it should be a 20deg cone angle "Skimmer" transducer. This will give you a cone shaped area of "detection", that will show bottom features within a circle that's about 1/3 as wide as the bottom is deep. Or in other words ... it shows a 7ft circle on the bottom in 21ft of water..10ft circle in 30ft of water ..etc.
Remember - your transducer is sending out a expanding "circle" of sound pulses ... in a cone shape ... towards the bottom. As the boat moves, the leading edge of that circle is the first thing to register on-screen. Your most accurate "picture" will come from the signal in the center of that circle. Your unit is the signal sender/receiver ... basically a computer that translates the signals it receives into pixels on the screen. The computing power of the unit determines how good the picture is ... and the smaller the pixels, the more detailed the picture. The Eagle 320 is a 320x320 pixel unit. That's 320 little squares (pixels) high and 320 wide ... should be a very sharp/accurate picture of things within that cone !! It advertises that you can get 60deg of "detection" by using high sensitivity settings. But, all the accessory transducers listed for it, are all 20deg cone angle. I'd depend more on the 20deg signal idea, myself.
Look here - http://www.eaglegps.com/products/Sonar/FishMark320.htm - for a Manual download, and other info. .......... cp