Less information on Garmin charts?

[BogMonster]

I was quite surprised when looking at some paper charts on a commercial vessel, to find that some large areas of a harbour are shown as being surveyed by lead-line in the 1840s ... there is no distinction on the electronic charts between that and ones done from modern (which here is <30yr old) surveys using modern methods. The paper chart set had a key showing when the survey was done that each chart was based on, which was quite enlightening!

[tonto]

Originally Posted by Copinsay

Hello Erin

Thanks for this - I quite agree re coverage... Indeed up here we still have areas that have never been surveyed and so are basically blank on the Admiralty paper charts (apart from the word "unsurveyed) or as they put it on the Garmin chart "inadequately surveyed area". Indeed, there's the pretty accurate saying that any map or chart is out of date as soon as it's printed.

1 Any chart or map is an interpretation of the real word based on certain rules on what to show and how.

2 For the paper charts were are used to (and love) those rules are pretty well known and documented

Steve

I agree to a certain extent, but have the following observations/experience.

The paper charts have had and still do have massive flaws, and limitations. Just this morning we passed a dangerous (to many commercial ships) on the congested entrance to Singapore straits a shallow bank, which has the words "reported to lie 2 cables NW" which in other words is 555m or over 1/2 a kilometer out of position. This paper chart has had that notation on it for many years, and despite later reprints has not been corrected. This is one example of hundreds I could quote.

The other was that paper charts were not always accurate across the entire chart. Let me explain. Again in this area, using the Malacca straits for navigation, and if you were using islands, headlands and other landmarks from one side of the straits to fix your position and then used the opposite side, they would not agree. The features were right, but in slightly the wrong places.

When we all started using Sat Nav 30 years ago it was realised that the earth was not as we thought it was, and was not the correct shape, or height, as we had expected, and we had to come up with a "best fit" model to standardise it, which is where WGS 78 came from, Then GPS came along with more accuracy, and we had to refine it further, so came up with WGS84, which is what we use today, but it is not perfect. A lot of the problems come from the position fixing systems in use, in relation to the datum of the chart.

In the old days, we would steer well clear of any dangers, as we would not route near them. With the advent of GPS, people rely on the fixing accuray to the nearest few meters and expect the chart to have that detail, when they just never did, paper or the ENC's of today.

As I have said before to get the level of detail as a full detail Admiralty ENC, there is an enormous amount of data, which would slow down and disable many "simple" chartplotters, so I guess a lot of the more data intensive features are simplified, and am guessing this is what makes them usable and more importantly Cheap! They will get there eventually, just not tomorrow!

I love the use of paper charts, and we have just started getting rid of a lot of them, it will be a sad day to see them go. but they do have innacuracies, far worse than quality ENC's. It all comes down to how to use each, and how to trust each.

[9D280]

I have only recently hit the dizzy heights of plotter. My trusty Garmin 12 has a set (well, OK, sets - I have to upload the appropiate set for where I'm going due to lack of memory in the machine) of waypoints which if dots are joined, will keep me well clear of anything solid in low visibility.

I also have a set of A5 laminates under the seat and one of these funny magnetic things on the top of the console.....

[BumbleAbout]

There is a very good explanation of both chart and GPS issues in Nigel Calder's book "How to Read a Nautical Chart" (significant features of seabed missed when surveying/ GPS assuming that the surface follows a mathematical shape/ changes in features compared to when a survey took place etc). I think that it is well worth reading just to understand the issues with chart accuracy, never mind the additional issues with displaying similar data on a moving map GPS.

Steve's comment above a plumb line survey in the 1840s highlights one of the classic issues where the QE2 (from memory) ran into a rock in the Atlantic a couple of years ago which were part of a mid ocean ridge between two survey lines (i.e. deep enough where they surveyed, but not between the survey lines)!

Never mind a GPS - a compass and charts won't help you if data is missing from the charts in the first place!

[tonto]

Originally Posted by BumbleAbout

There is a very good explanation of both chart and GPS issues in Nigel Calder's book "How to Read a Nautical Chart" (significant features of seabed missed when surveying/ GPS assuming that the surface follows a mathematical shape/ changes in features compared to when a survey took place etc). I think that it is well worth reading just to understand the issues with chart accuracy, never mind the additional issues with displaying similar data on a moving map GPS.

Steve's comment above a plumb line survey in the 1840s highlights one of the classic issues where the QE2 (from memory) ran into a rock in the Atlantic a couple of years ago which were part of a mid ocean ridge between two survey lines (i.e. deep enough where they surveyed, but not between the survey lines)!


Never mind a GPS - a compass and charts won't help you if data is missing from the charts in the first place!

The QE2 ran aground because of Squat, She sank bodily due to her speed, and in known shallow water. Yes it was an uncharted rock, but she was going too fast in that area, and the rock was not significantly more shallow than the rest, however that digresses from the OP!

However with ENC's it is much easier to get any data out there as a correction in digital format.

The last point is very apt too, as how many people get the NTM, and do all chart corrections on their paper charts every week? (including the laminated versions?) The electronic ones are a a lot easier to correct (if there is a correction available weekly.... and I am not sure if there is with Garmin???)

[BumbleAbout]

Originally Posted by tonto

The QE2 ran aground because of Squat, She sank bodily due to her speed, and in known shallow water. Yes it was an uncharted rock, but she was going too fast in that area, and the rock was not significantly more shallow than the rest, however that digresses from the OP!

Outrageous - you mean that there was some spin involved in how the incident was reported to the general public ?

[GordyP]

I know that the Garmin plotter I have is very inaccurate in a number of specific places, don't know whether this is the chart or the GPS signal at these points. Its very significant in the Cuan Sound where if you follow the 'safe route' on the plotter it actually takes you directly over the rock which should be avoided, similar situation at the McCormick Isles.

It is accurate most of the rest of the time, I certainly don't completely trust it and use it in conjuction with paper charts and pilot books.

[9D280]

There is the high level argument of course that if you are that close to shallow stuff, someone looking over the bow might be the most appropriate measure!

Prime example - Stirling Jubilee cruise - I had to trust that Kev (who I know has canoed the river dozens of times) knew where he was as I crossed brown & greeen bits of the chart at 25 knots on a falling tide as I followed in his wake. Somewhat unnerving to put it mildly, but Kev knows that river and I still have all the paint on my skeg!

Havng now added local knowledge to the discussion I'll stand aside & let the games begin!