john_morris_uk
Well-Known Member
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It's not just a vertical offset. The coast and depth lines would not join smoothly when offset.
Clearly there is aproblem with the chart as you have it. The electronic chart people have been a bit careless issuing the chart with this error. There may be an underlying error in the original data source - presumably UKHO.
It's probably better to have an obvious error than someone unknown doing a bit of "smoothing". At least you know you should keep a reasonably wide berth here.
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Keep a wide berth WHERE???!!!
Either the chart on the right or the chart on the left is approx 100m incorrect... and I've no way of working out which... I'll actually have to keep a wide berth throughout both to be safe... and how can I trust all the other charts if I can't trust these two.
This isn't gonna help me very much when I'm picking my way carefully along a narrow channel (that's less than 200m wide) in limited visibility!
[/ QUOTE ]I will try and answer a couple of points in your quoted reply and your comments.
One of the problems you have with the electronic chart is that you have no idea where the data has come from and what its provenance is. It will have been taken from UKHO sources, but you don't know what scale the original charting data on either side of the line was drawn to. With a plotter using vector charting data it is possible to zoom in to a scale that appears to give you accuracy that just doesn't exist in the original data. A paper chart is always drawn to as accurate a representation as possible for the scale used. Furthermore, the chart will have a diagram showing who did the surveys and when (and by what method). A sensible navigator will use this diagram to give some 'weight' to the accuracy of the part of the chart he/she is on.
Coastal areas which are rocky and away from port areas tend to get surveyed less frequently than sand or mud channels in ports and their approaches. Charted contour lines on paper charts 'jump' when they go from one survey data to another. The Cartographer uses the best data he/she has and can't do anything else. The jump on your chart amost certainly isn't due to survey data change.
The jump on your chart is probably due to it being the join between two charts that have been entered electronically. I have no idea which side is correct (if either!). The problem is exacerbated by you having no data on the charting that was entered. Its a bit unfair to say UKHO have got it wrong - we have no idea what the electronic charting people have done with the data when they converted it for your plotter.
The reason that you are taught to use contour lines for reduced visibility navigation is that they are usually very accurate (on paper charts) and that is one reason why charts aren't cheap.
[ QUOTE ]
It's not just a vertical offset. The coast and depth lines would not join smoothly when offset.
Clearly there is aproblem with the chart as you have it. The electronic chart people have been a bit careless issuing the chart with this error. There may be an underlying error in the original data source - presumably UKHO.
It's probably better to have an obvious error than someone unknown doing a bit of "smoothing". At least you know you should keep a reasonably wide berth here.
[/ QUOTE ]
Keep a wide berth WHERE???!!!
Either the chart on the right or the chart on the left is approx 100m incorrect... and I've no way of working out which... I'll actually have to keep a wide berth throughout both to be safe... and how can I trust all the other charts if I can't trust these two.
This isn't gonna help me very much when I'm picking my way carefully along a narrow channel (that's less than 200m wide) in limited visibility!
[/ QUOTE ]I will try and answer a couple of points in your quoted reply and your comments.
One of the problems you have with the electronic chart is that you have no idea where the data has come from and what its provenance is. It will have been taken from UKHO sources, but you don't know what scale the original charting data on either side of the line was drawn to. With a plotter using vector charting data it is possible to zoom in to a scale that appears to give you accuracy that just doesn't exist in the original data. A paper chart is always drawn to as accurate a representation as possible for the scale used. Furthermore, the chart will have a diagram showing who did the surveys and when (and by what method). A sensible navigator will use this diagram to give some 'weight' to the accuracy of the part of the chart he/she is on.
Coastal areas which are rocky and away from port areas tend to get surveyed less frequently than sand or mud channels in ports and their approaches. Charted contour lines on paper charts 'jump' when they go from one survey data to another. The Cartographer uses the best data he/she has and can't do anything else. The jump on your chart amost certainly isn't due to survey data change.
The jump on your chart is probably due to it being the join between two charts that have been entered electronically. I have no idea which side is correct (if either!). The problem is exacerbated by you having no data on the charting that was entered. Its a bit unfair to say UKHO have got it wrong - we have no idea what the electronic charting people have done with the data when they converted it for your plotter.
The reason that you are taught to use contour lines for reduced visibility navigation is that they are usually very accurate (on paper charts) and that is one reason why charts aren't cheap.