Which sonar channel to use to map vegetation? 200 khz or Downscan?

An update to the BioBase sonar file reader released in Feb 2021 included an update that allows the user to select which sonar channel they’d like to process (Primary or Downscan) for vegetation. We call the broadband, traditional sonar channel “Primary” which includes the recommended frequency 200 khz. Most Lowrance transducers have this frequency. If the user selects the .sl2 or .sl3 file format and they have a Downscan-compatible transducer, the file will also include the Downscan channel. Downscan differs from the Primary channel in the size and orientation of the soundwave beam (Figure 1).

Figure 1. Diagram of primary sonar beam (round hatch) vs downscan sonar beam (narrow oval hatch). From Patent US20130021876)

Because of the different orientations of the beam, the area that is processed by the echosounder and read into the file is slightly different and the technologies produce different images. In terms of the image, both have their strengths and drawbacks. Downscan generally gives the viewer better detail (Figure 2) and we often hear users tell us you can actually see individual plant leaves with downscan and visually separate out species. In fact, in the age of Artificial Intelligence, research has made strides in determining aquatic plant species from Lowrance downscan screenshots (Patel et al. 2019). Although robust species typing technology is still a ways from automation, we are making strides in this direction. The downside of downscan is that the image is quite sensitive to the speed and “yaw,” or turning of the boat. The benefits of the conical beam of the primary channel is that although it might not produce as detailed of an image of bottom as downscan, it is presumably a better integrator of the bottom conditions and plant growth. As far as we know, science has not delved much deeper into quantifying tradeoffs between the two sonar technologies. So we encourage our BioBase users to experiment and find what works best in the environment they are mapping!

Figure 2. Screenshot from a Lowrance HDS showing views of both 200 khz and downscan over a seagrass bed.

Downscan vs Primary: Lake Survey Example

By default, BioBase processes vegetation using the Primary channel. User’s can change this preference in User Preferences area of My Account. When they change settings here, each uploaded sonar log will have these settings automatically. There is another area within the Trip Viewer map that allows the user to change the sonar channel for a single trip and reprocess the map. Select Tools – Trip Reprocessing. We go into greater depth about settings and preferences here.

So for the example below, we processed a sonar file with Primary, downloaded the statistical report, and then reprocessed with Downscan. Note how similar they look visually (Figures 3 and 4).

Figure 3. BioBase vegetation map for Orchard Lake, MN processed with 200 khz primary channel. Gaps in the map are from the automated buffer feature which can be adjusted by the user.
Figure 4. BioBase vegetation map for Orchard Lake, MN processed with 200 khz primary channel. Gaps in the map are from the automated buffer feature which can be adjusted by the user.
Figure 5. Vegetation Reports created automatically by BioBase. The red boxes highlight statistics that were compared in this analysis.

Differences were also rather small statistically speaking. There was a only a 5% difference in one biovolume metric (BVw which is biovolume across the entire survey area) and a 10% difference in a different Biovolume metric (BVp which is biovolume only where plants exist). Downscan seemed to think plants were a little longer than Primary. However, Downscan also seemed to find just a wee bit fewer plants than Primary since PAC was 5% less with Downscan than Primary. These differences probably reflect the subtle differences in the areas of bottom that the transducer beams sample. As such, there probably isn’t a right or wrong answer to which channel gives the most accurate results and relies on the end user to find which setting works best for their needs. For more info on these or any other features and to get started using BioBase, check out our web page biobasemaps.com or contact us at info@biobasemaps.com.

Author: biobasemaps

BioBase is a cloud platform for the automated mapping of aquatic habitats (lakes, rivers, ponds, coasts). Standard algorithms process sonar datafiles (EcoSound) and high resolution satellite imagery (EcoSat). Depth and vegetation maps and data reports are rapidly created and stored in a private cloud account for analysis, and sharing. This blog highlights a range of internal and external research, frequently asked questions, feature descriptions and highlights, tips and tricks, and photo galleries.

2 thoughts on “Which sonar channel to use to map vegetation? 200 khz or Downscan?”

  1. In freshwater lakes with substantial water depth and a biovolume ranging from 2 to 70 percent, there will likely be no difference between the higher frequency downscan and the commonly used primary (200KHz) default. In fact, in many of my explorations the 200KHz track produced superior interpretations. However, the downscan soundings are preferred when biovolumes approach the water surface where discretization of the vege tops from surface water noise is challenging. This interpretation challenge is enhanced in shallower water, particularly when mapping seagrasses in 3 feet or less.

    1. This is great insight Rob, thank you! There is a shallow water enhancement we have on our feature development list that will hopefully reduce false detects in shallow water.

Leave a Reply

Translate »
%d bloggers like this: