The instrumental record of surface temperature change is based on a combination of land air, marine air, and ocean surface temperature changes recorded over roughly the past century and a half. While several different datasets exist, the most widely used version has been produced by the Climatic Research Unit (CRU) of the University of East Anglia in the UK. Issues of consistency and homogeneity of the measurements through time have been taken into account in constructing this global surface temperature database, and measures have been taken to ensure that all non-climatic inhomogeneities (including Urban Heat Island effects) have been removed.
An Urban Heat Island is a metropolitan area which is significantly warmer than its surroundings. As population centers grow in size , they tend to have a corresponding increase in average temperature. Not to be confused with global warming, scientists refer to this phenomenon as the “Urban Heat Island Effect” (UHIE). There is little controversy in the existence of the UHIE. What is more controversial is whether, and if so how much, this additional warmth affects the (global) temperature record. The current state of the science is that the effect on the global temperature record is small to negligible. More information can be found here.