Lake depth map from sonar logs

An online tool for making a depth map from sonar logs: it reads Lowrance .sl2 and .sl3, Humminbird and Garmin recordings, cleans out false bottom and fish echoes, and draws a bathymetric chart with contour lines, a 3D bottom, holes, humps and drop-offs. The finished lake map is saved as GPX for a chartplotter, KML or KMZ for Google Earth, a picture, a PDF or a GeoTIFF; logs of any size are read right in the browser.

How to make a depth map

1
Record and copy the log

Turn on sonar recording on the sounder, make passes over the water and copy the files from the memory card: .sl2/.sl3, .RSD or the Humminbird recording folder.

2
Open it and tune the map

Drop one or several logs. Pick the contour step and how wide a gap between passes may be filled, and set the water level of each day if it changed.

3
The map for a chartplotter, Google Earth or PDF

Download GPX for the chartplotter, KMZ for Google Earth, a picture or a PDF — or the soundings as CSV.

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Build a bathymetric chart of your lake from Lowrance, Humminbird or Garmin recordings

Drop sonar logs here or click to choose
Several logs of one lake make one map. For Humminbird, pick the .DAT file together with its recording folder.
Lowrance SL2 · SL3 · SLG · Humminbird DAT/SON · Garmin RSD · CSV · GPX · NMEA

Logs live on the sounder’s memory card: Lowrance — .sl2/.sl3, Garmin — .RSD, Humminbird — RecXXXXX.DAT with a folder of .SON files. Points with depth from CSV, GPX or NMEA work too.

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What it does

Lowrance, Humminbird and Garmin logs
Reads .sl2, .sl3 and the older .slg from Lowrance, Simrad and B&G, Humminbird recordings (.DAT with .SON files), Garmin .RSD, and points with depth from CSV, GPX or NMEA. A multi-gigabyte log is read in parts and never loaded whole.
Clean soundings, not raw pings
Fish echoes, weed, the false bottom at twice the depth and GPS jumps are filtered out. Lowrance and Humminbird repeat a once-a-second GPS fix in 8–15 pings; the tool spreads those pings along the path instead of piling them onto one spot.
Contour lines, holes, humps and drop-offs
Contour step from 0.25 m (1 ft) up. A hole deeper than its surroundings, a hump shallower and a steep drop-off are found and listed with their depths.
Accuracy measured on your own log
30 m stretches of your passes are hidden from the map and rebuilt from the rest; the error is shown next to the map, so you know how far to trust the bottom between passes.
The bottom in 3D
Rotate the lake bed with the relief stretched up to 20 times and look at the structure from any side.
To the chartplotter, Google Earth and paper
GPX with contours as tracks and spots as waypoints, KML/KMZ with the depth picture, PNG and PDF with a legend and a scale, the soundings as CSV and a GeoTIFF of depths for QGIS.

How accurate the map is — we checked

A depth map is exact only where the boat went: between passes the bottom is interpolated. We modeled a 640 × 420 m lake with a hump, a pit and a steep bank and surveyed it the way a real sounder does — passes 28 m apart, a GPS fix once a second, ±5 cm depth noise, fish echoes and false bottom. Against the true bottom the map was off by 4.9 cm on average and by no more than 9.7 cm at 90% of points; the pit and the hump were found within 5 m of where they were placed. The tool runs a similar check on every log you open: it hides 30 m stretches of your passes, rebuilds them from the rest and shows the error next to the map.

Questions about depth maps from sonar logs

How do I record a sonar log on a Lowrance, Garmin or Humminbird?

Put a memory card into the unit and start recording from the sonar menu: on Lowrance it is “Log sonar” (files .sl2, or .sl3 on units with ForwardScan or 3D), on Garmin ECHOMAP and GPSMAP — “Sonar Recording” (.RSD files), on Humminbird — recording from the snapshot and recording menu (a RecXXXXX.DAT file and a folder of .SON files). Drive slow, steady passes over the area you want to map.

Why are there blank areas between my passes?

The map draws the bottom only where neighboring soundings are close enough — by default no more than 60 m apart. Wider gaps stay blank on purpose: anything drawn there would be a guess. Run the passes closer together, or raise “Fill gaps between passes up to” if a coarser bottom there is acceptable.

What does the error shown next to the map mean?

30 m stretches are cut out of your passes, the map is rebuilt without them, and the depths it predicts there are compared with what the sounder actually measured. On our modeled lake with passes 28 m apart this error was 5–6 cm; on a log with sparse passes or a rough bottom it is larger — it is the number to trust for that particular map.

How do I load the depth map into a chartplotter?

Save it as GPX and copy the file to the unit’s memory card: contour lines come in as tracks named by their depth, holes and humps as waypoints. Units limit how many track points they keep, so for a big lake pick a contour step of 1–2 m. For Google Earth save as KMZ: it carries the depth picture as well.

Why do depths differ between days or from published charts?

A sounder measures from its transducer, and the level of a reservoir or a river changes by tens of centimeters or even meters. Enter the transducer depth in the settings and, for each log, how much the water was above or below the usual level that day: the logs are then brought to one level and stop contradicting each other.

Can I make a depth map without a sonar log file?

Yes, if the sounder or its app exports points with depth as CSV or GPX — latitude, longitude and depth on each row: such a file is mapped the same way. A plain GPS track does not work — it has no depths to draw the bottom from.

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Who is this tool for?
🎣 Hunters & anglers
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