A fish finder measures the depth several times a second, and its sonar log keeps every reading
with the GPS position where it was taken. This tool draws them as a depth map of the area the boat
covered, from .sl2, .sl3 and .slg logs of Lowrance, Simrad and B&G units.
What the map shows
- Every depth reading as a dot where it was taken, coloured from pale yellow (shallow) to dark blue (deep), over a thin line of the boat's track.
- A depth scale with the colours and depths, in metres or feet.
- A scale bar and north, and the date of the log when it is stored.
The depths are the unit's own readings from the 2D sonar, below the transducer. Where a log has no 2D readings, those of down scan or side scan are used.
Making a better map
- Cover the area in lines. A map from one trip across a lake is a single line of dots. Parallel passes a boat's length or two apart fill it in.
- Record at a steady water level. Readings are depths at the moment of logging; on tidal water or a reservoir that changes level, maps from different days don't match.
- Mind the transducer depth. Depths are measured from the transducer, not from the surface, unless the unit adds a keel offset. The CSV lists the offset when the log stores one.
Why there are no contour lines
Contours need depths all over an area. Between two passes there is no data, and drawn contours there would be invented. The map shows exactly what was measured instead. For gridded charts, the GPX track (with the depth at every point) and the CSV (every ping with its position) go straight into mapping software such as QGIS.
Good to know
A map needs the boat to have moved: a log recorded while anchored or drifting within about 25 m gets no map, and the result says so. Logs without GPS positions can't be mapped at all, but their sonar pictures still work.
More about what this tool reads and writes: Sonar Log Viewer.