Down Imaging vs. Down Scan

Sonar Imaging has become extremely popular over the last few years, and rightfully so. Sonar imaging gives you a perspective on the water below that is tremendously useful, and distinctly different than traditional 2D sonar. Here, we address some of the key differences between the offerings of the two available brands.

Prior to January 2011, both Lowrance and Humminbird both had their own version of down-looking sonar imaging as an addition to a side-looking sonar imaging. Lowrance had their “Down Scan” sonar imaging built into their LSS1 module, and Humminbird had their “Down Imaging” as a software development of their Side Imaging transducer. Then, in January of 2011, both companies launched less-expensive products featuring all new transducers with a single down-looking transducer element; bringing the once premium feature of sonar imaging into lower price points.

Both companies designed new transducers for the new DI/DS products: streamlined, low-profile transducers dedicated to the down-looking sonar imaging. Now you have four options that give you down-looking sonar imaging; two from each brand. Lowrance has “Down Scan” with the LSS1 module and “Down Scan” with the new Elite DSI models, and Humminbird has the “Down Imaging” created from the two Side Imaging beams, and now the new “Down Imaging” created from the new DI models. There are some key differences of which you should be aware.

Lets start with the original models. Humminbird was the first on the market with Side Imaging in 2006, putting the side-scan sonar technology, then used in the scientific community, into a small transom-mount package for the consumer market. At ICAST, Las Vegas 2009, Lowrance announced the release of their Structure Scan LSS-1 module as a network add-on to the HDS line. The LSS-1 module included both side-looking sonar imaging elements, and a down-looking sonar imaging element, thus SSI and DSI. Humminbird, having already been in the sonar imaging business for several years, responded with a free software release that created “Down Imaging” from the overlapping areas of the existing Side Imaging beams, thus SI and DI.

Humminbird’s solution was actually pretty clever, as this new “blended beam” Down Imaging had more coverage options than the Lowrance offering. The Humminbird software allows you to select Narrow, Medium, or Wide Down Imaging options. Narrow is an “and” math function, where target returns must show up in both the left AND the right Side Imaging beams. The Wide selection is an “or” math function where a target that shows up in the left beam OR the right beam will be displayed, although the full range of the return is truncated based on the depth range that you select in 2D sonar. The Medium selection is a hybrid of the other two options.



Referring to the image, the Narrow DI setting will show the trunk of the tree, because it is in both the left and right Side Imaging beams (B. and C.). The Wide setting will show the whole tree, because the whole tree is found in the left or the right beam (B. or C.).

The downfall of the blended beam Down Imaging from Humminbird is that the DI uses the weak part of the side-looking transducers, though there are two of them. That is to say, each SI element is angled outboard 45°, and in order to create Down Imaging, you must use the inboard weak fringe of the sonar beam (away from the main beam strength pointed at +/- 45°).

The Lowrance LSS-1 module has a dedicated down-looking transducer element, in addition to the side-looking elements. Their design gives stronger returns and more detail in a down looking beam, but the LSS-1 module is a $600 cost adder to the HDS line. Questionable customer service from the company has been a systemic issue, and customers are cautioned to do their homework before committing to a sale.

In short, Humminbird uses two weak beams and Lowrance uses one strong beam. The performance difference between the two is arguable, but it is generally accepted that Humminbird’s blended beam Down Imaging isn’t as strong as the dedicated down-looking Down Scan in the LSS-1.

Humminbird corrected this shortcoming with the new 2011 DI models, which have an all new transducer with a dedicated down-looking transducer element, and a traditional cone-shaped 2D sonar. The company now has two different “Down Imaging” products: one created with a blended beam that has ultra-wide coverage but is formed from the weak fringe of two beams, and one with narrower coverage but giving stronger returns using a single down-looking imaging beam.

The new models from Humminbird (HDDI) and Lowrance (DSI) have very similar performance, but you should note that Humminbird’s product offers an additional transducer element, providing traditional 2D sonar as well as dedicated down-looking sonar imaging. The chart below summarizes your options as they pertain the down-looking sonar imaging:


Note that these products refer to down-looking sonar imaging only, not side-looking sonar imaging. It is generally accepted among fishing professionals that DI and DSI are subordinate to SI and SSI because you can’t easily determine location of a target in the display—particularly with Humminbird’s blended beam DI on “Wide” setting. For information on SI and SSI, please click here.