Setting up and using sonar, Gsd 21 or gsd 20 is connected. see the, For more information – Garmin GPSMAP 278 User Manual

Setting up and using sonar, Gsd 21 or gsd 20 is connected. see the, For more information | Setting, Up and using sonar, Showing sonar on the map page | Garmin GPSMAP 278 User Manual | Page 98 / 138