Navionics charts accuracy question

[Hipchick]

We have a Raymarine E7 chart plotter and use Navionics on it and on the ipad. We recently sailed up the Essequibo river in Guyana and knew that the charts weren't going to be reliable. We followed waypoints to reach our destination 40 miles upriver which worked well/ Our track showed us crossing over land in several places and was out in all directions. We're planning to go to Colombia soon and know that south of Cartagena they are unreliable as before in Guyana. Can anyone explain in clear English why this is and what can we do about it apart from use paper charts which we don't yet have.

 

[Tempus]

I don't know the specifics of your area, but I suspect that paper charts would be no more accurate as the Navionics data should be lifted from them. The problem is that the chart data has not caught up with GPS.

Many areas are still relying on surveys done (often by the Royal Navy, or even Captain Cooke!), with good old fashioned triangulation and compass bearings. Therefore charted objects correct in relation to each other, but the difficulty of ensuring that each fix is 100% accurate as you progress down the coast, or even rely on star sights for a longitude means that the overall chart can be several (hundred) metres out. Cross checking your position with a 3 point fix and treating the GPS with caution is the order of the day.

 

[Buck Turgidson]

I don't know the specifics of your area, but I suspect that paper charts would be no more accurate as the Navionics data should be lifted from them. The problem is that the chart data has not caught up with GPS.

Many areas are still relying on surveys done (often by the Royal Navy, or even Captain Cooke!), with good old fashioned triangulation and compass bearings. Therefore charted objects correct in relation to each other, but the difficulty of ensuring that each fix is 100% accurate as you progress down the coast, or even rely on star sights for a longitude means that the overall chart can be several (hundred) metres out. Cross checking your position with a 3 point fix and treating the GPS with caution is the order of the day.

Exactly this. Pre GPS none would have noticed the mapping errors but it's obvious these days. Better off using google maps photo images as your GPS chart and pilotage with the paper/navionics charts.

 

[NormanS]

As the others have said, this is not a fault with or by Navionics, but is simply because the original surveys were done without the benefit of the accuracy of GPS.

 

[jdc]

I don't know the specifics of your area, but I suspect that paper charts would be no more accurate as the Navionics data should be lifted from them. The problem is that the chart data has not caught up with GPS...

You are most probably spot-on, but I think there are two questions here:
1. Are all charts of that area unreliable - it being far from diligent hydrographers; and
2. Notwithstanding 1, are there differences between different chart suppliers (eg Navionics vs Garmin or C-map, and any or all of these commercial suppliers vs official sources like UKHO, USHO or French etc.

Commercial chart suppliers don't do their own surveying and so have to source their data somewhere. Do they just pick the cheapest (is there a 'market' in hydrographic data?), or take charts which are out of copyright or do their own hand digitising in Indian sweat-shops? What QA or checking is done? I wish I knew as I'm in the market for some E charts of far-away places at the moment, and would dearly like some means of establishing the quality of different suppliers before purchase lest I find out the hard way!.

 

[PeterR]

We have a Raymarine E7 chart plotter and use Navionics on it and on the ipad. We recently sailed up the Essequibo river in Guyana and knew that the charts weren't going to be reliable. We followed waypoints to reach our destination 40 miles upriver which worked well/ Our track showed us crossing over land in several places and was out in all directions. We're planning to go to Colombia soon and know that south of Cartagena they are unreliable as before in Guyana. Can anyone explain in clear English why this is and what can we do about it apart from use paper charts which we don't yet have.

I assume you have checked that you are using the correct chart datum? Your plotter will default to WGS 84 but that may not be what is being used in that part of the world and may need to be manually reset it. If you are on the correct datum it is worth checking with Navionics if they know of any problems with the charts in use. I have come across some Navionics charts which have been compiled with an inbuilt fault. If none of this applies I am afraid, as the others have said, you just have to accept that the hydrographic surveys havn't caught up with GPS accuracy.

 

[TradewindSailor]

Be aware to that it is all too easy to believe that it is a chart datum error and correct for it by using the offset feature. Often when one error it corrected another one is created. This is probably why Navionics still produce charts with long known major errors.

 

[GHA]

Exactly this. Pre GPS none would have noticed the mapping errors but it's obvious these days. Better off using google maps photo images as your GPS chart and pilotage with the paper/navionics charts.

+1
I had the same going up the river in Suriname. Chart plotting on the laptop was out but google earth was useful. As were eyeballs, depth sounder and following locals

There's a prog which will create charts viewable on Opencpn from Google earth images.

http://www.yachtvalhalla.net/navigation/ge2kap/ge2kap.htm

 

[jwilson]

We have a Raymarine E7 chart plotter and use Navionics on it and on the ipad. We recently sailed up the Essequibo river in Guyana and knew that the charts weren't going to be reliable. We followed waypoints to reach our destination 40 miles upriver which worked well/ Our track showed us crossing over land in several places and was out in all directions. We're planning to go to Colombia soon and know that south of Cartagena they are unreliable as before in Guyana. Can anyone explain in clear English why this is and what can we do about it apart from use paper charts which we don't yet have.

Where did your waypoints come from?

I have seen GPS tracks go well over land in old-chart-survey areas: I try only to trust GPS the second time I've been somewhere, after seeing the accuracy the first time.

 

[Roberto]

The two "most detailed" paper charts available for that area are these.
The inset of this first chart, showing some details of the entrance, is to a 1/100 000 scale

The navionics (or whatever electonic format) charts of the rest of the river, after the entrance, are likely to be based on this other paper chart. Sorry I could not find the exact scale, but say 1:500 000 ?

This is a zoom of one section of this second chart: between the two latitude parallels there are *5 miles*, look at the "grain" of the lines... If the chart is to IHO standard, the drawing line width might be around 150m, that is about 40m *per pixel* with this resolution.
A NAvionic chart would probably show neater lines, but it cannot give any additional precision against the paper chart.

I'll see if I can do a GE chart overlay

 

[chrisb]

Navionics and C map for that matter will use the latest available chart and as already said if that is many years 0ld that's what you get . Half way through a circumnavigation I have found the charts to be on the whole reliable and it to be fairly obvious when they are not .
I use radar on my approaches to synchronise the actual with the chart and forward looking sonar which is of immense value and has saved the bacon several times . This data can be viewed at the wheel on your e series

 

[jdc]

Navionics and C map for that matter will use the latest available chart and as already said if that is many years old that's what you get ...

But whose? They are Italian or American companies after all. Do we know the terms of their commercial agreements (if any) with the Hydrographic Offices of all countries?

And if it's an old chart it was never created as digital, let alone vector, data. So how is the digitisation done, by whom, with what probability or error? In recent months Navionics have made significant errors in the Solent of all places! So quality control is not infallible.

I think an assumption that electronic charts are all the same and have the best available data is unproven at best and perhaps naive.

 

[Hipchick]

I'll try the google maps, thanks.

 

[Hipchick]

Navionics and C map for that matter will use the latest available chart and as already said if that is many years 0ld that's what you get . Half way through a circumnavigation I have found the charts to be on the whole reliable and it to be fairly obvious when they are not .
I use radar on my approaches to synchronise the actual with the chart and forward looking sonar which is of immense value and has saved the bacon several times . This data can be viewed at the wheel on your e series

I would dearlylove to have forward sonar. Good tips thanks.

 

[Hipchick]

Where did your waypoints come from?

I have seen GPS tracks go well over land in old-chart-survey areas: I try only to trust GPS the second time I've been somewhere, after seeing the accuracy the first time.

The waypoints came from two sources. Both boats who had sailed there, the second boat making small improvements on the first. Guyana and Suriname Facebook page for first information which led to detailed logs/blogs from Tiger Lilly.

 

[Hipchick]

Those charts look interesting. I suspect they are ancient. Navionics chart had 'prominent tree' in lots of places but the whole river bank was covered in trees and had probably grown a hundred feet since the chart was printed. It was clear that it was unreliable and the water was the colour of milky coffee so no use looking to see what was in there as it was impenetrable. It was exciting and a fantastic experience.

 

[Birdseye]

In the good old days of Admiralty, the navy had charts of most all of the world produced with the use of british ships both naval and commercial. Nowadays the arrangement is that british charts for france, for example, use french data. For the US they use US data etc. The question is whose data do they use then they get to places that dont have their own? The answer in many cases will depend on who colonised the country so for much of the globve that was pink the data will be old british data.

Problem was that in the old days the mapping was done by triangulation. So if you didnt know exactly where your island was, then the position of that rrock offshore was equally wrong. And in lots of cases they didnt know where the island was with the requisite accuracy - after all they would have had to use sextants and clocks. In the end, the introductioon of satellite mapping showed that some old british charts of islands in the pacifi8c were as much as 3 miles off where they were, and even nearer home in cornwall or scotland the errors were sometimes half a mile. As many of these as possible were put right when UKHO moved from OSGB datum to WSG and issued new charts. But that doesnt get around all the problems in the thiurd world where surveys are either very old or poor.

As for which electronic system gives the best accuracy, I do not know. Only thing I would be sure off is that the UKHO raster charts will have been produced to the highest standards they could without much regard to cost.

 

[chuckr]

We're planning to go to Colombia soon and know that south of Cartagena they are unreliable as before in Guyana.

Not sure how you know that unless you have been there. Have you been there? We have and our electronic charts were pretty much spot on -- now we did not try to enter the bay on the eastern side but went all the way down as the entrance on the side has a brick wall across it - (thanks to the spanish who were trying and successfully repelled the brits there) - and the bouys may not be in the right location -
oh - on our entrance we had to tell them what agent we would be using to clear customs and you have to use an agent - so just be prepared for that

 

[AntarcticPilot]

But whose? They are Italian or American companies after all. Do we know the terms of their commercial agreements (if any) with the Hydrographic Offices of all countries?

And if it's an old chart it was never created as digital, let alone vector, data. So how is the digitisation done, by whom, with what probability or error? In recent months Navionics have made significant errors in the Solent of all places! So quality control is not infallible.

I think an assumption that electronic charts are all the same and have the best available data is unproven at best and perhaps naive.

There are international agreements between Hydrographic organizations to avoid multiple charting of areas. This is mediated by the IHO (the INT charts). So, it is unlikely that there will be more than one "latest" source of mapping. Several HOs may produce maps of the same area, but what happens is that one is the "producer" organization, and the rest are copies (legal and above board!) of the chart produced by the "producer" organization, with text and annotation in the language of choice.

I'm afraid it is likely that other posters have it right - there are no recent charts, and the ones that are available used pre-GPS survey techniques, with built in errors of a few hundred metres. I note also that it is likely that the river estuaries change rapidly as well!