Dual frequency sounders that differ only in screen size. Both feature exceptional resolution, fast redraw rates and split-screen capability so you can view your position and sounder information at the same time. Their high repetition rate for transmit pulse creates a sharper, more accurate image. These units are WAAS capable for greater accuracy and have built-in worldwide base maps with coastline detail, U.S. tide data, and inland features. They also accept optional data cards with Garmin's BlueChart marine cartography for additional detail. GPSMAP 188: Screen Size is 5.5". GPSMAP 238: Screen size is 7.1"