Skip to content

Depth Sounder Setup

Bristol Maps can read depth from a sounder on your NMEA network, show it on the map, share it with your fleet, and record it into your own bathymetry layers. Getting the offset right matters -- an uncalibrated reading is worse than no reading.

This is not a sounder

Bristol Maps displays a number your sounder sent it. It does not replace a proper depth sounder, a paper or electronic chart, or your own eyes. Depth shown in the app can be stale, mis-calibrated, or absent entirely. Never navigate on it alone.

Connecting a sounder

The sounder has to reach the app over the boat's network like any other instrument:

  1. Set up either an NMEA connection to your gateway or a N2K Bridge.
  2. Confirm depth is arriving. The bridge setup wizard shows Depth ✓ when it sees depth on the bus; for a plain NMEA connection, the Debug button on the connection shows the raw sentences.
  3. In Settings > NMEA & Instruments, under Map Overlays, turn on Show Depth Overlay ("Show the current depth reading from your sounder on the map").

Calibrating the offset

Settings > NMEA & Instruments > Instrument Data > Depth Sounder Offset

In the app's words, this is a "Correction (in feet) for where your transducer sits -- use it to read depth from the waterline or keel."

The picker runs from -5 to 5 feet in half-foot steps.

Which sign you want depends on what your sounder is already reporting and what number you want to see:

  • Most sounders report depth below the transducer. If you want depth below the keel, subtract the distance from the transducer face to the bottom of the keel.
  • If you want depth below the waterline, add the depth of the transducer below the surface.

Check it against something you know

Tie up somewhere with a known depth, or drop a lead line over the side, and compare it with what the app shows. Adjust the offset until they agree. Do this once, in calm water, before you need to trust it.

Verifying the reading before you rely on it

The app watches for a sounder going quiet:

  • If no reading arrives for about 5 seconds, the depth on the map flashes red as a warning.
  • If no reading arrives for 60 seconds, the depth is removed from the map and depth sharing stops broadcasting -- stale depth is never uploaded.

So a red flashing number means the sounder has stopped talking, and a number that disappears means it has been quiet for a minute. A steady number means data is current.

Before you rely on the reading:

  • Watch it while you move across ground you know. It should change the way the bottom does.
  • Compare it against a chart or your sounder's own display.
  • Confirm the units are what you expect -- the offset is set in feet.

Sharing depth with your fleet

Settings > NMEA & Instruments > Instrument Data > Depth Data Sharing

"Share your depth readings with your fleet so group members can see them on their maps." Turn this on and your depth readings go out to your group; group members' readings show up on yours.

This is live sharing only. It is separate from recording depth into a bathymetry layer.

Recording depth for your own charts

Live display is one thing; keeping the readings is another. If you want to build up your own depth picture over a season, that is the bathymetry system -- Settings > Depth & Bathymetry, starting with Record Depth Data.

There's a Transducer Model setting there too (Generic, Furuno, Simrad, Garmin) which is used to interpret bottom hardness data correctly.

See Depth & Bathymetry for the full set.

Related