How does Jim memorize the map?

From the text:

"I brooded by the hour together over the map, all the details of which I well remembered. Sitting by the fire in the housekeeper's room, I approached that island in my fancy from every possible direction; I explored every acre of its surface; I climbed a thousand times to that tall hill they call the Spy-glass, and from the top enjoyed the most wonderful and changing prospects. Sometimes the isle was thick with savages, with whom we fought, sometimes full of dangerous animals that hunted us, but in all my fancies nothing occurred to me so strange and tragic as our actual adventures."