Using Depth SoundersPart 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.
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.