Those mushroom type antennas are usually associated with FishFinder/GPS Combos used on boats...they offer stronger reception power...while most hand held GPS are with built-in antennas, some with antenna finger exposed....usually bigger screen for combos & smaller screen for hand helds

There are two main streams of digital chart in the market..

"MapSource BlueChart...competible with Garmin GPS

http://www.gpscentral.ca/accessories/bluecharts.htm

http://www8.garmin.com/cartography/bluechart/region.jsp?RegionPK=25355 chart US019R is what most ppl use..when purchased each CD-ROM will be assigned with a code no. to unlock the specific region the buyer preferred, and is allowed to unlock two separate GPS for the same region, providing these two GPS are owned by the same buyer...in fact many purchases are shared between two users for cost reduction...some grey area here

http://www8.garmin.com/manuals/MapSource_BlueChartUsersGuide.pdf   (check disclaimer)

 "Navionic"...competible with Lawrance & Humminbird GPS

http://www.navionics.com/

大家新年好！

我需要这个gps能告诉我船的速度，水下有没有暗礁等。有没有pre-load水图的gps呢？象我买的汽车用的gps,里面已经预装了北美地图的。