But (from memory) they aren't: they reflect a very very few measurements from a few years in a span of decades. (tamino.wordpress.com)
Incorporating this kind of data into the models has been difficult in part because geostationary data provide fewer measurements for any given vertical slice of the atmosphere than do polar orbiters, which circle Earth at lower altitudes. (scientificamerican.com)
Both mostly use the same network of surface stations, but they differ in how they extrapolate temperature changes in areas with few measurement stations. (yaleclimateconnections.org)