As further protection for internal electronics, they are filled at the factory with dry nitrogen gas to keep moisture out. Map chips are currently available for the Great screen with the current position accurately overlaid on the map. Map chips are currently available for the Great Lakes and the US East Coast which more regions being added quickly. Few electronic land maps are available at this time. C-Map technology does not replace printed maarine charts or land maps for serious navigation.
Marine charts work best with ccordinates in degrees of latitude and longitude.
Again, depending on the map or chart and compass you're using, You may want headings shown in magnetic, true or grid form. To give the most accurate location, your GPS unit should also provide for changing the internal map datum reference to correspond to your map or chart. The best units provide for all of these variables. Six pages are devoted to this topic. According to these two men, the receivers are getting cheaper and the performance is improving.