Navionics--what is the 'real' accuracy?

[SirCranksalot]

I know the charts show contour lines with 2-3 ft interval, but I was fishing yesterday and my sonar showed me an area 10ft deeper than an area further out even though the chart does not show any depression in that area. I know they can't show every tiny little up and down checked a couple of points about 100 ft apart and they were both about 10 ft deeper than further out..

[Raf]

the 1ft contours seem to vary from not even close to poorly accurate depending on where i am fishing.

[porkpie]

I've found it to be up to 5 feet off, and other times I've found it exact to the foot. I was looking for a small depression on simcoe a week ago, dropping from 20 to 32 foot of water and back. I walked straight on top of it, shot through with the sonar, and found the exact depth, and my buddy and I laughed and thought it was pretty good. Other times it's been a bit off. They are only as good as the cartographic data they have obtained. I find them to be accurate or close enough more often than not!

[Terry]

the maps that have been redone recently are very accurate

 

others are not even close because of the way the determined the depth , as the got the depth every so far, then just grew lines between them

 

gps is never right on either and clouds and location can also lessen accuracy

[dracokaos]

I think you also have to keep in mind the current water levels. Simcoe was down this year. Im not sure how often the maps are updated.

[EC1]

There are certain lakes where the maps are slightly "off". The GPS might say you're 100m away from shore but in reality it may be half of that. Not a big problem, just use the numbers as a reference to the contours in a lake

[JoshS]

I've noticed it depends on the lake, some are more accurate then others. I've seen different versions give different info some reason too. I'm not sure why they can't group all the data. As an example, there's a few humps on a specific lake that show up on my gold chart that don't show up on navionics+ chart which is supposed to be conclusive. In terms of accuracy though, every lake differs from what I've noticed... some are close, sometimes it's way off. I take it with a grain of salt. It's good for getting an idea of an area, then use your fishfinder.

[BowSlayer101]

Off 1 foot Max for me.

[irishfield]

As Terry noted.. depends on the lake. Temagami is as much or more than 200 feet off location in places because they are relying on a late 1800's survey as to were the lake is. Some places you are driving your boat across the ground.. on the screen...

[BillM]

People also need to realize that their GPS isn't as accurate as they think.. Civilian vs military is a HUGE difference, lol. I think they say something like 7 metres @ 95% of the time?

 

That being said, I haven't had to mess with the offset on my HB 898 too much. It's usually pretty spot on.

[craigdritchie]

There is a lot of variance in chart water depths, largely because water levels vary from year to year, and throughout the course of a season. A spot that's 12 feet deep in the spring might have only 9 feet of water over it in the fall. Hence the phrase "at chart datum" ... which means that the depth was accurate when the soundings for that particular chart were taken.

 

Many Canadian charts are using base maps from the 1970s. They're obviously nowhere near as accurate as more recent ones.

 

Between that and the +/- variance of GPS on a given day, use it as a rough guide but don't expect it to be right on the button all of the time. It still beats holding your thumb up and trying to line up with trees on the shore.

[Terry]

I don't see the water level as an issue

I can tell pretty quick if levels are up or down and expect the depth to be different but I would still expect the depth between the top of the hump and the bottom to always be the same distance

and water levels don't cause a hump to not be where the map is, or drop offs to move

it is maps aren't accurate and gps is off

[Raf]

There are shoals, points that are not even mapped. Its not an issue of water levels. Look at the 1ft contour map of pigeon lake for instance.

[mike rousseau]

Navionics doesn't have everything

 

There are river channels on the st lawrence that aren't on navionics....

[manitoubass2]

Lol. Where I fish on rainy river the chart stops EXACTLY where I like to fish.

 

Oh well Ive figured it out but a proper chart coulda saved me some time lol.

 

It is what it is.

 

This is a good reminder to be careful using navionics on lakes filled with reefs, just in case.

Edited January 24, 2015 by manitoubass2

[fishindevil]

Most chart and nav data is always taken at low water datum !!!!! Just like govt charts are supposed to be as to always err on the side of caution for navigation as well as Wayne said above in his post some maps are on old surveys and not on the newer GPS coordinates it's a lot better than it was 15yrs ago when. The US military under orders from the president made GPS a lot more accurate for civilian use it's quite the story about it too It' was sketchy at best back then and if it was on that day it was pretty good and some days it was brutal !!!! They were scrambling the signal up to 35% it something like that ..... So it's def way way better today ....

[SirCranksalot]

Let me explain myself a bit better than my orig post:

 

I was not using a GPS with Navionics on it. I just printed out a small section of the lake where we would be fishing. Nevertheless, I had quite an accurate idea of where I was from looking around me in the small cove we were in.If I believe my sonar readings, there should be a 'dip' or depression in the area that I checked----indicated by concentric closed loops. But there is no such indication on the map. The map shows the depth increasing more or less uniformly with further dist from the shore.

 

So, while GPS accuracy is a pertinent topic for this forum, it's not really relevant to the issue on this thread.(I guess I should have elaborated a bit more in the first place ).

 

I don't see the seasonal variation of water level to be particularly important. I.e. I'm interested in the relative depth-----the 'holes', bumps, ridges---the 'structure'--- rather than the absolute depth.

 

I have to wonder where Navionics gets their very detailed 'data'. Has somebody really surveyed the lakes in such detail?? Or, did they take the cruder contours shown on gov't charts and interpolate them like crazy??