Raymarine chart is offset... please help

[EugeneR]

Currently chartering a boat in Croatia with a Raymarine RLC70C plotter. The plotter shows my position wrong by around 200 meters (rough guess) even after a while. GPS reports precision as 2m.

I have now configured the chart with an offset... right berth at least as opposed to out at sea... but I wonder if there is a known issue involved? Please advise because I have no guarantee hat my offset will remain constant over time so cannot rely on the plotter for accurate maneuvers.

I know the Croatian paper charts need to be adjusted to be wgs84 plotable but would have expected the raymarine, which is set for wgs84, to know this.

Thanks

 

[Piers]

Strange. So does the chart plotter show the chart it's using as WGS84? And is the plotter's datum set to WGS84?

 

[Ridax]

Have you tried changing the map datum to hermannskogel or european 1950 to see if it makes any difference?

 

[prv]

Is it possible the electronic charts for that part of the world are simply wrong? I know the paper ones were when I was last there a dozen or so years ago. Not a datum difference, just simple inaccuracy because they were last surveyed by the Royal Navy in the 1800s. Each bay or island would be accurate within itself, but its exact location could be off by a few hundred yards. There was a note to this effect on the chart.

Maybe they've improved things since (aerial photography seems like a cheap way to calibrate positions without sending out a survey ship), maybe they haven't.

Pete

 

[little_roundtop]

Is it possible the electronic charts for that part of the world are simply wrong? I know the paper ones were when I was last there a dozen or so years ago. Not a datum difference, just simple inaccuracy because they were last surveyed by the Royal Navy in the 1800s. Each bay or island would be accurate within itself, but its exact location could be off by a few hundred yards. There was a note to this effect on the chart.

Maybe they've improved things since (aerial photography seems like a cheap way to calibrate positions without sending out a survey ship), maybe they haven't.

Pete

+1
We certainly have that problem in a few places around Crete. The surveys are old and they're simply wrong. I know of one anchorage (Gramvousa Island) where there is an error close to 200m.

 

[prv]

+1
We certainly have that problem in a few places around Crete. The surveys are old and they're simply wrong. I know of one anchorage (Gramvousa Island) where there is an error close to 200m.

The interesting question then is whether whatever electronic map format Raymarine use is able to display the kind of notes that are printed on a paper chart.

Pete

 

[krissteyn]

I would be interested to know if any other raymarine (navionics) systems in your area had a similar "offset" , and if Garmin systems (using garmin charts) also had a problem.

Of course , you could always haul out the sextant and re-check it (just kidding)

Kris (about to buy either Raymarine or Garmin ???)

 

[EugeneR]

I have played with datums no luck. All are wgs84. Does seem accurate in the region so more comfortable. Garmin handheld and several Navionics phones and tabs are all accurate.

 

[AntarcticPilot]

Is it possible the electronic charts for that part of the world are simply wrong? I know the paper ones were when I was last there a dozen or so years ago. Not a datum difference, just simple inaccuracy because they were last surveyed by the Royal Navy in the 1800s. Each bay or island would be accurate within itself, but its exact location could be off by a few hundred yards. There was a note to this effect on the chart.

Maybe they've improved things since (aerial photography seems like a cheap way to calibrate positions without sending out a survey ship), maybe they haven't.

Pete

Aerial photography ain't cheap! Aircraft time plus post-processing makes it expensive. Accuracy requires stereo-analysis, which in its turn requires a scattering of accurate (better than stand-alone GPS; ideally you need centimetre accuracy, which is attainable but takes an hour or two observation at each point) ground control points. But it is cheaper than using surveying techniques for any significant area, and you are correct to suggest it is the most cost-effective way for a detailed survey. But cheap it isn't, and it will only get done as part of a comprehensive re-survey.

 

[prv]

Aerial photography ain't cheap!

I was actually thinking of using existing imagery (satellite or aerial) rather than commissioning new. Hell, even using Google Earth to drag an island 200 yards across a chart to where it actually appears to be would be an improvement. I know Google Earth isn't meant to be accurate for survey purposes, but just reducing the size of the error would help.

Pete

 

[LittleSister]

The plotter shows my position wrong by around 200 meters (rough guess) even after a while. GPS precision as 2m.

I have now configured the chart with an offset... right berth at least as opposed to out at sea... but I wonder if there is a known issue involved? Please advise because I have no guarantee hat my offset will remain constant over time so cannot rely on the plotter for accurate maneuvers.

Does it matter? Presumably you found your berth Ok? Pre-GPS it would have been rare to know where you were within 200m, but people managed to navigate quite easily.

 

[jordanbasset]

Have noticed a couple of places where my C-MAP NT+ is not accurate, for example the harbour in Corfu town. As others have said normally not an issue as I can see the harbour and am not normally looking at the plotter. Over 99% of the time it is accurate to within a couple of metres.
I suppose if there was fog, unlikely in Corfu, it woud be an issue but if it was that thick would anchor off until it cleared or use radar.

 

[AntarcticPilot]

I was actually thinking of using existing imagery (satellite or aerial) rather than commissioning new. Hell, even using Google Earth to drag an island 200 yards across a chart to where it actually appears to be would be an improvement. I know Google Earth isn't meant to be accurate for survey purposes, but just reducing the size of the error would help.

Pete

Well, we do that in Antarctica where there isn't much else. But you have to ask the question, how is the satellite image geo-referenced? Often it is referenced to existing maps! And if it isn't, it is referenced to the satellite's orbital parameters; these are good to something like 100m. So, you end up with something that is of very doubtful reliability. OK if you're working to an accuracy of a hundred metres, but not Ok otherwise.

Satellite imagery also has a resolution of a metre or so if you use the most expensive, modern Quickbird or similar stuff. Good enough for a lot of purposes, but it won't show isolated rocks. Landsat ETM+ has a resolution of around 15 m; that is mostly what Google uses.

Google also masks sea areas, so the ABSENCE of a shoal in Google doesn't mean there isn't a shoal!

High resolution images are expensive, and not available unless someone has specifically requested that they be obtained.

All of this is good enough for government work, as they say, and for a local correction to a region you know well, it may well suffice. But relying on it for more than reconnaissance of possibilities is not good practise.

 

[prv]

All of this is good enough for government work, as they say, and for a local correction to a region you know well, it may well suffice. But relying on it for more than reconnaissance of possibilities is not good practise.

I'm not suggesting that anyone try to draw a new chart from cheap satellite images. Just that if you overlay the images on the chart and find that an island on the chart is 2 inches to the left of the same shaped island on the photo (assuming the mainland more or less lines up) then maybe the island isn't where Captain Sir Humphrey Fuzzy-Sextant RN originally put it

Pete

 

[Tranona]

I would be interested to know if any other raymarine (navionics) systems in your area had a similar "offset" , and if Garmin systems (using garmin charts) also had a problem.

Of course , you could always haul out the sextant and re-check it (just kidding)

Kris (about to buy either Raymarine or Garmin ???)

You will find errors in any/all of the electronic charts in the Med, simply because they are all based on the same raw data which is often inaccurate and out of date. You even find that charting for some features uses different raw data depending on the level of zoom. For example the C Map chart for Fiskardo on Cephalonia is accurate for approches to the harbour and the basic harbour layout. Zoom into the detail chart and suddenly instead of being moored on the quay you are 100m or so inland behind the taverna. Similarly, enter Lakka on Paxos and you sail over a hill, through the school yard, but do end up on the quay. Your track out is the reverse.

Fortunately there is far less reliance on accurate chartwork because of the lack of tides, clear shore features and good visibility. An up to date paper chart, or an electronic version running on a PC plus a basic plotter with waypoints at the helm is more than adequate.

 

[nigelmercier]

Is it possible the electronic charts for that part of the world are simply wrong?

My C-Map charts of the area certainly were.

 

[RichardS]

I always have to apply a 100m or so offset to my chartplotter in Croatia whilst in the home marina and it is then usually very accurate for most of the area where we sail (Zadar - Dubrovnik).

The plotter is set to WGS84 and the Navionics chart is the same so I assume that there is something slighly adrift with the transcription from the original Croatian charts which I believe were not WGS84.

Richard

 

[EugeneR]

Does it matter? Presumably you found your berth Ok? Pre-GPS it would have been rare to know where you were within 200m, but people managed to navigate quite easily.

It is convenient for accurate maneuvers yes e.g. following a narrow and unmarked passage with several underwater obstacles between two islands, avoiding a 10 mile detour, or evaluating the depth within one's swing radius when anchored close to obstacles etc.

Not required but when I have it available, I am happy to use it within the bounds of it's accuracy. Just like any modern technology... sure, you can survive without anchor alarms, the internet and even the wheel for that matter.