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Old 01-11-2017, 11:44 AM   #1
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Default Navionics Web App - Accuracy in Saltwater?

I am in SC and I have been looking at the Navionics Web App on their website to try to find spots to fish. My questions is how accurate are the contours on sonar mode and are they based on low or high tide?
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Old 01-11-2017, 12:07 PM   #2
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the convention is:

"numbers on a nautical chart are water depths at “Mean Lower Low Water,”

which is the average depth at the lower of each day's two low tides. "
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Old 01-11-2017, 05:59 PM   #3
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Quote:
Originally Posted by Ditch the Paddle View Post
I am in SC and I have been looking at the Navionics Web App on their website to try to find spots to fish. My questions is how accurate are the contours on sonar mode and are they based on low or high tide?
The 1' contours are pretty accurate as far as showing where the bottom is changing by 1'. The depth number on the contour is subject to be 1 - 3 feet off of the actual depth. Because they collect all of this data from the public and they have no way to know for sure exactly how everyone has their transducer set up or if the data was collected at speed or idle or exactly what the tide level was at that particular spot etc., there is no way the algorithm they use to try to correct for all of these variables is perfect and must be based on some assumptions. The algorithm tries to take into account the exact tide but the problem is in so many areas the water moves in strange ways and there are often large expanses of areas of more than 20 miles between tide stations. So, in shallow water say 4' or less it's probably not a good idea to count on the accuracy of the depth indicated on a contour. They are useful to show where the bottom is changing but not necessarily to gauge exact depths. Thus, its useful in deeper areas to find structure and bottom movement. In those areas, it doesn't really matter what the exact depth is - it's just important the bottom is changing and how fast.
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Old 01-11-2017, 08:24 PM   #4
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So it sounds like Navionics is good for finding points, holes, drops, etc. but not at showing exact depth? Is the structure that shows up pretty accurate in terms of size and location? If there is a hole that is 8' deeper than the area around it and it shows up before a bend in the river, will it pretty easy to find that hole with the depth finder or could the hole be anywhere within a quarter mile of that spot?
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Old 01-12-2017, 03:32 AM   #5
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Originally Posted by Ditch the Paddle View Post
but not at showing exact depth
Nothing but a depth sounder operated in real time will show you the exact depth... The underwater "scenery" changes constantly... Sand bars move... Depth changes during the course of the day due to the tides... Seasons also influence the tidal fluctuation...
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Old 01-12-2017, 06:07 AM   #6
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Buddy has it on his phone. It's definitely accurate, the boat icon keeps up with no lag. Only thing he doesn't like is having to hold the phone with one hand and drive with the other
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Old 01-12-2017, 09:40 AM   #7
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I got the phone app before installing the chip in my plotter and it was 100% accurate on the location of depth changes. Like previous posters have said, the exact depth varies due to transducer placement and tides but the structure was right where it should be.
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Old 01-12-2017, 06:36 PM   #8
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As other posters have said, the numbers on the charts are mean (avg.) low water. Again, things change, but my charts are usually pretty accurate. Now, for finding the holes, drops, rises, structure, they are pretty dead on. Sonar Charts (1ft) contours are so tight, when it comes to deeper spots, they pretty much give the positions away. I can tell where to set up, how to position, how the bait will move in and out and how the target predator fish are going to set up. Pretty awesome stuff that I def. credit a lot of my success on the water to.

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Old 01-13-2017, 09:58 AM   #9
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General comment for whatever it's worth pertaining to chart depths vs depth finders. Where we boat, many times the bottom is covered with ell grass, some 4+ feet tall. The depth finders that I encounter always show the depth between the transducer and the tops of the grass, not the bottoms. I anchor in spots 5-7' deep at low tide (measured with a drop line) where my depth finder shows zero feet.
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