A survey won't produce enough sample points to be statistically significant. You'd get more statistically reliable numbers using Google Keyword Tool. For example, if you're thinking of making a voip app for the android, GKT shows 9,900 global monthly searches for "voip android," which is a lot lower than the 3.35M that voip searches get and the 13.6M that android searches get. Both are big markets, and I can't imagine "guesswork" that would somehow arrive at 10k by combining the 2 market numbers of 3M and 13M. Google Keyword Tool would be one of the most reliable ways to give you the confidence or warning signs you need to dive in or avoid implementation.