Q: Why don’t some depths on my map match the depths displayed on my fish-finder?
By Greg Huff and
Ever been fishing and noticed that depths displayed on your digital map don’t match actual depths determined by your sonar unit? Ron Lutes has. That’s why he subscribed to Insight Genesis to make his own custom lake maps with real one-foot contours.
“I used my iPhone with a [digital map] on Eagle Lake in northwest Ontario … the map info was absolutely worthless!” reports Lutes. “There was not one spot I checked with the depth/fish-finder in the boat that was close to the actual depths. That was one of the reasons I wanted to use Insight Genesis – to improve or make my own charts. I don’t know where [that map company] came up with thousands of bogus contour lines.”
What Lutes called “bogus” contour lines could be the result of bad data. A variety of user-initiated anomalies can cause a bad sonar signal that results in bad data. They include:
- High acceleration/deceleration rates
- Transducer misalignment
- Hitting bottom/debris
- Cavitation/turbulence near the transducer
- Surface interference
- Vegetation interference
- Poor GPS signal
- Transducer signal disruptions
- Sonar log corruption
Lake maps, contour charts, sonar charts – whatever you call them – can only be as good as the data from which they’re drawn. Bad data, therefore, will create bad maps. As the old saying goes, “Garbage in, garbage out.” This is as true with recording sonar logs as it is with taking photographs – if your camera lens is smudged, or it’s too dark out, you won’t get a good photo. Likewise, if your transducer is crooked, covered in weeds, or too close to heavy prop-wash, etc. you won’t get a good sonar signal. Because custom contour maps are made from sonar signals, a bad sonar signal will create a bad map – or at least a bad portion of a map, depending on where and when your sonar signal was compromised.
That’s why the Insight Genesis Quality Control team reviews thousands of sonar logs every week, searching out and removing bad data that resulted from user-initiated anomalies on the water. A trained professional on the QC team compares your custom map’s contour lines against your sonar log recording – a truth source that displays exactly what you saw on your sonar screen while on the boat – to verify that each map you make is accurate and precise.
If your map is not compared against your sonar log by a professional trained to understand sonar imagery and interpolation models, you might as well just eyeball depths and draw contour lines with a crayon on a can koozie. Without review by real, live, trained professionals, there’s no way to be confident in the validity of all the data from which your map was built – which means there’s no way to ensure the accuracy of your map.
That could be why Lutes was vexed by those “bogus” contour lines up in Canada. The company that makes the digital map he was using advertises that it can update sonar charts (i.e. lake maps) with GPX files collected from Lowrance sonar/GPS units. But they use a GPX-based work-around process that provides no truth source with which to compare their depths against. That means that maps made (or updated) with their process could include some bad data and therefore be inaccurate in places.
Here’s one way bad data could have caused “bogus” contour lines like Lutes found on that map app:
Let’s say we make a lead-line sounding on Spot X, determining with no doubt that the depth is exactly 14 feet. If your sonar transducer is crooked – perhaps bumped by a log or rock; perhaps the nut came loose on its mount – you will get an inaccurate depth reading at Spot X, say 24 feet for example. The depth reading is wrong because your sonar signal is not shooting straight down at the bottom like it should be, but rather at an extreme angle, thus making the bottom appear much farther away from the transducer than it actually is.
In this situation, a GPX-based mapping program would draw contour lines based on your latitude, longitude and that inaccurate depth reading. But such a system has no truth source to verify that depth reading. With the Insight Genesis sl2-file-based system, however, there is a truth source – your sonar log. A signal from a crooked transducer displays in your sonar log with visual indicators that any one of the trained professionals on the Insight Genesis QC team can recognize.
Trained to see telltale signs in your sonar log of the kinds of problems bulleted in the list above, our QC professionals will delete from your map any compromised data that would cause inaccurate contour lines. While this will leave a few holes in your map, it’s easy to drive back over those spots the next time you’re on the water and fill in the blanks. Merge the data from your follow-up trip log with your original trip logs and you’ll have the most accurate, most highly detailed custom contour map available anywhere.
We built the Insight Genesis Quality Control team after asking ourselves one simple question – would a customer prefer a map that’s 85% accurate or 99% accurate? The answer is clear, so we work every day to make Insight Genesis maps as perfect as possible for all subscribers, regardless of their subscription level. No other custom-mapping service employs a similar Quality Control team devoted to near perfection.
That’s why Insight Genesis custom contour maps are better than other DIY-mapping programs – with the others, you are truly doing it yourself; with Insight Genesis, our team of trained Quality Control professionals is always on hand to help.
The example of a crooked transducer is just one of many possibilities. Any one of the user-initiated anomalies in the bullet list above could cause bad data that, in turn, could cause the “bogus” contour lines that bothered Lutes. The Insight Genesis QC team, however, ensures that your data is both fresh and accurate.