Plotter slightly "out"

[Andy Bav]

We have a Garmin 4018 plotter and loaded a bluechart SD card. Haven't really used it a great deal in the Med yet.

When we were in the UK, if we zoomed in tightly it would pinpoint us within a matter of inches. When we went to a new marina in the Med and zoomed in, it showed us c15m East of a breakwater as opposed to our actual position which was tied up to a Quay West of the breakwater.

It's not a big deal but has anyone had a similar experience?

 

[Momac]

GPS is not a precise as you might think
https://www.ordnancesurvey.co.uk/bu...avigation-technology/gps-beginners-guide.html
and the chart is only as accurate as the survey data.

 

[mikemanor]

Have you checked the datum being used on the Med charts?

 

[colingr]

As Matyn has said, GPS accuracy can vary and the chart may be based on something created many years ago so not at all accurate to that level.

I think our assumption that GPS is spot on is skewed by the car GPS systems that have clever software to snap you to the nearest bit of road so avoiding it looking like you are driving through a field next to the road. Marine GPS units go not do this.

To be within 15m is OK.

Useful info here

Click the word "here" as the link doesn't seen to want to look like a link on here any more

 

[gjgm]

One of the more entertaining threads on here was many years ago with Gludy giving his version of this topic. In fact, as turned out like often he was technically right.
So the 95pct 15m idea is what we all feel comfortable with. AFAIR, that isn't 15m from a centre point though with a circle around it, but lots of circles of 15m.
Anyway, I glazed over well before post 999 I expect which is why I cannot now explain. Still, turns out you could be WAY off your centre point, but usually are not.

 

[Deleted User YDKXO]

It's not a big deal but has anyone had a similar experience?

Yup I've seen major errors in electronic cartography in the Med even on recently purchased cards such as not showing a massive new breakwater across the entrance to Ibiza Town harbour and similarly not showing the new entrance into Barcelona harbour. Basically I don't trust electronic charts and I always consult other sources of information such as traditional paper charts and pilot books as well when planning a passage. Of course the best tool you've got is your Mk1 eyeball

 

[Andy Bav]

Thanks all. It's a non OEM card which I thought may have been partly to blame, but if 15m(metres) is an acceptable margin of error I can live with that.

If we land and the locals are speaking French that may be a bigger issue!

 

[prv]

Basically I don't trust electronic charts and I always consult other sources of information such as traditional paper charts

The paper charts are significantly (half a mile or more) off in parts of the Ionian, or were when I was last there. If a place was last surveyed by a Royal Navy officer in a rowing boat with a sextant, it doesn't matter what kind of chart you look at.

Pete

 

[Deleted User YDKXO]

The paper charts are significantly (half a mile or more) off in parts of the Ionian, or were when I was last there. If a place was last surveyed by a Royal Navy officer in a rowing boat with a sextant, it doesn't matter what kind of chart you look at.

Pete

Correct but the electronic charts are often based on the same survey data so you'll get the same errors. Having said that I found some significant variations in Croatia between Navionics electronic cartography and the local charts produced by the Croatian Hydrographic service including a large clump of rocks outside a well known marina which weren't shown on the electronic chart at any scale. I suppose one shouldn't rely on just one single source of information whether it be a paper chart or an electronic one or a pilot book and the more sources of information you have the better your judgement can be

 

[AIDY]

Have you checked the datum being used on the Med charts?

My thought to is it set to wgs84

 

[prv]

Correct but the electronic charts are often based on the same survey data so you'll get the same errors.

Exactly, that's what I meant. Looking at different presentations of the same ancient survey.

Pete

 

[Vidas]

Was coming west in Loch Ness last November on a tug that had loads of Nav gear , unfortunately it was about 30m out when compared to my IPad navionics app
We ran aground in the dark!!

 

[Bandit]

As said above check you are set to the right datum WGS 84 or whatever it is for that area of the med.

 

[Andy Bav]

Thanks for the datum suggestions - time to read the manual I think.....

 

[henryf]

You must have a much smaller boat than us if you are trying to work to the nearest 15 metres. It would depend where abouts on the superyacht we placed the transponder.

Henry

 

[Andy Bav]

You must have a much smaller boat than us if you are trying to work to the nearest 15 metres. It would depend where abouts on the superyacht we placed the transponder.

Henry