It means.....

It is not down in any map; true places never are. ~Herman Melville

Well, San Diego is in fact on a map. You can find it on google maps and just about any other map. You probably can't find it on a treasure map unless someone decided to bury their treasure in San Diego, which wouldn't be the coolest place to hide your precious jewels and coin.

Tommorow I embark for the Great Diego. It'll be like a great voyage and I'll be the captain. Destination unknown, adventure everywhere and peril always at the helm. I'm not sure what that means but it sounds fun. Onward!