Using Depth Sounders

Part 4: Interpreting Sounder Images

In this instalment of our ongoing series about getting more from your depth sounder, we compare standard sonar and StructureScan images from our Lowrance HDS units and offer some thoughts on how they differ, as well as why and how to best use each.

A classic sonar image taken in 32 metres of water showing a large and tightly-packed school of slimy mackerel, yellowtial and barracouta stretching from a couple of metres beneath the surface down to about 12 metres, potentially with some slightly larger fish or isolated school members under them. Note also what might be weed on the bottom at the far left.
A very clear SideScan image taken in 10 metres of water in an estuary scenario. Note the mooring cables of a couple of navigation markers on the right and the texture and make-up of the bottom on the left, and how a depression in the bottom at the left is darker than raised areas (think about that spotlight analogy again).  There's quite a bit of cable or chain from the first (lower) channel marker lying on the bottom and I'm puzzled by the six targets hovering over the cable. They look VERY much like large fish returns (jewies?) but seem just a tad too regularly spaced to me. What do you think? Even the spotlight clarity of StructureScan can't answer every question unequivocally!
Another very crisp SideScan image from the estuary. See how the mooring line of a marker buoy casts a very clear shadow on the bottom? Shadows are a big part of interpreting the spotlight-bright imagery provided by StructureScan.
An interesting contrast between the "floodlight" of sonar (left) and the "spotlight" of StructureScan (right) on the same school of fish. SideScan shows that the school was to the right of the boat. Sonar would've left us guessing on this.
We've shown you this one before, but it's worth re-examining using the "floodlight" versus "spotlight" mindset. It would be easy to interpret the "targets" on the sonar signal (left side of screen) as some sort of structure lying on the bottom, or a dense weed bed. However, the narrower, more intense and clearer StructureScan image clearly shows that they are fish.  Have a look at the vertical scale and see if you can make an educated guess about the likely size range of the fish.
This graphic offers a valuable demonstration of the different shapes of the sonar "floodlight" versus the StructureScan "spotlight".

The images above and at left (or possibly below, depending on the device you’re using), are intended to demonstrate the differences between “standard” sonar and Lowrance’s StructureScan (including SideScan) system. In our opinion, each one has its place.

One of the best ways to think about this is to consider the difference between a relatively dim floodlight intended to illuminate a large area at night (that’s the sonar) and the intense, narrow beam of a powerful spotlight shone into the same area. The spotlight gives much more detail, but only down or along its very narrow, intense beam. (It’s also worth noting that StructureScan is much less useful in deeper water and, in our experience, doesn’t work too well beyond the 40 or 50 metre mark.)

Sonar gives a broad, general but often not entirely clear picture.


So, sonar gives a broad, general but often not entirely clear picture. And because the “cone” of the sonar is much larger and rounder than the narrow, tight beam of the StructureScan/SideScan, returns that show up on sonar could be coming from anywhere within a relatively large area… an area that increases with depth.

This floodlight versus spotlight analogy is a good one to keep in mind when considering the relative strengths and weaknesses of these two systems in day to day use.

Click on each of the images shown here to expand them and read the extended captions/descriptions for each. They should help you understand much better how these systems actually operate and the displays they generate.

Just as a powerful spotlight creates strong shadows when partially blocked by an object, so too do the returns from the StructureScan system.

The other point worth stressing is the importance of shadows in interpreting StructureScan (DownScan and SideScan) images on these sounders.

Just as a powerful spotlight creates strong shadows when partially blocked by an object, so too do the returns from the StructureScan system. Seeing, identifying and interpreting these shadows is a really big part of getting the most out of StructureScan or any similiar system used by other sounder makers. Those shadows tell us so much: about the size and shape of the object causing them, its distance off the bottom and so on.

Okay, now click on the images and read the captions… and PLEASE feel free to add your comments, thoughts and personal observations below. We certainly don’t claim to have all the answers and are still learning ourselves.

Any questions about this tutorial?

1 Comment

  1. Steve Peach avatar

    Great article Steve! That first blue sidescan image is an interesting one. I agree, I think those marks around the mooring line are probably some sort of block or structure used to anchor the mooring. There does appear to me to be at least a couple of good fish on the left side though, just where the darker hole meets the rock pile.

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