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Old 22-09-2020, 10:03   #1
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Avionics charted depths....

Are the depths displayed on Navionics screens datum or actual?
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Old 22-09-2020, 10:45   #2
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Re: Avionics charted depths....

Not actual but are the depths shown equivalent to the hydrographic charts?
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Old 22-09-2020, 10:56   #3
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Re: Avionics charted depths....

Any quality chart packages should give you the datum they use, just like paper charts. In many electronic chart packages, a right click will tell you what chart they are using.
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Old 22-09-2020, 11:00   #4
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Re: Avionics charted depths....

Quote:
Originally Posted by Rol1964 View Post
are the depths shown equivalent to the hydrographic charts?
generally yes . . . except it has been shown they have occasional (pretty significant) machine translation errors, and sometimes they have overwritten the charted depths based on user feedback (which has sometimes been accurate and other times not).
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Old 24-09-2020, 07:58   #5
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Re: Avionics charted depths....

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Originally Posted by Stu Jackson View Post
Any quality chart packages should give you the datum they use, just like paper charts. In many electronic chart packages, a right click will tell you what chart they are using.
I just purchased Navionics for Canada and USA. Right click doesn't yield any info. I must be overlooking something very obvious cause it doesn't indicate anywhere, what datum it's based on.
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Old 24-09-2020, 09:29   #6
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Re: Avionics charted depths....

If you are using the Navionics Android App (and perhaps Ios) :

App has a "water level correction" option. If you turn it on (I don't!), it corrects water levels based on the factor you enter in a map option below. If turned off or correction set to "0", then level indicated is usually same as official charts (at least for Canada), with small error occasionally due to conversion (eg if you view depths in feet but original chart is in metres).

I rarely "depend" on Navionics (i. e., I might view when planning, but do not navigate with them), but I do not use the water level "correction" feature. I could imagine an error where one sets one level and then forget to revise it when using at a later time with a different tide level. Best to stick with known datum.
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Old 24-09-2020, 10:00   #7
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Re: Avionics charted depths....

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Originally Posted by Breaking Waves View Post
generally yes . . . except it has been shown they have occasional (pretty significant) machine translation errors, and sometimes they have overwritten the charted depths based on user feedback (which has sometimes been accurate and other times not).
The user feedback (e.g., “sonar” chart view) is questionable, IMHO. You don’t know about the user depth sensor calibration or otherwise know how the user data is indexed to the vertical datum. Boater beware!
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Old 24-09-2020, 10:14   #8
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Re: Avionics charted depths....

I have zeen Navionic's "sonar charts" that are nonsensical: they were clearly/simply interpolations of official (and often widely spaced) soundings. Garbage.

As for user-contributed soundings: did they calibrate their sensor? Did they (and how) account for tide level?

No, I don't trust them. (And take all user-contributed data, such as Active Captain with a vrain of salt.)
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Old 24-09-2020, 10:17   #9
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Re: Avionics charted depths....

But I was trying answer Rol1964's (the OP) question about Navionics (rather than trash user contributions).

Not sure if Navionic's chart CARDS have same options/issues....
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Old 24-09-2020, 10:56   #10
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Re: Avionics charted depths....

I just started experimenting with Navionics this year and was wondering about that. The soundings and contour lines appear to be much more detailed than the official charts. I have to wonder where they got the data. Kinda doubt they went out and did their own surveys. And as others have said if it's user contributed you can't rely on any of it.
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Old 24-09-2020, 17:53   #11
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Re: Avionics charted depths....

And...my mobile devices indicate 32 ft of clearance for a particular bridge but the same bridge indicates 42 ft on my desktop...? Any ideas?
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Old 24-09-2020, 20:54   #12
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Re: Avionics charted depths....

Quote:
Originally Posted by Stu Jackson View Post
Any quality chart packages should give you the datum they use, just like paper charts. In many electronic chart packages, a right click will tell you what chart they are using.
This would be a good feature request. When you hit the question mark button for a location Navionics lists a number of properties, e.g. "Pacific Leatherback Conservation Area" that you can expand for more details. It seems reasonable to me that the expanded details for a sounding or depth zone should include the CATZOC or survey details.

Quote:
Originally Posted by fcallard View Post
The user feedback (e.g., “sonar” chart view) is questionable, IMHO. You don’t know about the user depth sensor calibration or otherwise know how the user data is indexed to the vertical datum. Boater beware!
It occurs to me that the the bathymetry from the sonar readings could be analyzed for a "best fit" to existing soundings based on position. You have soundings at specific coordinates on the official charts, so find the corresponding positions in the sonar data and calculate the average offset based on that. That way you needn't worry how the user calibrated their sensor. Just a random thought, of course.

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And...my mobile devices indicate 32 ft of clearance for a particular bridge but the same bridge indicates 42 ft on my desktop...? Any ideas?
No idea. Any bridge in particular?
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Old 24-09-2020, 21:35   #13
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Re: Avionics charted depths....

Quote:
Originally Posted by capt jgw View Post
I just started experimenting with Navionics this year and was wondering about that. The soundings and contour lines appear to be much more detailed than the official charts. I have to wonder where they got the data. Kinda doubt they went out and did their own surveys. And as others have said if it's user contributed you can't rely on any of it.
I don’t know anything about Navionics’ chart development process, but I do assist NOAA with maintenance of some of their hydrographic survey vessels. A NOAA chart doesn’t remotely show all of the bathymetric data collected by NOAA, but NOAA does make that data available. You can see what NOAA makes available at their website.

If you take a look at one of their “smooth sheets” overlaid on a chart you’ll see there are hundreds of actual soundings for each sounding label displayed on a chart. So, if Navionics uses the smooth sheets or BAG models (I don’t know that they do, only that data is available) they could provide much more detailed contours than found on standard charts.

Here’s a NOAA surface map image of part of San Francisco Bay that shows some of the additional data available:


And a smooth sheet showing all the soundings for a location overlaid on the actual chart (sorry, it’s a PDF, that’s what NOAA posted):

https://www.ngdc.noaa.gov/mgg/bathym...moothsheet.pdf
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Old 25-09-2020, 05:01   #14
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Re: Avionics charted depths....

Quote:
Originally Posted by requiem View Post
This would be a good feature request. When you hit the question mark button for a location Navionics lists a number of properties, e.g. "Pacific Leatherback Conservation Area" that you can expand for more details. It seems reasonable to me that the expanded details for a sounding or depth zone should include the CATZOC or survey details.



It occurs to me that the the bathymetry from the sonar readings could be analyzed for a "best fit" to existing soundings based on position. You have soundings at specific coordinates on the official charts, so find the corresponding positions in the sonar data and calculate the average offset based on that. That way you needn't worry how the user calibrated their sensor. Just a random thought, of course.



No idea. Any bridge in particular?
Pont Galipeault at Canal Sainte Anne - just west of island of Montreal.
Mobile gives 9.9 meters and desktop says 13.5 meters. But I found my error (the height entering the lock is higher than the rest of the bridge - difficult to pinpoint with phone screen) so looks like my boat will fit.
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