High-tech developments haven’t changed sonar basics

Modern fish finders offer nearly photographic images of what’s under your boat, but if you are letting the computer decide what it shows, you aren’t getting everything out of your investment.

Current-generation sonar units show the underwater world with a level of realism fishermen could only dream of 30 or 40 years ago.

Back then, we ran flashers at high boat speeds to find our fishing spots, and then switched to paper chart recorders to see if the fish were home.

It took skill developed through years of experience to read a flasher, and the paper chart recorder’s learning curve was confusing new territory for most of us.

Eventually, we realized that the new-fangled chart machines just printed out the picture we had been forming in our minds based on the dancing lights on our flashers. We no longer had to remember the trending flash patterns that consecutive soundings put on our flasher’s screen — the chart recorder printed it all out as an easy-to-read screen history that remembered it for us.

Most of the tools and features we used back then to get the clearest possible understanding of what we were seeing are still available, and they help just as much today.

To me, the goal in sonar development has always been to produce a unit that could deliver a picture so clear it would be like taking the water away and letting you look over the side of your boat and see just exactly what was between you and the bottom.

You could argue that today’s side- and down-scanning systems are very close to that level of performance. And, side-looking, 360-degree scanning and other steerable sonar systems answer the old question of where that fish on the screen is in relation to my bow fishing seat.

Still, we can often improve screen detail with a measure of old-school technique.

In sonar’s “olden days” getting the best detail and target separation with conventional down-looking sonar sometimes could not be done if you let your unit’s computer automatically set your depth range. This can still limit your detail, even with today’s broadband and CHIRP technologies.

Fortunately, this is easy to fix. Nobody likes to look at a screen that is constantly jumping between depth ranges as your boat moves over an uneven bottom, so your sounder’s computer is programmed to settle on a depth range setting deep enough to show all the depths you are passing over.

And, it just shows the bottom’s undulations with a rising and falling bottom indication on the screen.

This prevents the annoying screen jumps, but when you stop to fish, it often leaves the lake bottom’s indication near the center of your screen.

The lake bottom must appear near the bottom of your screen to give you all the screen resolution you paid for.

Let’s say you’ve spent just over $2,000 for a Humminbird ONIX unit sporting a display 1024 pixels wide by 768 pixels tall. Your fishing depth is 55 feet and you’ve selected the 0-60 foot depth range.

The bottom indication is right at the bottom of the screen and those 60 feet of depth are divided by the screen’s full 768 vertical pixel count. If you trust my math, each vertical pixel represents about 0.9 inches of depth.

However, if the computer moves the lake bottom’s indication up to the middle of the screen, those 60 feet are only being divided by 384 of those vertical pixels — so each pixel can only represent 1.9 inches of depth.

Now you have only half as much resolution.

Think of it as switching from drawing the underwater image with sharp colored pencils to drawing it with pencils having dull, flattened points. You could have spent about a fourth as much money and bought a Humminbird Helix unit with more vertical resolution than that.

So what is the lesson here? Select the depth range setting (manually if you have to) that puts the lake bottom at the bottom of the screen to get the sharpest top-to-bottom view.

This same principle applies to the new side- and down-imaging features that deliver photo-like screen images. Rocks look like rocks, stumps look like stumps, weeds look like weeds and fish and baitfish can easily be identified.

Maximizing your depth range or side range setting can show you what kind of weeds those are and if there is an exposed root system under that stump that could be holding fish.

Fish are sometimes shown as short, hard-to-see lines on longer-range settings and sometimes their shadows are easier to see than the fish themselves. Maximizing your screen detail increases the chance that you’ll spot them in and around different kinds of cover.

So, once again, choose the down-scan feature’s depth range setting that puts the lake bottom at the bottom of the screen. If you are looking sideways, choose the shortest side-scan range that reaches and includes the structure and cover you are examining.

If the target cover is off to the right of the boat, select the screen view that shows only the starboard side-scan view. This spreads the right-hand scan view across the whole width of the screen instead of just the right half of it and doubles your resolution. Everything appears twice as large and half as difficult to see.

Once you spot what might be fish, you can use the zoom feature to further magnify details. If you are using down-looking sonar, you can select a small depth segment and expand it to cover the whole screen for a magnified view.

Select the 2X zoom to double the size of everything or the 4X setting to quadruple its size. Some units allow you to set custom zoom windows, allowing you to put the full sonar view in one screen window and a constant zoom view in another.

If the fish are holding close to the bottom, you can set the zoom window to show the bottom and a few feet of water just above it. If the fish are holding at a thermocline, you can set the zoom window to show a narrow depth segment centered on the thermocline.

You can dedicate the whole screen to the zoom view for best detail, but I like to keep a boat-to-bottom view in one display window so I don’t miss seeing a school of fish or baitfish pass above or below my selected zoom window.

Most side-scan features allow zooming, too, and zoom can make the difference between seeing fish or bait on a potential hotspot or missing them completely. Longer-range settings help you examine more water quickly as you search for fish, but they make details like individual fish and baitfish appear smaller and more difficult to see.

Zoom lets you quickly magnify any area you think is showing fish but isn’t delivering enough detail for you to be sure.

Other sonar basics also apply to both new and old units. If you have multiple operating frequencies, you need to remember that higher frequencies deliver more detail but don’t reach quite as far, while lower frequencies reach farther through water but don’t offer as much detail or work as well at higher boat speeds.

If you’re not seeing fish, try turning up the sensitivity; it could be that current conditions are making your unit’s computer set the sensitivity too low.

You might have to look through a bit of screen interference to see fish. Run your interference rejection features at the lowest settings that give you a readable picture; it is still possible to filter important details right off your screen.

We have come a long way since the Lowrance green box flasher introduced anglers to electronic fishing more than 50 years ago, but the devil is still in the details.

Be the first to comment

Leave a Reply