Read any forum thread about lake mapping and you’ll find much discussion of “one-foot contours,” as if that’s the Holy Grail of fishing maps. But like legends of the Grail, many tales of one-foot contours seem to stem more from flights of fancy than fact.
Because many lake maps are merely glitzed up re-packagings of antique charts, even if a map you bought yesterday shows one-foot contours, many of those lines could just be estimates. That’s the case for many paper maps and charts on digital chips.
According to one published report, the charts available in the “vast majority of electronic cartography” are based on historical data from the National Oceanic and Atmospheric Administration (NOAA) and/or from governmental agencies like state departments of natural resources (DNRs). And some historical depth charts, according to the report, might be based on only “four or five” depth measurements “for the entire acreage” of a lake. (For more about the historical sources of many lake maps, read this blog post.)
For such a lake then, there would be only four or five known data points – in this case, depth measurements made, in many cases, by dropping a lead weight on a rope directly under a boat. From the measurements made at those four or five fixed points, a mathematical formula can estimate depths between the known points, and contour lines can be drawn based on those estimates.
Similar estimating went into many of the hand-drawn lake maps available from state agencies. By taking known-depth data points and using mathematical equations to estimate depths between actual known points, digital maps based on that data can sometimes give the appearance of “full lake coverage.” What’s hard to know after the fact, however, is where those old known points were located and what is just guesswork based on them.
A computer program can be written to fill in a map with one-foot contours, but many of those lines are often additional estimates, which may or may not depict the actual lake bottom – and in some cases might obscure humps, rockpiles, reefs, holes, ledges or drains that actually exist. That’s a major problem for fishermen scouting for fish-holding bottom structure.
Looking at estimated contour lines – one-foot or otherwise – can lead anglers to mistakenly believe that fish-holding structure does not exist in places that it actually does exist. That’s probably why you so often hear fishing tournament winners say something like they found a little hump out in the middle of nowhere that wasn’t on any map. If you looked for that same hump amongst estimated one-foot contours on a map, you wouldn’t likely find it.
Many top anglers have long used or made custom maps to fill in the blanks on maps used by weekend anglers and the general public. You can now get that pro advantage with Insight Genesis, a cloud-based service that empowers anglers to locate fish-holding structure on detail-rich, custom maps of the lakes they fish – no matter how big or small.
“Insight Genesis gives you the most detailed mapping available anywhere,” says Bassmaster Elite Series Pro Shaw Grigsby. “If you want to find structure, Insight Genesis is the best way.”
Check out the screen-grabs below to see for yourself. The “Before” image on the left is a screenshot of a section of an off-the-shelf chip map of Georgia’s Lake Lanier. The “After” photo on the right shows the same section of the same fishery, as mapped by Insight Genesis and viewed on a Lowrance HDS Gen 2 unit.
“As you can see from the pictures, the … chip doesn’t show the ditch on their map – they actually show it has a hump like an old road bed or pond dam,” says Lowrance Pro-Staffer Chase Simmemon, who first shared these images on a Georgia Outdoors News forum. “But with the map created on Insight Genesis, you can clearly see the contours of the ditch.”
We’ll dig more into such map comparisons above in an ensuing blog post. Stay tuned. Until then, if you haven’t yet liked Insight Genesis and Lowrance on Facebook, go ahead and click the preceding links and keep updated of great sales and the latest tips and tricks to use your electronics to catch more and bigger fish.
For more information about Insight Genesis, watch the video below and visit the website at https://insightstore.navico.com/insightgenesis.
C-MAP Fishing 
Great article! Information you can use.