Global temperature is difficult to measure. Think of the room you are in, the thermostat on the wall may give you the temperature. But if you move a thermometer near a window or by a heater you may read a different temperature. So you could put lots of themometers in the room and take the average but that a lot of work to get the temperature of one small room. Fortunately we are not interested in the global temperature but the change in temperature, or temperature anomaly, from year to year. So if we set up a thermometer and don't change anything and record the temperature consistently each day we can see the change in temperature over the years. However there are large areas of the earth where there are no measurements, for instance the oceans which cover about 75% of the earth do not have many stations. Also continents like Africa and Antarctica only have a few stations. To complicate things further many weather stations are in urban settings and there changes around them. For example forests may be cut down and replaced with concrete buildings or asphalt parking lots making heat islands. To get an estimate of the sea surface temperature (SST) measurements of water temperature from ships is utilized.  Of course all these measurements are recorded within a few meters of the earth's surface and tell us nothing about what is happening in the atmosphere as a whole. Since about 1950 weather balloons have circumvented some of these issues by passing up through the atmosphere they get away from heat islands and provide a full altitude temperature profile. Starting in 1979 satellites have been orbiting the earth and recording the temperature. They too measure the entire atmosphere and cover all areas of our planet.