Ran into an issue that has really made me question the accuracy of maps and GPS available in chart-plotter units and on apps like Navionics. First off, I'm not naive to the fact that I can't rely on a smart phone's built-in GPS to give me a perfectly precise location. I'm not marking a tiny rock pile at home and then getting mad that my phone can't put me directly on top of it. That's not what I'm talking about here. I'm talking about the maps themselves in relation to GPS coordinates.

So, I'm planning a trip in a motor boat up the Sag corridor. I've heard all the stories about Sag being a rock monster, so I figured I'd attempt to pre-load a "safe" route into the Lowrance chart plotter on my boat. Contour maps for Sag are not included in my Lowrance package (c-maps US base map), but it shows the outline of the lake and it has the camp sites as well. So I got the idea of creating the route using GPS waypoints in Navionics, exporting the route, saving it to my SD card, and importing into my Lowrance.

Everything seemed to work great, loaded in without issue, etc. I opened the route on my Lowrance and started following it around -- was kind of surprised to find that my route, which was all in at least 10 fow on Navionics, took me literally out of the lake and onto dry ground on the C-maps basemap. The discrepancies are significant and I'm curious to hear what others think here. Is this primarily just an issue with inaccurate maps, either by one or the other, or perhaps even both? I will try to upload photos to demonstrate. Thanks!

Sounds like a couple potential issues. One, which you already mentioned, is that one of the two maps is inaccurate. This can be especially acute if one of the maps dynamically resizes at different scales. I have found that in Garmin maps depending on my zoom level the outline of a water body can be heavily oversimplified.

Another possibility is that your Lowrance is reading the data incorrectly (or the data on your SD card is not in the right format for your Lowrance). This seems unlikely to me.

Try importing your data into another mapping application like Google Earth. That might give you a sense of where the issue is coming from (the Navionics or the Lowrance).

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geotramper: "Try importing your data into another mapping application like Google Earth. That might give you a sense of where the issue is coming from (the Navionics or the Lowrance)."

Thanks, I'll give this a try and report back.

FWIW, it definitely seems to importing in the correct format. It's a GPX file made out of a series of GPS waypoints. Each waypoint and the route all show up, it's just that they're in the wrong areas. Does not improve based on zooming in or out. One possibility is that, since C-Maps US basemap does not include contours for Saganaga, maybe the outline for that lake is more of a rough sketch as opposed to an actual representation of the shoreline. It does look more "square" in places.

Here are the screen shots I promised. This is right in between the 2nd and 3rd campsites as you head north. "CO359" on the c-maps screen shot corresponds to campsite 35 listed here.

Seems to look great imported into Google Earth, so I'm guessing this is an issue with CMAPS. Appreciate the idea, Geotramper!

This kinda stinks -- I have an iPhone 8 that I don't trust the GPS accuracy on, but I do have accurate maps via Navionics which match up well with Google Earth. My chart-plotter has a more accurate GPS, but the maps are bunk. Bummer. I could of course purchase the Navionics card for my plotter but I have heard horror stories about my particular unit not being able to read that map quickly enough from the SD while running around at speed in the boat.

Nothing's perfect!

thegildedgopher: "Ran into an issue that has really made me question the accuracy of maps and GPS available in chart-plotter units and on apps like Navionics. First off, I'm not naive to the fact that I can't rely on a smart phone's built-in GPS to give me a perfectly precise location. I'm not marking a tiny rock pile at home and then getting mad that my phone can't put me directly on top of it. That's not what I'm talking about here. I'm talking about the maps themselves in relation to GPS coordinates.

So, I'm planning a trip in a motor boat up the Sag corridor. I've heard all the stories about Sag being a rock monster, so I figured I'd attempt to pre-load a "safe" route into the Lowrance chart plotter on my boat. Contour maps for Sag are not included in my Lowrance package (c-maps US base map), but it shows the outline of the lake and it has the camp sites as well. So I got the idea of creating the route using GPS waypoints in Navionics, exporting the route, saving it to my SD card, and importing into my Lowrance.

Everything seemed to work great, loaded in without issue, etc. I opened the route on my Lowrance and started following it around -- was kind of surprised to find that my route, which was all in at least 10 fow on Navionics, took me literally out of the lake and onto dry ground on the C-maps basemap. The discrepancies are significant and I'm curious to hear what others think here. Is this primarily just an issue with inaccurate maps, either by one or the other, or perhaps even both? I will try to upload photos to demonstrate. Thanks!"

I have had the same exact problem using my Navionics on my Lowrance HDI-4. It does cause a problem when I'm trying to get right on a piece of structure to fish. So, really it just became a resource for looking for general structure and then I have to find the actual structure myself once we are near the area. I did a lot of research trying to fix it and I couldn't seem to make any adjustments on the GPS to get it to be perfectly accurate.

Nothing's perfect is right. Most marine chips include 'safe route' lines on big waters. My LakeVu chip shows them -- along with navigation buoys -- on LOW, Namakan, etc but does not have any such marking in the area you're looking at on Sag, nor any lake depths.

Seems to me that this sort of pre-trip route planning is helpful but certainly provides no guarantee that you will come back with a complete lower unit. That's a 90 ft wide opening you're navigating. Not a place I'd be watching my onboard maps. After going through a few times, I might learn to trust my previous tracks, but there's always that one rock that was narrowly missed the first few times...

Totally agree Schweady. "Safe route" was a poor choice really -- I would never just go blowing through wide open throttle just because I'm following a line on my screen. I just wanted to have the route in there for reference. At pinch points like that, I'd be taking it very slow and using my son as a spotter in the bow no matter what -- but I figured if it was off by that much there, it could be off in other places as well. Just made me think a little.

Lyontyl -- well at least I'm not the only one ;)

I agree with your assessment, it looks like CMaps is using a very simplified shape for the shoreline/lake. You can really see this comparing the shoreline between the two images and the Google Earth imagery.

thegildedgopher: "...I figured if it was off by that much there, it could be off in other places as well. Just made me think a little..."

Yeah, most times, devices are within 30 ft... sometimes as close as 10-15... good enough to motor up to the vicinity of the spot on a spot, but you need the sonar to zero in...

Once in a while, we're catching fish while tracing a shoreline back and forth at 15 ft or so and I tell my buddy in the bow, "Hang on... we're cutting across Lucky Peninsula again!"

There is a problem combining different maps and GPS co-ordinates. Maps are drawn in a variety of different data sets and GPS co-ordinates accumulate errors, adding error upon error.
Check your maps data sets, common is WGS84 but there are many others. If you compare 2 GPS co-ordinate sets, expect the error to double even when both data sets are the same.
All maps are dated by nature and cannot show current small changes, this just compounds the problem of accuracy. There is no substitute for on the spot observations.
With a bunch of time doing geocaches I have yet to find a cache only with GPS co-ordinates, it always ends with a visual inspection of the terrain involved.

butthead

Very interesting. Learning a lot today. Appreciate the feedback everyone!

butthead: "...GPS co-ordinates accumulate errors, adding error upon error.
If you compare 2 GPS co-ordinate sets, expect the error to double even when both data sets are the same."

Interestingly, I have an app on my phone to track when I'm on a trail. I use it when I go for my walk in the local park which has a number of benches on my 1.5 mile route. When I stop at a bench the app shows me wandering around the bench location up to 40-50 feet away. The app pings the GPS coordinates every few seconds. If you keep moving it gives a relatively accurate indication of the trail. When stationery the GPS error becomes apparent. Adding this to the map inaccuracies I can see where problems develop.

Another thing to consider is that most maps are existing maps (older than GPS use) and the map is then geo-referenced to a know set of co-ordinate established by survey crews.
In plain terms the map is stretched, rotated, to match up multiple GPS reference points. Then it is saved for later printing or display. You can do this yourself with the Google Earth program,

A large scale Fisher of Seagull Lake area geo-referenced and layered over Google Earth. By adjusting the transparency a combination of the Fisher map markings and Google Earth terrain can be printed on 1 map.
Is it accurate enough to navigate around a underwater rock 3 feet in diameter? No it's not, but still a very useful map for other uses.
Still the old problem, a flat map of a curved space. Mercator projection maps tried to correct this effect with limited success. Another attempt was Goode homolosine projection.

butthead

The same thing happened with my Lowrance Hook 4 and my Navionics chip. When I was up in Ontario fishing I could see it tracks being recorded on the water correctly. But when I was home I looked at the track logs and it showed I was fishing on dry land many times when I was near land. I emailed Lowrance and they basically said what Butthead said. Map datum on my Lowrance and Navionics was set to the same on each to WGS84 and UTM/UPS. So I was using the Lowrance map and overlaying a Navionics map to get this track log. And that still doesn’t explain why when on the water it is showing my track log correct but at home after the trip it showed me fishing on land from a boat. If it was correct in real time, why was it different when I got home?