RealClimate: 2009 temperatures by Jim Hansen
The GISS analysis assigns a temperature anomaly to many gridboxes that do not contain measurement data, specifically all gridboxes located within 1200 km of one or more stations that do have defined temperature anomalies.Yet Another Amazing Detail About Climate Science « The Unbearable Nakedness of CLIMATE CHANGE
The rationale for this aspect of the GISS analysis is based on the fact that temperature anomaly patterns tend to be large scale. For example, if it is an unusually cold winter in New York, it is probably unusually cold in Philadelphia too.
Here’s the root of it all, from 1987:The 1200-km limit is the distance at which the average correlatiom coefficient of temperature variations falls to 0.5 at middle and high latitudes and 0.33 at low latitude