Heading down to Santa Monica Pier today, I heard there is a crap ton of pokemon there.
so I just found out the exact spawns for Pokémon in this game (and it make perfect sense). the entire game is pulled directly off of Ingresses existing data meaning ALL pokestops and gyms you see are portals in Ingress with gyms being portals that were visited the most in Ingress.
Pokémon go uses the Ingress exotic matter (white dots on map) as the spawn points for all the Pokémon (although I am not aware of how frequently they will spawn from those locations). so obviously if you download Ingress and look at the map you will physically see all the possible Pokémon spawn points, and places where there are giant clusters of exotic mater will spawn more Pokémon.
how Niantic created this exotic mater in ingress though is entirely based off of Googles assets (as they previously owned Niantic). dots are placed where people with mobile phones use any bit of Google functionality (googleing, maps, g-mail, etc.) off of Wi-Fi, and after so much data has been used in a certain spot it will create a dot. so what becomes obvious knowing this is that places where a LOT of people congregate see a REDICULOSE amounts of Pokémon spawns. so places like santa monica pier where there are no areas to connect to the internet and have a lot of foot traffic are the BEST places to go to find lots of pokemon and rare ones.
Here in my town this became clear when I went down to the Mark and the Expo center, Which are venues for concerts, sports, conventions, etc. that are frequented by lots of people. only 1 pokestop each but anywhere you walk there are 5+ spawns.