bitbaltic
Well-Known Member
I'll take those two together. I'm not even talking about 'looking at a chart while navigating'. When pre-planning, I want to look over a chart in detail because I used to make geological maps for a living (and the government), so I understand the way in which representations of the earth, based on isolated data, are put together. Geological maps are in fact way more complex that hydrographic ones, but the way in which the guy drawing the linework works is the same (even when partly computer aided)- the final interpretation he gives you tells you something about his confidence. How have the contour lines been drawn- sweeping smooth lines or complex wiggles? What's the density of soundings, how much data is he interpolating through (or, how much of it is he presenting to you)? What's the variation in the values- might this be a dipping planar surface or something more like a lunar topography with only six soundings across it? Might a string of isolated deep water blobs across a drying grounds actually be representations of connected channels below the resolution of the sounding spacing? (this is actually not uncommon for older surveys). What's the cartographer's confidence- how certain is he that his final presented linework is a probable representation of the ground versus one of two or three possible interpretations, or one of a dozen, or one of a hundred? What confidence should I take from it, and how high should I expect the potential for error to be? How will I use all of this to plan my route? How good is this chart I'm using?
This is what I mean by wanting to assess the errors in the cartography seperately from the depth being adjusted for height of tide. It is the first step in proper navigation; it's also a process you'll re-iterate underway if you know how to read a chart in this way rather than just reading a depth off it and taking that as gospel. If you are not going through this step then you are not getting the most out of your charts (or yourself as navigator). Using charts in the way you describe is several steps down the path of navigating a passage for me.
This brings me to the next point which is that if you are reading any chart with this sort of an eye, then you are going a LONG way down the road to being sure you will not fall foul of the modern problem of 'old and sparse surveys being converted into crisp certainty'. Whether raster or vector (excl Navionics sonar for the moment) or paper, the shape of contours, style of linework, density of soundings won't change- the cartographer is telling you what the uncertainty is. You don't have to 'believe' stuff about chart inaccuracies, you can make an estimate for yourself.
I agree that tidal uncertainties are less- although I am less familiar with this type of modelling- but they are still not perfect models. There are parts of the Bristol Channel where flows are way outside anything you'll get in the tidal atlas. GRIB data like MyOcean does better, but my point is that the uncertainties in the tidal model, if important to the navigator, will be more difficult to assess if you have to back out the depth from CD in the first place.
Again you are talking about navigating in one particular place and/or by staring at a chartplotter as you go. If you take a large-scale overview of a long coastal passage, the height of tide will differ along the coast at any given point in time. Say, therefore, that your destination has more tide than you do at the arbitrary time, two days before, when you do your passage planning. The navigator will see the sandbanks lying offshore from his harbour of departure but all will be blue in the harbour of arrival, and he'll either have to work hard to check for those sandbanks, or go get a chart drawn to datum in any case in order to assess them, or when he arives tired just after dark, be happy to hit them and find out about them at that point. I can't see this is a good idea.
Because you are suggesting replacing CD with a datum of the sea surface, which is no datum at all because it moves up and down, not to mention and at different speeds in different places. The whole point of a datum is that it allows you to compare like for like at any scale, as I point out above- not just at a given local point of interest at a time relevant to what you're doing there right now. That's why people starting to navigate now should (and do) care about it. Yes it's necessary for 'static' charts but becuase tide height variations are more closely distributed than the origin and destinations of coastal passages, but you need a datum even for 'dynamic' (?- electronic) charts at small (large area) scales. Electronic or not, what you are suggesting is still a confusing datum shift over large distances: just like throwing away the necessary abstraction for paper charts and drawing a series across the south coast of the UK and using CD, HAT, and OD successively as you go then expecting someone to formulate a plan from Falmouth to Dover with it.
For the reasons above, I'm not. Perhaps if someone made a plotter which only ever displayed a heads up rolling road with 1.5Nm of cartography either side of the boat, it might be a selling point. But as most manufacturers know that safe navigation is done in context, I doubt they are up for it.
Moore's Law has already solved that one. Modern plotters are essentially Linux PCs in a waterproof box, the performance increase over, say, a Raymarine C80 from 10+ years ago must be enormous.
I should hope so but my NSS Simrad (admittedly a coupe of years old) doesn't fill me with confidence
