Global Warming Thermometers

How do we know if the earth is warming if we don't know what the actual temperature is?  It seems simple enough, but if there are only 4 agencies that measure global temperatures and they don't agree with each other, how do we know what the truth is?  NASA declares their data isn't really trustworthy according to this article, and that leaves the CRU, which is at the center of the climate-gate scandal.  Additionally, according to one site, SurfaceStations.org, 90 percent of the thermometers are less than 100 feet from "biasing influences", like garbage incinerators, asphalt roads, industrial centers, etc.  So, again, how are these scientists really certain what the temperature really is?