Survey drop chance is a flat rate regardless of level, dlc ownership etc.
You don't get survey maps for dlc zones you don't own or have access to so, if you only own the base game, you will only get survey maps for base game zones. For example before I bought the High Isle chapter I didn't get any for High Isle but as soon as I bought the chapter, many months after release, I started to get High Isle survey maps. Same experience with previous chapters or dlc zones I still don't own or have access to.
edit: disambiguation
redlink1979 wrote: »The drop chance ratio of the surveys isn't related to the content owned by the player.
Players are able to get survey reports of any zone where they are available as a reward but if the player doesn't own the dlc zone he/she just won't be able to pick them up (just as it happens with leads for scrying/excavating antiquities: players are able to get the leads but without the required dlc zones players aren't able to scry/excavate the leads gathered while playing.)
The drop rate does not depend on DLC ownership. It's a flat 1 in 8 chance of "getting survey" then the game picks a random zone for it to be in (and then a random one from that zone, if it has multiple, such as Craglorn, Wrothgar or Coldharbour)