The Zero Line is the place where the water meets the shore. It's also the starting point for building your own maps with AutoChart. With these all-important Zero Lines as your foundation, you can begin creating your own accurate and detailed maps of any fishing spot, even where contour coverage doesn't exist. Anglers can purchase additional Zero Line Map Cards to view the AutoChart created maps on multiple Humminbird units. Simply export the map from the PC program to the extra Zero Line Map Cards and you'll have multiple charts of your favorite fishing spots ready to view on a range of compatible* Humminbird fish finders.*Optional for select SOLIX, ONIX, ION, HELIX, 1100, 900, 800, and 600 Series models. Compatibility subject to change without notice. 32 GB SD card containing zero lines with over 140,000 named water bodies and over 1.75 million unnamed water bodies in the United States and over 96,000 named water bodies and over 3.85 million unnamed water bodies in Canada. Plus there is 16 GB of free space to maintain your custom maps.Card must be present to get the detailed zero line base map to display in AutoChart PC All Humminbird units will require a software update for this option