Sonar imaging is an important tool for recreational and professional fishermen, as well as marine surveyors. The two main types of sonar imaging technologies are DownScan and Sidescan. While they both use sound waves to image the underwater environment, there are some key differences between the two.
What is DownScan Sonar?
DownScan sonar utilizes a narrow, high-frequency sonar beam that is projected directly downward from a transducer mounted on the bottom of a watercraft. The beam has a very tight cone angle, typically less than 10 degrees. As the beam encounters objects, it reflects back to the transducer. The transducer converts the reflected sound waves into electrical signals that are used to display graphical information about what is directly under the boat on a fish finder screen.
DownScan sonar operates at a high frequency, usually around 455kHz or 800kHz. The higher frequency allows for excellent target separation and definition. It can crisply identify small objects like individual rocks, small baitfish, or even fishing lures. Higher frequencies also yield better resolution images, almost photographic quality. However, the tradeoff is the sonar signal does not travel as far through water. Maximum depth range is usually around 150 feet in ideal conditions.
Key Features of DownScan Sonar
- Narrow cone angle straight down beam
- High operating frequencies – 455kHz to 800kHz
- Excellent target separation and definition
- Photographic quality, ultra high resolution images
- Maximum depth range around 150 feet
What is SideScan Sonar?
SideScan sonar uses narrow sonar beams projected horizontally from transducers mounted on the sides of a watercraft. As the boat moves forward, the transducer beams scan the water and underwater terrain out to each side in a sweeping motion. Objects detected in the sidescan beams are displayed as images that provide a top-down view of the bottom on the left and right sides of the boat.
Sidescan operates at a lower frequency than DownScan, typically around 455kHz or 800kHz. The lower frequency allows the sonar beams to travel farther through water, up to 450 feet or more from the transducers. However, target definition is not as sharp as DownScan imaging. Sidescan is better for getting a big picture overview of bottom contours and structure rather than focusing on smaller targets.
Key Features of SideScan Sonar
- Narrow horizontal sonar beams from side mounted transducers
- Lower frequencies – 455kHz or 800kHz
- Wider coverage area up to 450 feet from transducers
- Displays top-down image of seafloor contours and structure
- Target definition not as sharp as DownScan
Side-by-Side Comparison
Here is a side-by-side overview comparing some of the key characteristics between DownScan and SideScan sonar technologies:
| Characteristic | DownScan | SideScan |
|---|---|---|
| Transducer Mounting | Under watercraft pointing down | On sides of watercraft pointing horizontally |
| Beam Width | Narrow, less than 10 degrees | Narrow, less than 10 degrees |
| Frequency | High, 455kHz to 800kHz | Moderate, 455kHz or 800kHz |
| Range | Up to 150 feet | Up to 450 feet |
| Image View | Below the boat | To the sides of the boat |
| Target Separation | Excellent | Good |
| Resolution | Photographic | Good |
DownScan vs SideScan Sound Frequencies
One of the main technical differences between DownScan and SideScan sonar is the operating frequencies used. DownScan utilizes higher frequencies in the ~455kHz to 800kHz range while SideScan operates at lower frequencies around 455kHz or 800kHz. This impacts two key performance characteristics – range and resolution.
Higher frequencies, like those used in DownScan transducers, yield better resolution and target separation. This provides a crisp, photographic view of the terrain directly under the boat. However, higher frequencies do not travel as far through water. So the maximum depth range is limited to about 150 feet under ideal conditions.
SideScan operates at lower frequencies that are able to travel farther through water – up to around 450 feet from the transducers. However, operating at lower frequencies means SideScan resolution is not as sharp. The images will show structure, vegetation, and bottom contours well, but with less detail compared to DownScan.
DownScan Frequency Advantages
- 455kHz to 800kHz frequency range
- Excellent target separation
- Photographic, ultra high resolution images
- Crisp view of small objects like individual rocks or baitfish
SideScan Frequency Advantages
- Lower 455kHz or 800kHz frequency
- Longer range up to 450 feet
- Wider coverage of bottom terrain
- Good for overview of structure and vegetation
DownScan vs SideScan Beam Width
Both DownScan and SideScan sonar use narrow beam widths to generate their images. Typical beam widths are less than 10 degrees for both technologies. This focused beam yields better resolution compared to wider beam traditional sonars.
For DownScan, the narrow beam is pointed straight down. This allows an angler or boater to get a very detailed look at the immediate area directly under the boat. SideScan’s narrow beams point horizontally to the sides, sweeping the bottom in a wide swath covering potentially hundreds of feet.
DownScan Beam Width Characteristics
- Less than 10 degree width
- Focused straight down under transducer
- High resolution view below boat
- Can see small targets like individual rocks or fish
SideScan Beam Width Characteristics
- Less than 10 degree width
- Focused horizontally from sides of boat
- Wider coverage of bottom terrain
- See contours, structure, vegetation over wider area
DownScan vs SideScan Imaging
The DownScan and SideScan narrow beams generate different views of the underwater environment. DownScan produces images of the high resolution terrain directly under the transducer. SideScan provides wider coverage images of the bottom off to the sides of the vessel.
DownScan Imaging
- Photographic images directly below transducer
- See small objects like individual rocks and fish
- View vertical structure like drop-offs or ledges
- Coverage limited to below boat
SideScan Imaging
- Maps wider swath of bottom terrain off transducer sides
- See contours, structure, vegetation over wider area
- Wider bottom coverage to locate potential fishing spots
- Less detailed target separation than DownScan
Common DownScan & SideScan Features
Although there are several differences between DownScan and SideScan, they do share some common attributes:
- Display sonar images on fish finder screens
- Allow viewing of underwater features and structure
- Help anglers and mariners navigate waters
- Aid in finding potential fish holding locations
- Narrow beam widths give better resolution than wide beam sonars
- High frequency transducers for good target separation
- Grant visual picture of unseen underwater environment
DownScan vs SideScan – Which is Better?
DownScan and SideScan both have pros and cons and excel in different applications. Neither is necessarily “better” overall. Here are some guidelines on selecting between them:
DownScan Advantages
- See every detail directly under your boat
- Spot small targets – great for finding individual fish or rocks/structure to fish
- Perfect for vertical fishing like under docks or ledges
- Crisp photographic images of structure immediately below
SideScan Advantages
- Scan large areas to either side of your boat
- Find fish holding structure and vegetation over wider range
- Map bottom contours, substrate types, obstacles over distance
- Aid in navigation and hazard identification
Many high end fish finders offer both DownScan and SideScan capabilities since they work well together. DownScan checks directly under your boat while SideScan searches the surrounding area. Having both technologies onboard allows anglers to capitalize on the unique advantages of each.
DownScan vs SideScan – Uses and Applications
Here are some common uses and applications for DownScan and SideScan sonar technologies:
DownScan Uses
- Vertical jigging beneath boat – see fish react to lures
- Fishing structure like drop-offs, docks, ledges, weedlines
- Checking for snags or debris before casting or anchoring
- Scouting bottom hardness and composition for insect hatches
SideScan Uses
- Finding fish holding structure – brush piles, rocks, channels
- Searching areas for vegetation like weedbeds
- Mapping bottom contours and substrate over distance
- Avoiding hazards when navigating
- Marine surveyors scanning wrecks or underwater infrastructure
DownScan’s ultra high resolution view under your boat helps pinpoint small targets like individual fish or tiny bits of cover. SideScan scans much wider areas, ideal for getting the big picture view around your boat to find promising fishing spots.
Conclusion
In summary, DownScan and SideScan sonar imaging technologies have differing strengths:
- DownScan – Excellent for targeting small objects directly below your boat with crisp, photographic images and target separation.
- SideScan – Provides wider scanning coverage of bottom terrain and structure off the sides of your vessel.
Many high performance fish finders now incorporate both DownScan and SideScan capabilities to take advantage of the benefits of each. DownScan checks the immediate high resolution area under your boat while SideScan maps the surrounding bottom over greater distance. Together they provide anglers and mariners a detailed view both near and far of the unseen underwater world.
