Side scan shows the bottom to either side of the boat, like an aerial photo made of sound:
drop-offs, weed edges, rocks, wrecks and fish show up tens of metres away from the boat's path.
Lowrance, Simrad and B&G units with side scan save it in their .sl2 and .sl3 sonar logs, and
this tool draws it as pictures you can look at and share.
How the pictures are laid out
- Time runs downwards, one ping per row (short logs are stretched to stay readable).
- Left of the boat is on the left, right on the right. The range scale along the top gives the distance to either side.
- The dark band in the middle is the water under the boat. Its width on each side is the depth: nothing comes back until the sound reaches the bottom. The bright line down the centre is right at the transducer.
- Time marks on the left (clock time in UTC when the log stores it), and the distance travelled on the right.
Reading what you see
Hard things, such as rocks and wrecks, give bright echoes; the shadow behind them, on the side away from the boat, shows their height. Soft mud is darker than sand or gravel. Fish are small bright dots, often with a little shadow.
Pitfalls
- Speed. Each ping is one row, so at slow speed things look stretched along the track and at high speed squashed. Side scan looks best at a steady speed of a few knots.
- Turns. While the boat turns, the outer side is smeared and the inner side overlaps itself. Straight passes give the clearest pictures.
- Range. The log keeps what the unit was set to. A side scan logged at a range of a few feet shows only a narrow strip around the boat.
- Long logs. Each picture holds up to 2,400 pings; longer logs make several, and very long ones combine neighbouring pings (keeping the strongest echo) so there are at most 24 per channel.
Down scan and 2D sonar in the same log get their own pictures too, and the track comes as GPX and KML.
More about what this tool reads and writes: Sonar Log Viewer.