Most fish finders can record and save sonar log files — a complete time-stamped record of your sonar screen, GPS track, and all sonar returns from a trip. Reviewing these logs later lets you study water you've fished, share what you've found with others, and identify structure you missed in real time.
How Sonar Recording Works
On Garmin units, sonar recording is called "Record Sonar." On Humminbird, it's "Side Imaging Snapshot" for still images or track logging for GPS + sonar review. On Lowrance, it's "Structure Map Recording." In all cases, the recording saves to a microSD card inserted in the unit — not internal memory. Use a class 10 or faster microSD card in the appropriate size for your unit (32GB is more than enough for most recording sessions).
The recording captures: GPS coordinates linked to each sonar ping, sonar returns from all active sonar views (2D, DI, SI depending on what's displayed), depth readings, and timestamps. The resulting file can be replayed on the same unit — you literally relive the trip, watching the sonar scroll exactly as it did when you were fishing, with your GPS track on the chartplotter.
Reviewing Recordings
The value is in reviewing recordings at normal or faster speed to find structure that scrolled past too quickly on the water. Side imaging recordings especially reward careful post-trip review — you can see logs, sunken boats, rock piles, and structural features at 100% zoom on a large screen that you only had a few seconds to interpret at 5 mph on the water.
Some units allow replaying recordings on a computer — Garmin Homeport software reads Garmin sonar log files (.slg extension) on PC/Mac. Humminbird's AutoChart PC software handles Humminbird recordings. Reviewing on a larger screen with the ability to pause and zoom adds significant analytical capability.
Class 10 or U1 speed rating microSD for sonar recording — slower cards cause lag in sonar recording. 32–64GB covers dozens of hours of recording.