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  1. #1
    Join Date
    Aug 2013
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    SW Leicestershire
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    1,157

    Default Tablet and chart plotter disagree - mapping or hardware?

    Made an exploratory trip up River Lynher yesterday on half-tide ebb up to Ince Point and back. I noticed at times when Navionics (updated July 2019 on Samsung Tab A) showed I was on lightest i.e deeper area, the chart plotter Garmin GPS750 blue charts updated 2018) showed position on shallower water. In the past I have compared GPS positions from all devices onboard including Garmin GPS 128 and found slight differences. I am wondering if the Navionics mapping now might be more accurate? I also wonder whether it is possible or desirable to split the signal from the 128's external GPS aerial to feed the chart plotter?

    On a separate question I want to create a track in Navionics and get it "validated" is there any way I can import that into my imac and print it off or put in a PDF for export?

  2. #2
    Join Date
    Feb 2004
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    Home: Saffron Walden... boatless
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    2,507

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    First point, if you were relying on the colour coding it might simply be set differently. Also, Navionics shows actual depths when you have a tidal flow sensor selected.

    Or, are you saying that the two plotters actually show you in different physical places? That's more concerning.

    Second point - yes with the Samsung for sure, use the Web App on your Mac and log in using the same credentials. Routes sync between them. I use this a lot (or did, when I had a boat... )
    Graham. "Scala" now sold. Boatless.

  3. #3
    Join Date
    Aug 2011
    Location
    On the Celtic Fringe
    Posts
    14,396

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    How accurate do you expect a survey to be? In a tidal river there can be significant changes in the sea/river bed day by day. We are working in a very dynamic environment and need to trust our depth sounder/lead and line/long stick.
    Cynical Scottish very nearly retired engineer who sails a Gib'Sea 96.

  4. #4
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    36,975

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    Quote Originally Posted by eddystone View Post
    I also wonder whether it is possible or desirable to split the signal from the 128's external GPS aerial to feed the chart plotter?
    I don't think so, as I recall it's a passive antenna.

    Like Scala, I'm not sure whether you saying that your two GPS devices are giving different positions. Are the charts working on the same datum?

  5. #5
    Join Date
    May 2007
    Posts
    38,801

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    The survey data may be different.
    The 'colouring in' may be done using different estimates of contour lines from limited soundings.

    First thing to do is look at the echo sounder if you want to know what depth you're in.
    Second thing, check your GPS position from either source by getting to a well charted point, like in line with the end of the breakwater, or a channel marker.
    Then you have to realise that GPS positions are not always accurate to the nearest metre.

  6. #6
    Join Date
    Jul 2018
    Location
    Chichester
    Posts
    51

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    Quote Originally Posted by eddystone View Post

    On a separate question I want to create a track in Navionics and get it "validated" is there any way I can import that into my imac and print it off or put in a PDF for export?
    Not sure what you mean by create a track and get it validated. Do you mean create a route (i.e. a series of waypoints) or record a track (i.e. a record of where you have been) ?

    If you mean you want to export a record of a track that you have sailed, then Navionics allows you to export a GPX file which is a standard mapping record. You can email the file to your Imac an open it using any standard mapping software. Or if you have Navionics on your Imac the track will already be there.

  7. #7
    Join Date
    Feb 2004
    Location
    Home: Saffron Walden... boatless
    Posts
    2,507

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    Yes, wot Benny said ^

    I missed the Track vs Route distinction.
    Graham. "Scala" now sold. Boatless.

  8. #8
    Join Date
    Aug 2013
    Location
    SW Leicestershire
    Posts
    1,157

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    Yes sorry I meant route in order to ask boatyard whether that accords with most recent information - I understand that the river bed is dynamic. I will check all devices working to same datum and recognise that inaccuracies could be of greater magnitude than variations between devices. It sound like the issue is mapping variations - if I have to go with one I'd be inclined to trust Navionics more.

  9. #9
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    36,975

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    Quote Originally Posted by eddystone View Post
    It sound like the issue is mapping variations - if I have to go with one I'd be inclined to trust Navionics more.
    Garmin BlueCharts are based on Navionics cartography, aren't they? After all, Navionics is owned by Garmin.

  10. #10
    Join Date
    Aug 2013
    Location
    SW Leicestershire
    Posts
    1,157

    Default Re: Tablet and chart plotter disagree - mapping or hardware?

    Quote Originally Posted by pvb View Post
    Garmin BlueCharts are based on Navionics cartography, aren't they? After all, Navionics is owned by Garmin.
    I didn't think so - Navionics have been around for many years and only just bought by Garmin. Interesting to see how that affects Raymarine who are direct competitor in marine electronic market and use Navionics in their plotters... for the moment.

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