If you're looking for a complete navigation unit, the Garmin GPSMAP 178C Sounder may be just what you want. Its a GPS chartplotter with integrated sonar that lets you plot your way and locate fish, old wrecks, or any other underwater objects.

Whether you're on a lake or an ocean, being able to see below the water surface is at least a help and sometimes a necessity. The Garmin 178C Sounder goes beyond regular GPS receivers to do just that. The unit comes with either a single frequency or duel frequency transducer. With the duel frequencies, you get better resolution for the same coverage area. Either way, you get a real-time image of the water and anything in it as it passes under your boat.

Sound waves from the transducer bounce off any objects below the surface and the returning sound waves are displayed as different colors according to their intensity. Water can have layers with different rates of temperature change and are called "thermoclines". Fish are often attracted to thermoclines and the Garmin GPSMAP 178C lets you identify both the thermoclines and the fish. The unit can even detect whether the bottom is hard or soft.

The depth range on the 178C can be set to track the bottom or stay at one specified distance. You also can adjust the reception sensitivity to filter out unwanted detail or use the zoom feature to check out areas of interest. When you do find something of interest, the 178C Sounder can store it as an underwater waypoint. These waypoints are tagged with the date, time and water temperature. This makes them particularly useful for returning to spots where fish gather. And with the receiver's split screen you can view the navigation chart while also watching what's happening below.

All this information would be useless without a good display and the Garmin 178C delivers. The 3.2" square display (4.5" diagonal) is a reliable 256-color LCD. A 200 MHz processor creates fast chart redraws and sonar scans. The high-contrast image stands out even in bright sunlight. At night or low light levels, the adjustable fluorescent backlighting keeps the display just as easy to read. The waterproof case fully protects the display and receiver, no matter where its mounted.

While the sonar helps you locate fish, the basemap of the 178C GPS receiver helps you get to where they are. The built-in worldwide map covers the roads to get you to the water and the shorelines of rivers, lakes, and oceans. You can name and make comments for up to 3000 waypoints. And if you ever are just out exploring and want to return to your starting point, the TracBack feature can help. The track log automatically records turns as waypoints, so your path will have been identified and you merely retrace your steps.

If you need maps with greater detail, the Garmin GPSMAP 178C uses memory cards to hold the map data. You transfer map data from your computer to the memory card using the supplied interface cable. The map data can come from Garmin's MapSource or BlueChart data series, for either inland or offshore boating.