Originally Posted by Copinsay
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 Indeed, there's the pretty accurate saying that any map or chart is out of date as soon as it's printed.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
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.