You are absolutely correct. There is no way we can calculate an average global temperature. A statistically huge number of points would have to be monitored to be close to accurate. Like "relative bearing" where the direction to port or starboard is measured in degrees relative to the direction the ship is heading, the only way to accurately to determine if there is any change in the global "average" temperature is to use a large set of points, not effected by human development, over a long time period. BUT, an attempt is already in progress to provide relatively consistent data and it shows no statistical deviation in the planets temperature for the last 19 years. Funny about that.