Navionics and Sonarcharts

[noelex 77]

Electronic charting is starting to include detail that is not available on paper charts. I wonder how long it will be before it is considered poor seamanship not to have electronic charts available .

Some of this information can be very useful, some less so. The latest is Sonarcharts. These add a lot more depth detail than is available on the normal electronic or paper chart. It makes the map too cluttered for normal navigation, but this sort of depth detail can be very useful when zoomed in, say approaching an anchorage. The sonar detail can be turned off and on.

The big question is how accurate is this data? How is it derived? The information is not on the equivalent paper chart.

I suspect it is a computer generated best fit, or prediction based on the available depths. Useful, but with obvious limitations. However, some of the information is actually different which does not fit with this theory.

Knowing how the data is produced would greatly help in choosing what level of confidence to place in the information. Does anyone know?

For those who have not seen the Sonarcharts here is a random anchorage with the sonar detail turned off and on. At this level of zoom the sonar chart is becoming a bit cluttered, but as you zoom in that problem disappears. The depths are in metres.

[AnchorageGuy]

If you go to Navionics website, it explains the process. Chuck

[Garrettw]

Originally Posted by noelex 77

The big question is how accurate is this data? How is it derived? The information is not on the equivalent paper chart.

It is derived from boaters like you and me. My current generation Raymarine plotter has a software function to enable sonar logging. It stores pings at a short interval and I can export them to Navionics. They integrate it into the Sonarcharts. When I do an upload I get an email back in a few days saying that it has been processed and added so that I know I can do a download and have my latest data (and everyone elses that had been processed in that time). This chart data isn't suitable for navigation purposes though because it requires me, as the data gatherer, to insure that my GPS positioning is accurate, my depth transducer is working properly and that my depth offset is set correctly to read depth from waterline. Any of those things are messed up and you are going to pollute the Sonarcharts' database. A true depth survey takes pains to get all those parameters unquestionably set. With that said, it is really useful as another datapoint to use in your overall decisionmaking. It is particularly helpful when trying to find shifting channels and areas of shoaling, rather than looking for the absolute value of depth.

[noelex 77]

Originally Posted by Garrettw

It is derived from boaters like you and me. My current generation Raymarine plotter has a software function to enable sonar logging. It stores pings at a short interval and I can export them to Navionics. They integrate it into the Sonarcharts.

Thanks for all the replies.

It would be great if all this information is really gathered like that. It makes these electronic charts a lot more advanced than anything that has been available before.

Forgive my skepticism, but there is sonar data that covers very remote areas. I know these locations would be lucky to see a boat every couple years. Even then it will be a local fishing boat. The chances that a boat that equipped to download this information has traversed the whole anchorage seems rather unbelievable.

Surely there must be a great deal of extrapolation, but I must admit I found them accurate so far.

Thoughts?

[Garrettw]

Originally Posted by noelex 77

Thanks for all the replies.

It would be great if all this information is really gathered like that. It makes these electronic charts a lot more advanced than anything that has been available before.

Forgive my skepticism, but there is sonar data that covers very remote areas. I know these locations would be lucky to see a boat every couple years. Even then it will be a local fishing boat. The chances that a boat that equipped to download this information has traversed the whole anchorage seems rather unbelievable.

Surely there must be a great deal of extrapolation, but I must admit I found them accurate so far.

Thoughts?

Gap areas appear to be populated with their standard cartography. In my area there is water that no one goes to for various reasons and the data there matches the standard bathymetry. On the other hand, areas that are heavily traversed, especially with commercial traffic, seem to differ the most or have the most granular data.

[noelex 77]

Originally Posted by Garrettw

Gap areas appear to be populated with their standard cartography. In my area there is water that no one goes to for various reasons and the data there matches the standard bathymetry.

On the European maps, at least in my area, that is not the case.

This is an example. The conventional Navionics data is very sparse with only the 20m contour line and single 4m depth. Both the 4m depth and the 20m contour lines are quite accurate, but SonarCharts have placed a series of depth that are wrong. It is unlikely that any boat capable of reporting sonar data has been into the anchorage.

I have anchored a couple of times here apparently crossing the 0.5m contour line. We have a 6 foot draft and there is negligible tide in area so this obviously impossible. I am not trying to pick isolated errors in the maps, rather point out large areas where it seems very unlikely that any measurements have been made at all.

It seems to me that a large portion of the information on the SonarCharts is a computer prediction. This is still useful, but the maps don't seem to differentiate between the areas of hard data and those where only a computer model exists.

Perhaps this should be clearer. It is of concern if people believe all the data is based on actual measurements.

[Garrettw]

Originally Posted by noelex 77

On the European maps, at least in my area, that is not the case.

This is an example. The conventional Navionics data is very sparse with only the 20m contour line and single 4m depth. Both the 4m depth and the 20m contour lines are quite accurate, but SonarCharts have placed a series of depth that are wrong. It is unlikely that any boat capable of reporting sonar data has been into the anchorage.

I have anchored a couple of times here apparently crossing the 0.5m contour line. We have a 6 foot draft and there is negligible tide in area so this obviously impossible. I am not trying to pick isolated errors in the maps, rather point out large areas where it seems very unlikely that any measurements have been made at all.

It seems to me that a large portion of the information on the SonarCharts is a computer prediction. This is still useful, but the maps don't seem to differentiate between the areas of hard data and those where only a computer model exists.

Perhaps this should be clearer. It is of concern if people believe all the data is based on actual measurements.

Interesting. I agree that we should be able to view, in some way, where actual data has been incorporated.