To the best of my knowledge many departments are sent the data from NASA.Terra and Aqua orbit the earth every 12 hours and can read the UV light emitted from the surface.From that the data is compiled.The temperature can change in a very short order before or after the measurements are taken.Clouds and water vapour affect the readings.Any missing data is made up to fill the gaps its called homogenising and is accepted.land readings are very poor vast areas of Africa and Australia do not have readings at all.The seas are even more difficult to measure.My local beach goes from 23deg to 17deg C in any given calender year.From this the different departments try to calculate the average and come up with vastly varying results.East Anglia results are a lot less than GISS.Never guess which data is used its GISS every time.And the results are given to .00 of a degree.More than a little suspicious.I watched a Canadian scientist give a speel and his speciality is how temperature affects material at the molecular level.He has always had an interest in the atmosphere and just shrugs and declares it is too complicated to work out.There are too many variables to consider.NASA do not have the luxury of spending billions of dollars then stating they could not do it.They have to have a result regardless.Do not start me on how satellites alledgedly measure sea level its even more guesswork.Remember Satellites are at least 9000 kms out or further or gravity drags them back down.They have to be boosted anyhoo.Something between 9000 and 10,000 kms away can measure to a millimetre on the surface?Civil GPS is calibrated to plus minus 100 foot which is why your car sometimes tells you to do a U turn and you are parked where you wish to be.Happens to me all the time.