Navionics depth offset bug they won't fix?

duncan_m

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So a couple of interesting points. On the whether they know it's a bug, I'd assume at least one person at Navionics has a sailing boat and uses their app so while the support team might not be regular sailors who use lots of tech, someone must be.

Though as I said I'm not sure this a 'global' issue...but I also cannot see how it couldn't be given there's no way to set an offset and indicate whether that is from the bottom of the keel or to the waterline.

Updated scenarios..
So there are a couple of possible scenarios from what's been suggested:

- Navionics ignore offset in the DPT sentence for the app and their sonar chart - This sounds like the most likely and I am yet to meet any boater who's interested in the transducer's depth rather than the waterline or keel/hull bottom;

-Navionics ignore offset in the DPT sentence only for the app but do feed the full data back - This seems unlikely and as mentioned many people offset to keel bottom or lower for safety rather than waterline or whatever their preference may be so sonar would be even more invalid;

- This is only an issue with my hardware set-up - I await someone to say otherwise but NM0183 sentences are pretty standard, would actually prefer if I was wrong;

Resolutions..
Either way there are a couple of fairly simple fixes available:

- Use the offset in the sentence and ask the user to indicate if this is bottom of keel or waterline in the app. This prompt could be preset with the current offset as most owners are probably have a vague recollection as to how they have their depth sounders set.
- Ask user to confirm if it's keel/other then prompt to update a value to get it to waterline for sonar chart;

- Alternatively if there's some sort of consistency issue around DPT sentences from different multiplexers/brands etc that Navionics are aware of, then ask the user to set the offset in the app one to what they want it to show in-app and one to the waterline;

For me as a sailor..
From a person goals perspective of accurate instruments any resolution is good.

For Navionics sonar charts
From a Navionics getting accurate sonar chart data perspective, both of the suggests above require some sort of 'trust' from the users to put in accurate values so that the output of the charts is accurate.

So for data normalisation you probably want an extra data point or two to work with. I'd go for boat make and model. Then if Navionics spot a Bravaria 37 with an offset that's outside of the normalised range for the above setting for that boat make/model then they could automatically discount it's data from sonar charts from a depth perspective.

I'm also assuming that they have some pretty decent bathymetry people at Navionics and that they can normalise the data in other ways but have no idea what happens behind the scenes. As Seven Spades has mentioned if the chart is just plain wrong too often then that's hard to argue.

...
Btw not sure if I'm going OTT with detail here but I genuinely think it's important to have good data when you're on a boat. If it's not accurate and you or one of the crew rely on it albeit accidentally if you know it's wrong then bad things can and do happen. For a new navionics user / sailor etc then they might think they're wrong....
 

Sandy

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Not as far as I know. This thread is about their software.

Pete
True, but the initial data comes from a sensor and is transmitted to a display unit. Navionics are intercepting the data, via WiFi and we are adding AIS into the equation (no mention of it working with AIS off or using another transmission media, e.g. wire) and displaying it on a totally different system. I would not expect them to deal with the nuances of data sentences that were never intended to be used by a third party system. Then add into the mix the implementation of NMEA 0183 that the major manufactures have done over the years and it gets messy.
 

Sandy

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Btw not sure if I'm going OTT with detail here but I genuinely think it's important to have good data when you're on a boat. If it's not accurate and you or one of the crew rely on it albeit accidentally if you know it's wrong then bad things can and do happen. For a new navionics user / sailor etc then they might think they're wrong....
I think you are missing one important step. No company would/should publish safety critical data with just one source, I am sure @AntarcticPilot will have more experience in this area, for user acquired data there must be several of not tens of data trails. Simple things like what was the atmospheric pressure at the time of the reading, was there an offset on the sensor, did the skipper set the reading to total depth of water or depth under the keel and was there a lot of water flowing down a river/estuary comes to mind.

P.S. I spend the last five years of my working life looking at safety critical data, but not this type of data.
 

AntarcticPilot

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I think you are missing one important step. No company would/should publish safety critical data with just one source, I am sure @AntarcticPilot will have more experience in this area, for user acquired data there must be several of not tens of data trails. Simple things like what was the atmospheric pressure at the time of the reading, was there an offset on the sensor, did the skipper set the reading to total depth of water or depth under the keel and was there a lot of water flowing down a river/estuary comes to mind.

P.S. I spend the last five years of my working life looking at safety critical data, but not this type of data.
I was in the unfortunate position that we often had to publish maps with inadequate data sources simply because nothing else was available; Antarctica is like that. But, of course, we did as many sanity checks as we could on the data we used. Not all Antarctic charting agencies did this - I once (and I am pleased to say, only once) saw a map published by another national Antarctic survey that had an obviously incorrect representation of the ice surface; they had clearly accepted the results of an automated routine without checking against other sources!

I'm not a big fan of the Navionics sonar charts, but I know that they DO take into account the offset of the transducer when depths are reported, but it's inpit into the system when you register to return sonar data, not taken from the NMEA sentences. However, there are various issues, and these fora have noted examples where the Sonar Chart of an area has had gross errors obviously caused by an incorrect offset and possibly also an incorrect time being used to compute depth with respect to chart datum. Coupled with the automated contour generation routines used, this can (and sometimes does) result in misleading depths. For these reasons, I am glad that my chart-plotter is incapable of using the SonarChart data! It can work well in areas frequented by many reporting vessels, but in areas where there are few vessels, it can be very poor indeed. Further, it appears that Navionics don't do any manual checking of the resulting data - some of the errors that have been reported would be immediately apparent to a human check.

Hydrographic Agencies follow a variety of policies to ensure a) that the data are as good as possible and b) that they err on the shallow side as far as possible. Obviously, where the seabed is mobile, even the latest survey is only correct on the day it is done!
 
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