The excellent article on radar by Twain Braden ("Plotting skills," Issue No. 91) describes the methods of plotting radar data on a maneuvering board.
However, the article makes no mention of the fact that the same techniques can be used for plotting directly on the radar screen using a suitable marker, straightedge, and scale. A suitable straightedge and scale can be a three-by-five-inch index card marked with distances taken directly from the radar screen. The marker used may be a grease pencil or felt-tip marker. The mark must, of course, be visible to the user, and must be readily erasable.
Compared to using a separate maneuvering board, on-screen plotting is faster, you can watch the other vessel’s motion develop in direct comparison to the plot, and you can more easily keep track of multiple vessels at the same time. It also avoids the possibility of making a mistake transferring information to a maneuvering board.
The major drawback of on-screen plotting is that the range scale of the radar must not be changed before the plot is complete. Changing the range scale voids the information of the plot.licensed mariner and retired commander in the U.S. Navy.Editor’s note: The reason the Coast Guard teaches on-screen plotting is that most commercial ships carry radar units that have a special screen for this purpose: a flat sheet of Plexiglas that illuminates plotting marks made by grease pencil or dry-erase marker. These types of radar sets are not available to the recreational vessel. Mr.Collins is correct that on-screen plotting is easier, quicker, and potentially more accurate if the radar is so equipped.