Most marine electronic devices with a display are promoted by giving the diagonal length of the display. A better method is to compare the screen area. A new article in the REFERENCE section explains how to calculate display area from a diagonal length and the screen aspect ratio. See
Screen Display Dimensions
http://continuouswave.com/whaler/reference/displayDimensions.html
I wrote this article because I was giving some thought to upgrading the display device on my boat. To compare displays by only their diagonal length can be misleading if the aspect ratio of the two displays is different. The article presents a method for finding area from diagonal length and aspect ratio.
Any follow up questions about the article and its methods that is posted in this thread will be answered.