It depends on what you want to measure. Big cities are hotter than small are hotter than villages are hotter than forests.

If you wish to show that baseline temperature increases, you let more measuring stations in urban areas. If you intend to compare apples with apples, you must consider the kind of location where your measuring station is.

It's like bringing more and more Hobbits to Gondor, and then complaining that an average height of the Gondorian citizens decreased over time. But did the height of an ethnic Gondorian really decrease?