My grandfather had photographic memory, and he was once trying to explain to me how he is able to retain so much data. He was also a savant, in the memory and mathematics area, so it was way beyond anything I could ever comprehend. But it was cool because he said that he remembered things by colors and distorted types of images, rather than just by raw numbers or data.
My uncle is also a savant, and knows every road, street, avenue etc in the entire world. How he does it, I don't even know. But the guy is like a GPS/Mapquest program. Put it this way, he was in France about 10 years ago and some fellow Americans asked him for directions, and not only did he give them precise ones, but he had never been to where they wanted to go, and was able to also tell them which lanes to stay in to avoid traffic, what exits or things to look at on the road to assist them as well. He's fucking brilliant.