a class with no GNSS.

 

[zoidberg]

........I've rarely found a chart of any nationality that doesn't use wgs84 these days.

The first chart I pulled at random from one of my navbags - BA32 Falmouth Harbour - declares 'Projection - Gnomonic. Positions are on Ordnance Survey of Great Britain ( 1936 ) Datum'.

C'mon. You knew SOMEONE would....

 

[DAVAIS]

yes, but always keep a paper map in the boat

 

[xyachtdave]

I’ve heard paper charts are being phased out but apparently there’s no plans at present to stop producing the economy sailors go to chart folio, all the UK in one publication.

 

[AntarcticPilot]

The first chart I pulled at random from one of my navbags - BA32 Falmouth Harbour - declares 'Projection - Gnomonic. Positions are on Ordnance Survey of Great Britain ( 1936 ) Datum'.

C'mon. You knew SOMEONE would....

Any chart of the UK before about 2000 would be on that datum. I'm a bit surprised about the projection - Gnomonic has no redeeming features unless you're crossing oceans (Great Circles are straight lines, which might be handy) or dealing with plate tectonics (transform faults are usually great circles), and the OS datum is designed for use with a Transverse Mercator projection.

 

[zoidberg]

The Air Force planned to go to war using Lamberts Conformals..... and did.

 

[Goldie]

I’m another one with Consol chart tucked away somewhere…. I’ve thrown away my RDF, and AFAIK, no one has mentioned VHF lighthouses on Ch 88. Remember them?

 

[Goldie]

The Air Force planned to go to war using Lamberts Conformals..... and did.

Would that be ‘Paddy‘ Lambert, ex ssssssh Squadron? ??

 

[zoidberg]

That name rings a faint bell of addled recognition.

 

[capnsensible]

I’m another one with Consol chart tucked away somewhere…. I’ve thrown away my RDF, and AFAIK, no one has mentioned VHF lighthouses on Ch 88. Remember them?

If that was the system where you counted the number of 'beats" to get a bearing, yes I do.

If not, no I don't.