When there's limited visibility, and there are shipping lanes nearby, you don't want to find yourself compressed under the bow of a freighter or tanker. Thus, it's a good idea to know exactly where you are in relation to those lanes. On the C-Map chart, shipping lanes show up as two parallel lines with dots between them on the four-mile scale. At the same scale, the shipping lanes on the Navionics chart use two parallel red lines to show their location. The effect at that scale is that you can see and find things more readily on the C-Map chart if your attention is divided.
Switch to other scales on C-Map, however, and the shipping lane is marked only by a single line that represents the middle of the lane. This gives the advantage to Navionics, explains Bernstein, who favors the two-mile scale when following a tanker out of the bay. "I use the Navionics chart to do my approach to a ship because I get the double line on the traffic lane, which is more useful to me. That's because before I make my approach I want to keep to the edge of the traffic lane because the ship is in the center of the lane."
Although there are some times when C-Map doesn't display the same data found on a Navionics chart for Penobscot Bay, overall, C-Map shows far more data than Navionics. That's true for the names of islands and mainland locations as well as for potential navigational hazards. For instance, the Rockland breakwater is identified on C-Map, but you have to put the cursor on the breakwater shown on the Navionics display to have it identified. C-Map also has far more depth markings than Navionics. On one level, all of C-Map's information is helpful, showing the identity of various islands, coves, and harbors, but at the same time, it can be confusing because a lot of information, displayed in a number of different colors, gets jammed onto a small screen. In addition, the C-Map chart of Penobscot Bay left out information in a couple of places where the electronic chart was built from two differently scaled government charts that abut each other, and the digitizing process used to make up the electronic charts didn't compensate for the change in scale ranges. The information for the area that's blank on one scale can be obtained by zooming to another C-Map scale. Navionics provides enough information to be usable without having to zoom in or out.
For instance, in the Fox Island Thoroughfare, the passage between North Haven and Vinalhaven Islands, using the four-mile scale, C-Map shows a blank area without depth marks or contour lines. At the same scale, Navionics shows some depth marks, contour lines, and has a buoy you can set a course to. To get navigating information on the C-Map chart, you have to zoom down to two miles. Some people may find shooting courses by way of the cursor easier on the Navionics chart because they won't have to zoom in to find their next waypoint.