Navionics sonar depth calibration?

[Seven Spades]

My iPad is connected to my boat network and so picks up the sonar information to record the depth that my boat sees. However I can’t see anywhere instructions on how to calibrate it. As far as I can see it will either take the information from the Raymarine display and convert that to what is thinks is chart datum or it is taking the raw feed from the transducer in which case it needs to know the depth of the transducer below the water level. I have my RAYMARINE set to display the depth below the keel if it is taking this as depth then it is recording depth 2.2m shallower than it is.

I cannot find either a setting nor instructions on how this is supposed to be set up. If it isn’t clibrted properly it is collecting bad data.

 

[[70521]]

In all the years, and boats, I have sailed on all calibration of depth readings have been done at the chart plotter not on an iPad.

You should be able to do the calculation in your head. Just add the depth of your keel + 1m to whatever figure you see and you should be safe. If the chart data is correct.

 

[TSB240]

Navionics will be logging the $SD DBT nmea sentence.
This sentence carries the measured depth below the sonar transducer and the offset of the transducer below the waterline. These added together will give the overall depth of water observed by the op.

If the op has set this offset correctly on his depth instrument or chartplotter he has no calculations to do and no calibration required on the navionics app or tablet.

I would be more worried about how they account for local tidal height and barometric pressure at the time of data collection!

 

[Roberto]

I would be more worried about how they account for local tidal height and barometric pressure at the time of data collection!

They don't, see older threads for countless examples of Navionics integrating raw depth data without any reduction to chart datum. Many drying areas are shown by sonar charts in ''always covered'' light blue as, of course, user depths could only be recorded while the area had some water over it, which is totally misleading.
Add the gps rollover date problem -no correct time stamp for a number of depth measurement with older instruments- as an additional difficulty.
Also, their depth interpolation method makes smaller features (rocks, tiny islets) literally disappear; use your kayak or dinghy with a recording sounder then circle around a small rock, it will be replaced by very nice and tight contours which of course make no sense.