Magellan Navonics Gold Chart Accuracy

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jackho

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Not my best boating weekend ( see my other post). Purchased an SD chart SG396XL and installed in my Magellan FX at weekend for sailing holiday to Scotland next week. Chart seems to be about 50m out - had me going through harbour wall in fog at the weekend - got quite a scare but survived. don't think i can alter anything and suggest the chart is not accurate. What is the best remedy - return to Magellan to be checked against some standard. Anyone else had this experience??
 
The chart is almost certainly ok, but the GPS is accurate to only about what your 'error ' is. This 'error' varies from time to time, and place to place - but I have been anchored 50 or 60 metres off a beach, and my position was shown as being on the edge of the road, a good 80m out. This was after the GPS system was made more accurate when selective availability was turned off by the US military.
Try your new system some more, preferably in better conditions, so you can check it's accuracy and develope some confidence - or otherwise!
Cheers
Ian
 
Other way around. GPS accuracy is excellent. In most places you will be getting well under 15m accuracy, and usually under 7 to 5m. With WAAS or Egnos, you can expect under 2m most of the time (not available in NZ)

Normally the problems seen with chart accuracy is the chart itself, or people not setting their GPS or plotter to the correct datum for the chart. You should not be seeing 50m errors unless the chart is incorrect, or the chart datum has not been set correctly
 
I agree with that Brendan and can add that I have a commercial fleet operator client who has up to 200 m charting error (varies over the same chart) on the non official electronic charts they use. That even though their location (not here in NZ) is well surveyed and was until recently an important naval base.

In my view, charts whether electronic or paper, especially the non official ones, should only be trusted for navigation close to dangers if the accuracy of the chart over the route has been proven by oneself before.

The other thing that people often do not consider is the inaccuracy of the depth contours on the chart. While above water dangers are normally quite accurate (but as said not necessarly so) the depth contours are much less so and in my view they should never be considered reliable for shallow water operation even if the bottom is rocky unless the route has been proven by oneself.

John
 
WAAS Accuracy

Correction.

WAAS, in my experience, halves inaccuracy, from 60-40' TO 30-15'.

That's a long way from <2m.

My old chartplotter had a calibration routine, which allowed me to accomodate any inaccuracy, the new one doesn't and is a consistent 12m West of actual position

I'd agree, the charts are far more likely to be the source of the inaccuracy than the GPS receiver and using GPS alone for close-work in poor visibility is definitely inviting disaster.
 
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In my view, charts whether electronic or paper, especially the non official ones, should only be trusted for navigation close to dangers if the accuracy of the chart over the route has been proven by oneself before.

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I would agree with that too John. All our 'potentially critical' routes have been checked over the years and waypint positions adjusted to fit onto the ACTUAL groundtracks as recorded by the plotter. This is for narrow channels and so on where we may need to get in/out/through in bad visibilty and especially with crosstides. Fortunately these are relatively few and it is easy enough to do once you have done each route a few times in good visibility and staying dead centre down the channels between the buoys etc. We have even run through one of them with the pilot set to follow 'track' as a check, it gives us confidence for when we need to do it in anger in fog. Our home marina is only 10 years old so newly charted, yet our track in/out consistently runs down the drying area on one side then neatly though the middle of the first row of boats in their berths, about 15m error I guess but at least it is always the same 15m in the same direction, and yes I have allowed for the GPS sensor location!

Robin
 
I am inclined to side with you. My FX324 chart plotter is fixed ie. no facility to alter chart datum as far as I know. It is mostly Wass enabled and is pin point accurate in the narrow river I navigate. When I said 50m out I suspect it closer to 100 but will have to measure when I get back to boat. I will talk to Mantsbrite (UK Magellan Dist.) to see what they say - I hope they suggest burning me another SD card with corrected chart.
 
I would set the map datum first............
map%20datum.JPG


then see if you still have a problem.
 
Aside from the Datum setting which Duncan has mentioned and shown where to chose it, WAAS is not available in Europe which will use EGNOS when it finally is available. Most sets only give you an on or off option so if you have 'WAAS' selected, it will actually run on EGNOS this side of the pond. However currently EGNOS is only running in test mode and it can result in worse accuracy rather than better. We have WAAS/EGNOS GPS plotters on board and are running them in standard mode for that very reason.
 
Re: WAAS

Yes I get that too if I switch 'WAAS' on but it is the EGNOS system satellites it is reading data from not the WAAS one where the satellites are over the USA, there is no option on our set to select WAAS and reject EGNOS, only to switch satellite differential (whichever it sees) on or off. Sometimes the results are better sometimes they are worse than standard, I have seen the position go 100m off for several minutes with the satellite differential 'on' and each time it cuts in or out it beeps -ergo it gets switched off until EGNOS is fully in service.
 
Re: WAAS Accuracy

That's because you shouldn't be using WAAS outside the US. The ground stations that measure inaccuracies, and send corrections back to the satellites, are designed to measure inaccuracies locally,ie in the US, not in the Med. Using WAAS outside the US is more likely to introduce inaccuracies than to increase accuracy.

Accuracy using correction overlays like WAAS and Egnos(is Egnos actually what you were using in the Med?)have been consistently been shown to give accuracies of well under 5m most of the time, and under 2m commomnly. Likewise, non corrected reception is usually under 10m, often under 7m
 
I have a Raymarine and navionics and found several errors in their Biscay chart but have not found any in the UK or Ireland ones.
The problem is that some charts Eire for instance are not WGS 84 compliant and so are 60 oto 100 yards out from the Gps position. The paper chart tells you this BUT not the electronic one!
 
Plotting accuracy ....

I have SHOM charts in BSB that consistently put me about 3 nm NNW of my true position here in Ventspils ...... that is with all datums etc. set correctly etc.

Error such as that is ok deep sea - but coastal is lethal !!
 
Misconception .....

Any electronic item - whether echo-sounder / GPS / Speed log etc. etc. is deemed AID TO NAVIGATION - not to be relied upon and should only be used in conjunction with visual and other proven methods of traditional navigation, pilotage of vessel or craft.

It is NOT a replacement for traditional .....

GPS by nature is NOT so accurate in terms of display of position in relation to its chart or data drawn on screen .... why ? Because the GPS uses modern datum that many a chart is compromise on or is only recently updated or corrected to that datum. The chart is still no matter how 'new' based on the original chart surveyed long time ago in the days of horizontal sextant angles, sun / moon and star sights etc. etc. All these had inherent errors .......... so how to accurately remove these ?

You have to build up familiarity with your area and machine used to navigate in that area .... if you sail outside of it - then you have to exercise extreme caution and NOT rely on the machine ....

Sorry if I seem to be teaching suck eggs etc. - but this sort of topic often wanders off down the GPS is God's bible of navigation ----- It is NOT.

/forums/images/graemlins/shocked.gif
 
Re: Misconception .....

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GPS by nature is NOT so accurate in terms of display of position in relation to its chart or data drawn on screen .... why ? Because the GPS uses modern datum that many a chart is compromise on or is only recently updated or corrected to that datum. The chart is still no matter how 'new' based on the original chart surveyed long time ago in the days of horizontal sextant angles, sun / moon and star sights etc. etc. All these had inherent errors .......... so how to accurately remove these ?

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Yes of course. However since the USA stopped degrading the civil GPS signals and removed SA the positions given by the GPS system have become very much more consistant (ignoring for the moment that the CHART might be inaccurate), that gives us what we used to have in the old Decca system - repeatabilty. Repeatabilty says that if the position as plotted, be that on a paper chart or an electronic one, is in position 'X' today it will be in postion 'X' next week, next year whatever. Therefore, ignoring Lat/Longs taken from the Chart (or plotter chart) and taking the Lat/Longs displayed from the GPS whilst following say the centre of a channel by mark one eyeball, we can create a repeatable route that we can rely on much more.

This is what we have done for example for a route through Chenal Du Four (between Ushant and NW France), through Le Raz De Sein (next gate south) and also for the entire channel for Poole from #1 buoy through to our home marina. In these cases where fog can be a serious problem we have adjusted original waypoint positions taken from charted sources with ones which fit exactly what occurs in practice when following a safe course through. In practice this is easiest with a chart plotter since you can set it to display the ground track as you follow the channel, then later nudge the waypoints to fit the displayed trackline. We have then followed the plotter/GPS waypoints precisely, using the off track error as well as COG etc, as a check, even to the point of letting the autopilot take us in 'on track' where all we have done is to 'approve' it's requested course change as each waypoint was reached. As a result we have high (but not total) confidence in these routes should we need to transit them in zero visibility with the help also of radar. All have now been used in foggy conditions at least once so it was worth the effort of doing it and checking the results. For the record we had done the same in the days before GPS and using the repeatabilty of Decca.

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It is NOT a replacement for traditional .....

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Well yes OK. But it IS in terms of priority of use! I'm well capable of navigating by traditional means (and even have a sextant) and was sailing in the days when there was not an option. If you asked me if I would believe my GPS position over my DR position, given no other reference - YES!

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You have to build up familiarity with your area and machine used to navigate in that area .... if you sail outside of it - then you have to exercise extreme caution and NOT rely on the machine ....

[/ QUOTE ]

Yes of course. But the 'circle of uncertainty' is a little bit smaller than it used to be on DR!


Robin
 
Just for the record ....

I was a ships Deck Officer for 17 yrs ..... navigating with Decca, Sextants, Loran, RDF, Transit Satnav, GPS Satnav, Syledis ......

should I go on ?? I do not know all the answers - am still learning ....

Decca repeatability - that is a debatable point !! given lane slip and decca corrections due to night, evening and day effects !! Only place in the world that used Decca for port entry WAS Rotterdam / Europort - Pilots had a super sensitive "Brown lane" box that they used to plug into the back of our Mk21 or later .... It used the lane that sat slap bang through the middle of Europort approach channel ..... and I can tell you that was the most accurate Decca I ever saw .... BUT it was not gospel and did wander at times .....

I commend your "tracing" a path to guide you in when fog ... but also note that you use radar as well. I would be very reluctant to do it with one item such as GPS in fog .... it is trusting the gods a little bit much for me ....
 
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