Bingo! Good answer. I believe there has also been a reduction in the number of rural data collection stations and that only increases the effect of the data from the “heat islands”.
I live in a rural area where the official Weather Service station is located at the small local airport among sand, rocks and sage brush. It is always at least 7 degrees hotter out there than anywhere else around.
That's part of it. The computer modelers also resampled and adjusted historic temperature data. In many cases their adjustments resulted in lower "temperatures" for locations in, for example, the 1930s than were actually recorded by the weather observers. Needless to say that "adjustment" makes the increase in temperature seem greater than it would be if the real data was used.
Global warming believers point to weather data series that show warming, like the data from Blue Hills Observatory in Massachusetts, but they ignore other data from the same recording stations, like average wind speed data, which are inversely correlated with the temperature data but don't fit the AGW models.